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** Programmer's Technical Reference for MSDOS and the IBM PC ** C H A P T E R E I G H T DOS DISK INFORMATION C O N T E N T S The DOS Area .......................................................... 8**1 The Boot Record ....................................................... 8**2 DOS File Allocation Table (FAT) ....................................... 8**3 Media Descriptor Byte ......................................... 8**4 12 Bit FATs ................................................... 8**5 16 Bit FATs ................................................... 8**6 DOS Disk Directory .................................................... 8**8 The Data Area ......................................................... 8**9 Floppy Disk Types ..................................................... 8**10 Hard Disk Layout ...................................................... 8**11 System Initialization ................................................. 8**12 Boot Record/Partition Table ........................................... 8**13 Hard Disk Technical Information ....................................... 8**14 Determining Hard Disk File Allocation ................................. 8**15 BIOS Disk Functions ................................................... 8**16 00h Reset 01h Get Status 02h Read Sectors 03h Write Sectors 04h Verify 05h Format Track (floppy disk) 06h Hard Disk - format track 07h Hard Disk - format drive 08h Read Drive Parameters 09h Initialize Two Fixed Disk Base Tables 0Ah Read Long (Hard disk) 0Bh Write Long (Hard disk) 0Ch Seek To Cylinder 0Dh Alternate Hard Disk Reset 0Eh Read Sector Buffer 0Fh Write sector buffer 10h Test For Drive Ready 11h Recalibrate Drive 12h Controller RAM Diagnostic 13h Controller Drive Diagnostic 14h Controller Internal Diagnostic 15h Get Disk Type 16h Get Disk Change Status (diskette) 17h Set Disk Type for Format (diskette) 18h Set Media Type For Format (diskette) 19h Park Hard Disk Heads 1Ah ESDI Hard Disk - Low Level Format 1Bh ESDI Hard Disk - Get Manufacturing Header 1Ch ESDI Hard Disk - Get Configuration THE DOS AREA├────────────────────────────────────────────────────────── 8**1 All disks and diskettes formatted by DOS are created with a sector size of 512 bytes. The DOS area (entire area for a diskette, DOS partition for hard disks) is formatted as follows: ┌────────────────────────────────────────────────────────────┐ │ D O S A R E A │ ├────────────────────────────────────────────────────────────┤ │ partition table - variable size (hard disk only) │ │ boot record - 1 sector │ │ first copy of the FAT - variable size │ │ second copy of the FAT - same size as first copy │ │ root directory - variable size │ │ data area - variable depending on disk size │ └────────────────────────────────────────────────────────────┘ The following sections describe each of the allocated areas: THE BOOT RECORD├─────────────────────────────────────────────────────── 8**2 The boot record resides on track 0, sector 1, side 0 of every diskette formatted by the DOS FORMAT program. For hard disks the boot record resides on the first sector of the DOS partition. It is put on all disks to provide an error message if you try to start up with a nonsystem disk in drive A:. If the disk is a system disk, the boot record contains a JMP instruction pointing to the first byte of the operating system. If the device is IBM compatible the first sector of the first FAT must be located at the same sector for all disk types. This is because the FAT sector is read before the disk type is actually determined. The information relating to the BPB for a particular media is kept in the disk's boot sector. The format of the boot sector is: ┌──────────────────────────────────────────────────────────────────────────────┐ │ D O S B O O T R E C O R D │ ├───┬───────┬──────────────────────────────────────────────────────────────────┤ │00h│3 bytes│ JMP to executable code. For DOS 2.x, 3 byte near jump (0E9h). │ │ │ │ For DOS 3.x, 2 byte near jump (0EBh) followed by a NOP (90h) │ ├───┼───────┼──────────────────────────────────────────────────────────────────┤ │03h│8 bytes│ optional OEM name and version (such as IBM 2.1) │ ├───┼───────┼──────────────────────────────────────────────────────────────────┤ │0Bh│2 bytes│ bytes per sector │ ├───┼───────┼─────┬────────────────────────────────────────────────────────────┤ │0Dh│ byte │ │ sectors per allocation unit (must be a power of 2) │ ├───┼───────┤ ├────────────────────────────────────────────────────────────┤ │0Eh│2 bytes│ B │ reserved sectors (starting at logical sector 0) │ │ │ │ │ 01 for 1.x-3.31, 02 for 4.0+ │ ├───┼───────┤ ├────────────────────────────────────────────────────────────┤ │10h│ byte │ │ number of FATs │ ├───┼───────┤ ├────────────────────────────────────────────────────────────┤ │11h│2 bytes│ │ maximum number of root directory entries │ ├───┼───────┤ P ├────────────────────────────────────────────────────────────┤ │13h│2 bytes│ │ number of sectors in logical image (total number of │ │ │ │ │ sectors in media, including boot sector directories, etc.)│ │ │ │ │ If logical disk size is geater than 32Mb, this value is 0 │ │ │ │ │ and the actual size is reported at offset 26h (DOS 4.0+) │ ├───┼───────┤ ├────────────────────────────────────────────────────────────┤ │15h│ byte │ B │ media descriptor byte │ ├───┼───────┤ ├────────────────────────────────────────────────────────────┤ │16h│2 bytes│ │ number of sectors occupied by a single FAT │ ├───┼───────┼─────┴────────────────────────────────────────────────────────────┤ │18h│2 bytes│ sectors per track │ ├───┼───────┼──────────────────────────────────────────────────────────────────┤ │1Ah│2 bytes│ number of heads │ ├───┼───────┼──────────────────────────────────────────────────────────────────┤ │1Ch│2 bytes│ # of hidden sectors (sectors before this volume) (1st part) │ └───┴───────┼──────────────────────────────────────────────────────────────────┤ │ EXTENDED BOOT RECORD (DOS 4.0+) │ ┌───┬───────┼──────────────────────────────────────────────────────────────────┤ │1Eh│2 bytes│ # of hidden sectors (sectors before this volume) (2nd part) │ ├───┼───────┼──────────────────────────────────────────────────────────────────┤ │20h│4 bytes│ # sectors in this disk (see offset 13h, if 0) │ ├───┼───────┼──────────────────────────────────────────────────────────────────┤ │24h│2 bytes│ physical drive number (max 2 for DOS 4, max 8 for DOS 5) │ ├───┼───────┼──────────────────────────────────────────────────────────────────┤ │26h│ byte │ extended boot record signature (29h) │ ├───┼───────┼──────────────────────────────────────────────────────────────────┤ │27h│4 bytes│ volume serial number (assigned with a random function) │ ├───┼───────┼──────────────────────────────────────────────────────────────────┤ │2Bh│11 byte│ volume label │ ├───┼───────┼──────────────────────────────────────────────────────────────────┤ │36h│7 bytes│ file system ID (FAT12 ), (FAT16 ) etc. ("reserved") │ └───┴───────┴──────────────────────────────────────────────────────────────────┘ The three words at the end return information about the media. The number of heads is useful for supporting different multihead drives that have the same storage capacity but a different number of surfaces. The number of hidden sectors is useful for drive partitioning schemes. DOS 3.2 uses a table called the BIOS Parameter Block (BPB) to determine if a disk has a valid File Allocation Table. The BPB is located in the first sector of a floppy disk. Although the BPB is supposed to be on every formatted floppy disk, some earlier versions of DOS did not create a BPB and instead assumed that the FAT begins at the second sector of the disk and that the first FAT byte (Media Descriptor Byte) describes the disk format. DOS 3.2 reads in the whole of the BPB and tries to use it - although strangely enough, it seems as if DOS is prepared to cope with a BPB that is more or less totally blank (it seems to ignore the descriptor byte and treat it as a DSDD 9-sector disk). DOS 3.2 determines if a disk has a valid boot sector by examining the first byte of logical sector 0. If that byte it a jump instruction 0E9h, DOS 3.2 assumes the rest of the sector is a valid boot sector with a BPB. If the first byte is not 0E9h DOS 3.2 behaves like previous versions, assumes the boot sector is invalid and uses the first byte of the FAT to determine the media type. If the first byte on the disk happens to be 0E9h, but the disk does not have a BPB, DOS 3.2 will return a disk error message. The real problems occur if some of the BPB data is valid and some isn't. Apparently some OEMs have assumed that DOS would continue to ignore the formatting data on the disk, and have failed to write much there during FORMAT except the media descriptor byte (or, worse, have allowed random junk to be written there). While this error is understandable, and perhaps even forgiveable, it remains their problem, not IBM's, since the BPB area has always been documented as containing the format information that IBM DOS 3.2 now requires to be there. When the BPB problems first became evident with DOS 3.2 a number of reports circulated claiming DOS looked for the letters "IBM" in the OEM ID field. This was incorrect. IBM DOS 4.0 *did* check for the letters "IBM" and would refuse to recognize hard drives formatted under MSDOS 4.0. IBM corrected this with their 4.01 revision. THE DOS FILE ALLOCATION TABLE (FAT)├─────────────────────────────────── 8**3 The File Allocation Table, or FAT, has three main purposes: 1) to mark bad sectors on the media 2) to determine which sectors are free for use 3) to determine the physical location(s) of a file on the media. DOS uses one of two schemes for defining the File Allocation Table: 1) a 12-bit FAT, for DOS 1.x, 2.x, all floppies, and small hard disks 2) a 16-bit FAT, for DOS 3.x+ hard disks from 16.8 to 32Mb This section explains how DOS uses the FAT to convert the clusters of a file into logical sector numbers. It is recommended that system utilities use the DOS handle calls rather than interpreting the FAT, particularly since aftermarket disk partitioning or formatting software may have been used. The FAT is used by DOS to allocate disk space for files, one cluster at a time. In DOS 4.0, clusters are referred to as "allocation units." It means the same things; the smallest logical portion of a drive. The FAT consists of a 12 bit entry (1.5 bytes) for each cluster on the disk or a 16 bit (2 bytes) entry when a hard disk has more than 20740 sectors as is the case with fixed disks larger than 10Mb. The first two FAT entries map a portion of the directory; these FAT entries contain indicators of the size and format of the disk. The FAT can be in a 12 or 16 bit format. DOS determines whether a disk has a 12 or 16 bit FAT by looking at the total number of allocation units on a disk. For all diskettes and hard disks with DOS partitions less than 20,740 sectors, the FAT uses a 12 bit value to map a cluster. For larger partitions, DOS uses a 16 bit value. The second, third, and fourth bit applicable for 16 bit FAT bytes always contains 0FFFFh. The first byte is used as follows: Media Descriptor Byte ................................................. 8**4 ┌──────────────────────────────────────────────────────────────────────────────┐ │ M E D I A D E S C R I P T O R B Y T E │ ├──────────┬──────────────────────────────────┬────────────────────────────────┤ │hex value │ meaning │ normally used │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ 00 │ hard disk │ 3.3+ extended DOS partition │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ ED │ double sided 9 sector 80 track │ Tandy 2000 720k (5¼) │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ F0 │ double sided 18 sector diskette │ PS/2 1.44 meg DSHD │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ F8 │ hard disk │ bootable hard disk at C:800 │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ F8 │ 720k floppy, 9 sector 80 track │ Sanyo 55x, DS-DOS 2.11 (5¼) │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ F9 │ double sided 15 sector diskette │ AT 1.2 meg DSHD │ │ │ double sided 9 sector diskette │ Convertible 720k DSQD │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ FA │ IBM Displaywriter System disk │ 287k │ │ │ Kodak "4 meg" (Pelican) │ 4.4 meg (5¼) │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ FB │ IBM Displaywriter System disk │ 1 meg (5¼) │ │ │ Kodak "6 meg" (Pelican) │ 5.5 meg (5¼) │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ FC │ single sided 9 sector diskette │ DOS 2.0, 180k SSDD (5¼) │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ FD │ double sided 9 sector diskette │ DOS 2.0, 360k DSDD (5¼) │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ FF │ double sided 36 sector diskette │ Practidisk 2.88mb DSED (3½) │ │ │ single sided 8 sector diskette │ DOS 1.0, 160k SSDD (5¼) │ │ │ double sided 8 sector diskette │ DOS 1.1, 320k SSDD (5¼) │ │ │ hard disk │ Sanyo 55x with DS-DOS 2.11 │ ├──────────┴───────────┬──────────────────────┴────────────────────────────────┘ │for 8 inch diskettes: │ ├──────────┬───────────┴──────────────────────┬────────────────────────────────┐ │ FD │ double sided 26 sector diskette │ IBM 3740 format DSSD │ ├──────────┼──────────────────────────────────┼────────────────────────────────┤ │ FE │ single sided 26 sector diskette │ IBM 3740 format SSSD │ │ ├──────────────────────────────────┼────────────────────────────────┤ │ │ double sided 8 sector diskette │ IBM 3740 format DSDD │ └──────────┴──────────────────────────────────┴────────────────────────────────┘ The third FAT entry begins mapping the data area (cluster 002). NOTE: These values are provided as a reference. Therefore, programs should not make use of these values. Each entry contains three hexadecimal characters for 12-bit FATs or four for 16-bit FATs. The possible entries are: 12-bit | 16-bit | 000h | 0000h if the cluster is unused and available 0FF7h | 0FFF7h bad cluster (if not part of the allocation chain) | 0FF0h-0FF7h | 0FFF0h-0FFF7h to indicate reserved clusters | 0FF8h-0FFFh | 0FFF8h-0FFFFh to indicate the last cluster of a file (EOF) | xxxH | xxxxH any other hexadecimal numbers are the cluster | number of the next cluster in the file. The | cluster number is the first cluster in the file | that is kept in the file's directory entry. The file allocation table always occupies the sector or sectors immediately following the boot record. If the FAT is larger than 1 sector, the sectors occupy consecutive sector numbers. Two copies of the FAT are written, one following the other, for integrity. The FAT is read into one of the DOS buffers whenever needed (open, allocate more space, etc). 12 Bit File Allocation Table .......................................... 8**5 Obtain the starting cluster of the file from the directory entry. Now, to locate each subsequent sector of the file: 1. Multiply the cluster number just used by 1.5 (each FAT entry is 1.5 bytes long). 2. The whole part of the product is offset into the FAT, pointing to the entry that maps the cluster just used. That entry contains the cluster number of the next cluster in the file. 3. Use a MOV instruction to move the word at the calculated FAT into a register. 4. If the last cluster used was an even number, keep the low order 12 bits of the register, otherwise, keep the high order 12 bits. 5. If the resultant 12 bits are (0FF8h-0FFFh) no more clusters are in the file. Otherwise, the next 12 bits contain the cluster number of the next cluster in the file. To convert the cluster to a logical sector number (relative sector, such as that used by int 25h and 26h and DEBUG): 1. Subtract 2 from the cluster number 2. Multiply the result by the number of sectors per cluster. 3. Add the logical sector number of the beginning of the data area. 12-bit FAT if DOS partition is smaller than 32,680 sectors (16.340 MB). 16 Bit File Allocation Table .......................................... 8**6 Obtain the starting cluster of the file from the directory entry. Now to locate each subsequent cluster of the file: 1. Multiply the cluster number used by 2 (each FAT entry is 2 bytes long). 2. Use the MOV word instruction to move the word at the calculated FAT offset into a register. 3. If the resultant 16 bits are (0FF8h-0FFFFh) no more clusters are in the file. Otherwise, the 16 bits contain the cluster number of the next cluster in the file. DOS Disk Directory .................................................... 8**8 The FORMAT command initially builds the root directory for all disks. Its location (logical sector number) and the maximum number of entries are available through the device driver interfaces. Since directories other than the root directory are actually files, there is no limit to the number of entries that they may contain. All directory entries are 32 bytes long, and are in the following format: ┌───────┬─────────┬──────────────────────────────────────────────────────────── │offset │ size │ DISK DIRECTORY ENTRY ├───────┼─────────┼──────────────────────────────────────────────────────────── │ 00h │ 8 bytes │ Filename │ ├─────────┴──────────────────────────────────────────────────────────── │ │ The first byte of the filename indicates the file status. │ │ The file status byte may contain the following values: │ ├──────┬─────────────────────────────────────────────────────────────── │ │ 00h │ Directory entry has never been used. This is used to limit │ │ │ the length of directory searches, for performance reasons. │ │ 05h │ Indicates that the first character of the filename actually │ │ │ has an 0EDh character. │ │ 0E5h │ Filename has been used but the file has been erased. │ │ 2Eh │ This entry is for a directory. If the second byte is also │ │ │ 2Eh, the cluster field contains the cluster number of this │ │ │ directory's parent directory. (0000h if the parent directory │ │ │ is the root directory). Otherwise, bytes 00h-0Ah are all │ │ │ spaces and the cluster field contains the cluster number of │ │ │ the directory. │ ├──────┴─────────────────────────────────────────────────────────────── │ │ Any other character is the first character of a filename. Filenames │ │ are left-aligned and if necessary padded with blanks. ├───────┼─────────┬──────────────────────────────────────────────────────────── │ 08h │ 3 bytes │ Filename extension if any │ ├─────────┴──────────────────────────────────────────────────────────── │ │ Three characters, left-aligned and padded with blanks if necessary. │ │ If there is no file extension, this field contains all blanks ├───────┼─────────┬──────────────────────────────────────────────────────────── │ 0Bh │ 1 byte │ File attributes │ ├─────────┴──────────────────────────────────────────────────────────── │ │ The attribute byte is mapped as follows: │ ├─────┬───┬──────────────────────────────────────────────────────────── │ │ hex │bit│ meaning │ ├─────┼───┼──────────────────────────────────────────────────────────── │ │ 00h │ │ (no bits set) normal; can be read or written without │ │ │ │ restriction │ │ 01h │ 0 │ file is marked read-only. An attempt to open the file for │ │ │ │ output using int 21h/fn 3Dh will fail and an error code │ │ │ │ will be returned. This value can be used with other values │ │ │ │ below. │ │ 02h │ 1 │ indicates a hidden file. The file is excluded from normal │ │ │ │ directory searches. │ │ 04h │ 2 │ indicates a system file. The file is excluded from normal │ │ │ │ directory searches. │ │ 08h │ 3 │ indicates that the entry contains the volume label in the │ │ │ │ first 11 bytes. The entry has no other usable information │ │ │ │ and may exist only in the root directory. │ │ 10h │ 4 │ indicates that the file is a subdirectory │ │ 20h │ 5 │ indicates an archive bit. This bit is set to on whenever │ │ │ │ the file is written to and closed. Used by BACKUP and │ │ │ │ RESTORE. │ │ │ 6 │ reserved, set to 0 │ │ │ 7 │ reserved, set to 0 │ ├─────┴───┴──────────────────────────────────────────────────────────── │ │ note 1) Bits 6 and 7 may be used in OS/2. │ │ note 2) Attributes 08h and 10h cannot be changed using int21/43h. │ │ note 3) The system files IBMBIO.COM and IBMDOS.COM (or customized │ │ equivalent) are marked as read-only, hidden, and system │ │ files. Files can be marked hidden when they are created. │ │ note 4) Read-only, hidden, system and archive attributes may be │ │ changed with int21h/fn43h. ├───────┼─────────┬──────────────────────────────────────────────────────────── │ 0Ch │ 10 bytes│ Reserved by DOS; value unknown ├───────┼─────────┼──────────────────────────────────────────────────────────── │ 16h │ 2 bytes │ File timestamp │ ├─────────┴──────────────────────────────────────────────────────────── │ │ These bytes contain the time when the file was created or last │ │ updated. The time is mapped in the bits as follows: │ ├───────────────────────────────┬───────────────────────────────┐ │ │ B Y T E 16h │ B Y T E 17h │ │ ├───────────────────────────────┼───────────────────────────────┤ │ │ F E D C B A 9 8 │ 7 6 5 4 3 2 1 0 │ │ ├───────────────────┬───────────┴───────────┬───────────────────┤ │ │ H H H H H │ M M M M M M │ D D D D D │ │ ├───────────────────┼───────────────────────┼───────────────────┤ │ │ binary # hrs 0-23 │ binary # minutes 0-59 │ bin. # 2-sec incr │ │ ├───────────────────┴───────────────────────┴───────────────────┘ │ │ note: The time is stored with the least significant byte first. ├───────┼─────────┬──────────────────────────────────────────────────────────── │ 18h │ 2 bytes │ File datestamp │ ├─────────┴──────────────────────────────────────────────────────────── │ │ This area contains the date when the file was created or last │ │ updated. The mm/dd/yy are mapped in the bits as follows: │ ├───────────────────────────────┬───────────────────────────────┐ │ │ B Y T E 18h │ B Y T E 19h │ │ ├───────────────────────────────┼───────────────────────────────┤ │ │ F E D C B A 9 8 │ 7 6 5 4 3 2 1 0 │ │ ├───────────────────────────┬───┴───────────┬───────────────────┤ │ │ Y Y Y Y Y Y Y │ M M M M │ D D D D D │ │ ├───────────────────────────┼───────────────┼───────────────────┤ │ │ 0-119 (1980-2099) │ 1-12 │ 1-31 │ │ ├───────────────────────────┴───────────────┴───────────────────┘ │ │ note: The date is stored with the least significant byte first. ├───────┼─────────┬──────────────────────────────────────────────────────────── │ 1Ah │ 2 bytes │ First file cluster number │ ├─────────┴──────────────────────────────────────────────────────────── │ │ * (reserved in DOS 2, documented in DOS 3+) │ │ This area contains the starting cluster number of the first cluster │ │ in the file. The first cluster for data space on all fixed disks and │ │ floppy disks is always cluster 002. The cluster number is stored │ │ with the least significant byte first. ├───────┼─────────┬──────────────────────────────────────────────────────────── │ 1Ch │ 4 bytes │ File size │ ├─────────┴──────────────────────────────────────────────────────────── │ │ This area contains the file size in bytes. The first word contains │ │ the low order part of the size. Both words are stored with the least │ │ significant byte first. └───────┴────────────────────────────────────────────────────────────────────── The Data Area ......................................................... 8**9 Allocation of space for a file (in the data area) is done only when needed (it is not preallocated). The space is allocated one cluser (unit allocation) at a time. A cluster is always one or more consecutive sector numbers, and all of the clusters in a file are "chained" together in the FAT. The clusters are arranged on disk to minimize head movement for multisided media. All of the space on a track (or cylinder) is allocated before moving on to the next track. This is accomplished by using the sequential sector numbers on the lowest-numbered head, then all the sector numbers on the next head, and so on until all sectors of all heads of the track are used. Then the next sector used will be sector 1 of head 0 on the next track. An interesting innovation that was introduced in MS-DOS 3.0: disk space that is freed by erasing a file is not re-used immediately, unlike earlier versions of DOS. Instead, free space is obtained from the area not yet used during the current session, until all of it is used up. Only then will space that is freed during the current session be re-used. This feature minimizes fragmentation of files, since never-before-used space is always contiguous. However, once any space has been freed by deleting a file, that advantage vanishes at the next system boot. The feature also greatly simplifies un-erasing files, provided that the need to do an un-erase is found during the same session and also provided that the file occupies contiguous clusters. However, when one is using programs which make extensive use of temporary files, each of which may be created and erased many times during a session, the feature becomes a nuisance; it forces the permanent files to move farther and farther into the inner tracks of the disk, thus increasing rather than decreasing the amount of fragmentation which occurs. The feature is implemented in DOS by means of a single 16-bit "last cluster used" (LCU) pointer for each physical disk drive; this pointer is a part of the physical drive table maintained by DOS. At boot time, the LCU pointer is zeroed. Each time another cluster is obtained from the free-space pool (the FAT), its number is written into the LCU pointer. Each time a fresh cluster is required, the FAT is searched to locate a free one; in older versions of DOS this search always began at Cluster 0000, but in 3.x it begins at the cluster pointed to by the LCU pointer. For hard disks, the size of the file allocation table and directory are determined when FORMAT initializes it and are based on the size of the DOS partition. Floppy Disk Types ..................................................... 8**10 The following tables give the specifications for floppy disk formats: IBM PC-DOS disk formats: # of FAT size DIR total sides (sectors)(entries) sectors │ sectors │ DIR │ sectors│ │ /track │sectors│/cluster│ │ │ │ │ │ │ │ ┌─────┬──┬───────┬─┴─┬───┴───┬─┴─┬─┴─┬─┴─┬─┴─┬──┴─┬──────────────────────────── │ 160k│5¼│DOS 1.0│ 1 │ 8 (40)│ 1 │ 4 │ 64│ 1 │ 320│Original PC-0, 16k mbd │ 320k│5¼│DOS 1.1│ 2 │ 8 (40)│ 1 │ 7 │112│ 2 │ 360│PC-1, 64k mbd │ 180k│5¼│DOS 2.0│ 1 │ 9 (40)│ 2 │ 4 │ 64│ 1 │ 640│PC-2, 256k mbd │ 360k│5¼│DOS 2.0│ 2 │ 9 (40)│ 2 │ 7 │112│ 2 │ 720│PC/XT │ 1.2M│5¼│DOS 3.0│ 2 │15 (80)│ 7 │14 │224│ 1 │2400│PC/AT, PC/RT, XT/286 │ 720k│3½│DOS 3.2│ 2 │ 9 (80)│ 3 │ 7 │112│ 2 │1440│Convertible, PS/2 25+ │1.44M│3½│DOS 3.3│ 2 │18 (80)│ 9 │14 │224│ 1 │2880│PS/2 50+ └─────┴──┴───────┴───┴───────┴───┴───┴───┴───┴────┴──────────────────────────── various MS-DOS disk formats: ┌─────┬──┬───────┬─┴─┬───┴───┬─┴─┬─┴─┬─┴─┬─┴─┬──┴─┬──────────────────────────── │ 200k│5¼│ * │ 1 │10 (40)│ │ │ │ │ │ │ 400k│5¼│ * ** │ 2 │10 (40)│ │ │ │ │ │ │ 800k│5¼│ * │ 2 │10 (80)│ │ │ │ │ │ │ 720k│2 │ │ │ │ │ │ │ │ │Zenith SuperSport 2-inch │ 720k│5¼│DOS2.11│ 2 │ 9 (80)│ 3 │ 7 │112│ 2 │1440│Tandy 2000 (discontinued) │2.88M│3½│ │ 2 │36 (80)│ │ │ │ │5760│Practidisk 2.88mb floppy │2720k│5¼│ *** │ 2 │17(192)│ 8 │ │272│ 4 │5440│Pelican (Kodak 3.3Mb)(disc.) │5570k│5¼│ *** │ 2 │17(384)│ 8 │ │272│ 4 │10880Pelican (Kodak 6.6Mb)(disc.) └─────┴──┴───────┴───┴───────┴───┴───┴───┴───┴────┴──────────────────────────── * Michtron DS-DOS 2.11 Plus and one version of MS-DOS 3.11 (vendor unknown) ** TallTree JFormat program *** Pelican driver source calls for 2 sectors/cluster, Norton Utils reports 4. ┌─────┬──┬───────┬─┴─┬───┴───┬─┴─┬─┴─┬─┴─┬─┴─┬──┴─┬──────────────────────────── │ 400k│5¼│DOS2.11│ 1 │10 (80)│ │ │ │ │ 800│DEC Rainbow SS/HD (disc.) │ 720k│5¼│DOS2.11│ 2 │variable number of sectors │Victor 9000 PC (discont'd) └─────┴──┴───────┴───┤per track, more sectors on ├──────────────────────────── │outer tracks than inner │ │tracks. Special DSDD drive. │ └────────────────────────────┘ Some oddball DOS versions specify "zero" heads in their data area. HP's single-sided disk format (16 256-byte sectors/track, model unknown) uses a zero-based parameter for the number of heads. Without special device driver software to "fix" this, these disks are basically unusable by normal DOS. A couple of people have reported that the IBM "Gearbox" industrial PC uses an 800k 3.5 inch floppy with 10 sectors and 80 tracks. I've been unable to confirm this. Files in the data area are not necessarily written sequentially. The data area space is allocated one cluster at a time, skipping over clusters already allocated. The first free cluster found is the next cluster allocated, regardless of its physical location on the disk. This permits the most efficient utilization of disk space because clusters freed by erasing files can be allocated for new files. Refer back to the description of the DOS FAT in this chapter for more information. SSDD single sided, double density (160-180k) 5¼ DSDD double sided, double density (320-360k) 5¼ DSQD double sided, quad density (720k) 5¼, 3½, 2 DSHD double sided, high density (1.2-1.44M) 5¼, 3½ DSED double sided, extra high density (2.88M) 3½ Much of the trouble with AT 1.2 meg drives has been through the inadverdent use of quad density disks in the high density drives. The high density disks use a higher-coercivity media than the quads, and quads are not completely reliable as 1.2Mb. Make sure you have the correct disk for your application. ROTATION SPEEDS: 720k, 3½" (unknown) note: Zenith has discontinued 2" floppies 720k, 3½" 300 RPM 1.44Mb, 3½" 300 RPM 360k, 5¼" 300 RPM 720k, 5¼" 300 RPM 1.2mb, 5¼" 360 RPM (even when reading 360k diskettes) all 8" 360 RPM The Victor 9000's 5¼" floppies could vary their rotational speed as required. This allowed them to put 720k on a standard 360k floppy, using a constant density throughout. The Central Point CopyIIPC Option Board emulates the Macintosh GCR recording format by varying the data rate instead of the rotational speed. HARD DISK LAYOUT ...................................................... 8**11 The DOS hard disk routines perform the following services: 1) Allow multiple operating systems to be installed on the hard disk at the same time. 2) Allow a user-selected operating system to be started from the hard disk. I) In order to share the hard disk among operating systems, the disk may be logically divided into 1 to 4 partitions. The space within a given partition is contiguous, and can be dedicated to a specific operating system. Each operating system may "own" only one partition in DOS versions 2.0 through 3.2. DOS 3.3 introduced the "Extended DOS Partition" which allows multiple DOS partitions on the same hard disk. FDISK (or a similar program from other DOS vendors) utility allows the user to select the number, type, and size of each partition. The partition information is kept in a partition table that is embedded in the master hard disk boot record on the first sector of the disk. The format of this table varies from version to version of DOS. II) An operating system must consider its partition to be the entire disk, and must ensure that its functions and utilities do not access other partitions on the disk. III) Each partition may contain a boot record on its first sector, and any other programs or data that you choose, including a different operating system. For example, the DOS FORMAT command may be used to format and place a copy of DOS in the DOS partition in the same manner that a diskette is formatted. You can use FDISK to designate a partition as "active" (bootable). The master hard disk boot record causes that partition's boot record to receive control when the system is initialized. Additional disk partitions could be FORTH, UNIX, Pick, CP/M-86, OS/2 HPFS, Concurrent DOS, Xenix, or the UCSD p-System. SYSTEM INITIALIZATION ................................................. 8**12 The boot sequence is as follows: 1. System initialization first attempts to load an operating system from diskette drive A. If the drive is not ready or a read error occurs, it then attempts to read a master hard disk boot record on the first sector of the first hard disk in the system. If unsuccessful, or if no hard disk is present, it invokes ROM BASIC in an IBM PC or displays a disk error message on most compatibles. 2. If initialization is successful, the master hard disk boot record is given control and it examines the partition table embedded within it. If one of the entries indicates an active (bootable) partition, its boot record is read from the partition's first sector and given control. 3. If none of the partitions is bootable, ROM BASIC is invoked on an IBM PC or a disk error on most compatibles. 4. If any of the boot indicators are invalid, or if more than one indicator is marked as bootable, the message "INVALID PARTITION TABLE "is displayed and the system stops. 5. If the partition's boot record cannot be successfully read within five retries due to read errors, the message "ERROR LOADING OPERATING SYSTEM" appears and the system stops. 6. If the partition's boot record does not contain a valid "signature", the message "MISSING OPERATING SYSTEM" appears, and the system stops. NOTE: When changing the size or location of any partition, you must ensure that all existing data on the disk has been backed up. The partitioning program will destroy the data on the disk. System programmers designing a utility to initialize/manage a hard disk must provide the following functions at a minimum: 1. Write the master disk boot record/partition table to the disk's first sector to initialize it. 2. Perform partitioning of the disk - that is, create or update the partition table information (all fields for the partition) when the user wishes to create a partition. This may be limited to creating a partition for only one type of operating system, but must allow repartitoning the entire disk, or adding a partition without interfering with existing partitions (user's choice). 3. Provide a means for marking a user-specified partition as bootable and resetting the bootable indicator bytes for all other partitions at the same time. 4. Such utilities should not change or move any partition information that belongs to another operating system. BOOT RECORD/PARTITION TABLE ........................................... 8**13 A boot record must be written on the first sector of all hard disks, and must contain the following: 1. Code to load and give control to the boot record for one of four possible operating systems. 2. A partition table at the end of the boot record. Each table entry is 16 bytes long, and contains the starting and ending cylinder, sector, and head for each of four possible partitions, as well as the number of sectors preceding the partition and the number of sectors occupied by the partition. The "boot indicator" byte is used by the boot record to determine if one of the partitions contains a loadable operating system. FDISK initialization utilities mark a user-selected partition as "bootable" by placing a value of 80h in the corresponding partition's boot indicator (setting all other partitions' indicators to 0 at the same time). The presence of the 80h tells the standard boot routine to load the sector whose location is contained in the following three bytes. That sector is the actual boot record for the selected operating system, and it is responsible for the remainder of the system's loading process (as it is from the diskette). All boot records are loaded at absolute address 0:7C00. The partition table with its offsets into the boot record is: ┌──────────┬──────────┬──────────┬────────┬──────────────────────────────────── │ Offset │ Offset │ Offset │ │ │from Start│from Start│from Start│ Size │ Description │ of Disk │ of Entry │ of Disk │ │ ├──────────┼──────────┼──────────┼────────┼──────────────────────────────────── │ │ 00h │ 0BEh │ 1 byte │ boot indicator │ │ 01h │ 0BFh │ 1 byte │ beginning head │ 1BEh │ 02h │ 0C0h │ 1 byte │ beginning sector │ (part 1) │ 03h │ 0C1h │ 1 byte │ beginning cylinder │ 16 bytes │ 04h │ 0C2h │ 1 byte │ system indicator │ │ 05h │ 0C3h │ 1 byte │ ending head │ │ 06h │ 0C4h │ 1 byte │ ending sector │ │ 07h │ 0C5h │ 1 byte │ ending cylinder │ │ 08h │ 0C6h │ 4 bytes│ relative (starting) sector │ │ 0Ch │ 0DAh │ 4 bytes│ number of sectors ├──────────┼──────────┼──────────┼────────┼──────────────────────────────────── │ │ 00h │ 0DEh │ 1 byte │ boot indicator │ │ 01h │ 0DFh │ 1 byte │ beginning head │ 1CEh │ 02h │ 0E0h │ 1 byte │ beginning sector │ (part 2) │ 03h │ 0E1h │ 1 byte │ beginning cylinder │ 16 bytes │ 04h │ 0E2h │ 1 byte │ system indicator │ │ 05h │ 0E3h │ 1 byte │ ending head │ │ 06h │ 0E4h │ 1 byte │ ending sector │ │ 07h │ 0E5h │ 1 byte │ ending cylinder │ │ 08h │ 0E6h │ 4 bytes│ relative (starting) sector │ │ 0Ch │ 0EAh │ 4 bytes│ number of sectors ├──────────┼──────────┼──────────┼────────┼──────────────────────────────────── │ │ 00h │ 0FEh │ 1 byte │ boot indicator │ │ 01h │ 0FFh │ 1 byte │ beginning head │ 1DEh │ 02h │ 0100h │ 1 byte │ beginning sector │ (part 3) │ 03h │ 0101h │ 1 byte │ beginning cylinder │ 16 bytes │ 04h │ 0102h │ 1 byte │ system indicator │ │ 05h │ 0103h │ 1 byte │ ending head │ │ 06h │ 0104h │ 1 byte │ ending sector │ │ 07h │ 0105h │ 1 byte │ ending cylinder │ │ 08h │ 0106h │ 4 bytes│ relative (starting) sector │ │ 0Ch │ 010Ah │ 4 bytes│ number of sectors ├──────────┼──────────┼──────────┼────────┼──────────────────────────────────── │ │ 00h │ 010Eh │ 1 byte │ boot indicator │ │ 01h │ 011Fh │ 1 byte │ beginning head │ 1EEh │ 02h │ 0110h │ 1 byte │ beginning sector │ (part 4) │ 03h │ 0111h │ 1 byte │ beginning cylinder │ 16 bytes │ 04h │ 0112h │ 1 byte │ system indicator │ │ 05h │ 0113h │ 1 byte │ ending head │ │ 06h │ 0114h │ 1 byte │ ending sector │ │ 07h │ 0115h │ 1 byte │ ending cylinder │ │ 08h │ 0116h │ 4 bytes│ relative (starting) sector │ │ 0Ch │ 011Ah │ 4 bytes│ number of sectors ├──────────┼──────────┴──────────┼────────┼──────────────────────────────────── │ 1FEh │ │ 2 bytes│ 055AAh signature └──────────┴─────────────────────┴────────┴──────────────────────────────────── Boot indicator (boot ind): The boot indicator byte must contain 0 for a non- bootable partition or 80h for a bootable partition. Only one partition can be marked as bootable at a time. System Indicator (sys ind): The sys ind field contains an indicator of the operating system that "owns" the partition. IBM PC-DOS can only "own" one partition, though some versions of MSDOS allow all four partitions to be used by DOS. The system indicators are: ┌─────────────────────────────────────────────────────────────┐ │ System Indicator (sys ind) │ ├───────┬─────────────────────────────────────────────────────┤ │ 00h │ unknown or no partition defined │ ├───────┼─────────────────────────────────────────────────────┤ │ 01h │ DOS 12 bit FAT (DOS 2.x all and 3.x+ under 16 Mb) │ │ │ less than 4086 clusters │ ├───────┼─────────────────────────────────────────────────────┤ │ 02h │ Xenix │ ├───────┼─────────────────────────────────────────────────────┤ │ 03h │ Xenix │ ├───────┼─────────────────────────────────────────────────────┤ │ 04h │ DOS 16 bit FAT (DOS 3.0+. Not recognized by 2.x) │ │ │ less than 65,536 sectors │ ├───────┼─────────────────────────────────────────────────────┤ │ 05h │ extended DOS partition, some 3.2 and all 3.3+ │ │ │ (pointer to further partition table) │ ├───────┼─────────────────────────────────────────────────────┤ │ 06h │ Compaq DOS 3.31, DOS 4.0+ partitions over 32 megs │ │ │ Digital Research DRDOS 3.4, 3.41 over 32 megs │ ├───────┼─────────────────────────────────────────────────────┤ │ 06h │ PC-MOS/386 partitions over 32 megs (NOT compatible │ │ │ with the DR, Compaq, and MSDOS big partitions! │ ├───────┼─────────────────────────────────────────────────────┤ │ 07h │ OS/2 High Performance File System │ ├───────┼─────────────────────────────────────────────────────┤ │ 051h │ Ontrack Disk Manager "read/write" partitions │ ├───────┼─────────────────────────────────────────────────────┤ │ 0DBh │ DRI Concurrent DOS (>32mb partitions?)│ │ │ DRI Concurrent CP/M? │ ├───────┼─────────────────────────────────────────────────────┤ │ 0E4h │ Speedstor, small partitions (?) (under 1024cyl?) │ ├───────┼─────────────────────────────────────────────────────┤ │ 0F2h │ 2nd DOS partition, some OEM customized DOS 3.2 │ ├───────┼─────────────────────────────────────────────────────┤ │ 0F4h │ Speedstor, large partitions (?) │ ├───────┼─────────────────────────────────────────────────────┤ │ 0FEh │ Speedstor, partitions >1024 cylinders │ └───────┴─────────────────────────────────────────────────────┘ There are ID bytes for proprietary formatting schemes. Some manufacturers (such as Zenith, Wyse, and Tandon) diddle with these system bytes to implement more than one DOS partition per disk. note 1) Xenix doesn't like extended DOS partitions a'la DOS 3.3, limiting you to a DOS partition of 32Mb. Xenix doesn't recognize DOS 4.0x at all, so to use it you need to boot from a floppy. Early versions of OS/2 also have this problem. 2) I have found one source listing Minix partitions as "40" and some Unix partitions as "63". I don't know if these are decimal or hexadecimal figures. Cylinder (CYL) and Sector (S): The 1 byte fields labelled CYL contain the low order 8 bits of the cylinder number - the high order 2 bits are in the high order 2 bits of the sector (S) field. This corresponds with the ROM BIOS interrupt 13h (disk I/O) requirements, to allow for a 10 bit cylinder number. The fields are ordered in such a manner that only two MOV instructions are required to properly set up the DX and CX registers for a ROM BIOS call to load the appropriate boot record (hard disk booting is only possible from the first hard disk in the system, where a BIOS drive number of 80h corresponds to the boot indicator byte). All partitions are allocated in cylinder multiples and begin on sector 1, head 0, with the exception that the partition that is allocated at the beginning of the disk starts at sector 2, to account for the hard disk's master boot record. Relative (starting) Sector: The number of sectors preceding each partition on the disk is kept in this 4 byte field. This value is determined by counting the sectors beginning with cylinder 0, sector 1, head 0 of the disk, and incrementing the sector, head, and then track values up to the beginning of the partition. This, if the disk has 17 sectors per track and 4 heads, and the second partition begins at cylinder 1, sector 1, head 0, then the partition's starting relative sector is 68 (decimal) - there were 17 sectors on each of 4 heads on 1 track allocated ahead of it. The field is stored with the least significant word first. Number of sectors (#sects): The number of sectors allocated to the partition is kept in the "# of sects" field. This is a 4 byte field stored least significant word first. Signature: The last 2 bytes of the boot record (55AAh) are used as a signature to identify a valid boot record. Both this record and the partition boot record are required to contain the signature at offset 1FEh. HARD DISK TECHNICAL INFORMATION ....................................... 8**14 Western Digital's hard disk installation manuals make the claim that MSDOS can support only 2 hard drives. This is entirely false, and their purpose for making the claim is unclear. DOS merely performs a function call pointed at the hard disk driver, which is normally in one of three locations; a ROM at absolute address C:800, the main BIOS ROM if the machine is an AT, or a device driver installed through the CONFIG.SYS file. Two hard disk controller cards can normally not reside in the same machine due to lack of interrupt arbitration. Perstor's ARLL controller and some cards marketed by Novell can coexist with other controllers. Perstor's technical department has had four controllers and eight hard disks in the same IBM XT functioning concurrently. A valid hard disk has a boot record arranged in the following manner: db drive ; 0 or 80h (80h marks a bootable, active partition) db head1 ; starting head dw trksec1 ; starting track/sector (CX value for INT 13) db system ; SYS IND ID from table above db head2 ; ending head dw trksec2 ; ending track/sector dd sector1 ; absolute # of starting sector dd sector2 ; absolute # of last sector The master disk boot record invokes ROM BASIC if no indicator byte reflects a bootable system. When a partition's boot record is given control, it is passed its partition table entry address in the DS:SI registers. DETERMINING HARD DISK ALLOCATION ...................................... 8**15 DOS determines disk allocation using the following formula: D * BPD TS - RS - ─────────── BPS SPF = ────────────────────────────── BPS * SPC CF + ────────────── BPC where: TS Total number of sectors on the disk RS The number of sectors at the beginning of the disk that are reserved for the boot record. DOS normally reserves 1 sector. D The number of directory entries in the root directory. BPD The number of bytes per directory entry. This is always 32. BPS The number of bytes per logical sector. Typically 512, but you can specify a different number with VDISK. CF The number of FATS per disk. Usually 2. VDISK is 1. SPF The number of sectors per FAT. Maximum 64. SPC The number of sectors per allocation unit (cluster). BPC The number of bytes per FAT entry. BPC is 1.5 for 12 bit FATs. 2 for 16 bit FATS. To calculate the minimum partition size that will force a 16-bit FAT: CYL = (max clusters * 8)/(HEADS * SPT) where: CYL number of cylinders on the disk max clusters 4092 (maximum number of clusters for a 12 bit FAT) HEADS number of heads on the hard disk SPT sectors per track (normally 17 on MFM) DOS 2.0 through 3.3 limit partition sizes to 32 megabytes. The limit arises from the fact that DOS does things by sector number, and each sector is stored as a word. So the largest sector number DOS can count to is 64k. As each sector is 512 bytes long, 64k * .5k = 32Mb. The easiest way for an aftermarket disk handler to break the 32Mb barrier is probably to increase the sector size - with 2k sectors, maximum partiton size increases to 128Mb. However, the BIOS boot routines and IBMBIO.COM are hardwired for 512 byte sectors, so you won't be able to boot from a drive with oversize sectors. That's why Disk Manager formats a small boot partition by default. DOS 2.x uses a "first fit" algorithm when allocating file space on the hard disk. Each time an application requests disk space, it will scan from the beginning of the FAT until it finds a contiguous peice of storage large enough for the file. DOS 3.x+ keeps a pointer into the disk space, and begins its search from the point it last left off. This pointer is lost when the system is rebooted. This is called the "next fit" algorithm. It is faster than the first fit and helps minimize fragmentation. In either case, if the FCB function calls are used instead of the handle function calls, the file will be broken into pieces starting with the first available space on the disk. BIOS Disk Routines .................................................... 8**16 ┌─────────────────────────────────────────────────────────────────────────────┐ │Interrupt 13h Disk I/O - access the disk drives (floppy and hard disk) │ └─────────────────────────────────────────────────────────────────────────────┘ (0:004Ch) 1) These calls do not try rereading disk if an error is returned. 2) In the IBM OS/2 Tech Ref Volume 1, page 7-33, under "DOS Environment Software Interrupt Support", it lists: 13h disk/diskette for non-removable media only, these functions are supported: 01h read status 02h read sectors 0Ah read long 15h read DASD (disk) type 3) On hard disk systems these calls may be vectored through the int 40h hard disk BIOS. Function 00h Reset - reset the disk controller chip entry AH 00h DL drive (if bit 7 is set both hard disks and floppy disks reset) 00h-7Fh floppy disk 80h-0FFh hard disk return AH status (see 01h below) note 1) Forces controller chip to recalibrate read/write heads. 2) Some systems (Sanyo 55x, Columbia MPC) this resets all drives. 3) This function should be called after a failed floppy disk Read, Write, Verify, or Format request before retrying the operation. 4) If called with DL >= 80h (i.e., selecting a hard drive), the floppy controller and then the hard disk controller are reset. 5) Function 0Dh allows the hard disk controller to be reset without affecting the floppy controller. Function 01h Get Status of Disk System entry AH 01h DL drive (hard disk if bit 7 set) 00h-7Fh floppy disk 80h-0FFh hard disk return AH 00h AL status of most recent disk operation 00h successful completion, no errors 01h bad command 02h address mark not found 03h tried to write on write-protected disk (floppy only) 04h sector not found 05h reset failed (hard disk) 06h diskette removed or changed (floppy only) 07h bad parameter table (hard disk) 08h DMA overrun (floppy only) 09h attempt to DMA across 64K boundary 0Ah bad sector detected (hard disk) 0Bh bad track detected (hard disk) 0Ch unsupported track or media type not found (floppy disk) 0Dh invalid number of sectors on format (hard disk) 0Eh control data address mark detected (hard disk) 0Fh DMA arbitration level out of range (hard disk) 10h uncorrectable CRC/EEC on read 11h ECC corrected data error (hard disk) 20h controller failure 40h seek failed 80h timeout 0AAh drive not ready (hard disk) 0BBh undefined error (hard disk) 0CCh write fault (hard disk) 0E0h status error (hard disk) 0FFh sense operation failed (hard disk) note 1) For hard disks, error code 11h (ECC data error) indicates that a recoverable error was detected during a preceding int 13h fn 02h (Read Sector) call. Function 02h Read Sectors - read one or more sectors from diskette entry AH 02h AL number of sectors to read BX address of buffer (ES=segment) CH track (cylinder) number (0-39 or 0-79 for floppies) (for hard disk, bits 8,9 in high bits of CL) CL sector number (1 to 18, not value checked) DH head number (0 or 1) DL drive (0=A, 1=B, etc.) (bit 7=0) (drive 0-7) 00h-7Fh floppy disk 80h-FF0h hard disk ES:BX address to store/fetch data (buffer to fill) [0000:0078] dword pointer to diskette parameters return CF clear successful AL number of sectors transferred set error AH status (00h, 02h, 03h, 04h, 08h, 09h, 10h, 0Ah, 20h, 40h, 80h) note 1) Number of sectors begins with 1, not 0. 2) Trying to read zero sectors is considered a programming error; results are not defined. 3) For hard disks, the upper 2 bits of the 10-bit cylinder number are placed in the upper 2 bits of register CL. 4) For hard disks, error code 11h indicates that a read error occurred that was corrected by the ECC algorithm; in this case, AL contains the burst length. The data read is good within the limits of the ECC code. If a multisector transfer was requested, the operation was terminated after the sector containing the read error. 5) For floppy drives, an error may result from the drive motor being off at the time of the request. The BIOS does not automatically wait for the drive to come up to speed before attempting the read operation. The calling program should reset the floppy disk system with function 00h and retry the operation three times before assuming that the error results from some other cause. Function 03h Write Sectors - write from memory to disk entry AH 03h AL number of sectors to write (1-8) CH track number (for hard disk, bits 8,9 in high bits of CL) CL beginning sector number (if hard disk, high two bits are high bits of track #) DH head number (head 0=0) DL drive number (0-7) 00h-7Fh floppy disk 80h-FF0h hard disk ES:BX address of buffer for data return CF clear success AL number of sectors written set error AH status (see 01h above) note 1) Number of sectors begins with 1, not 0. 2) Trying to write zero sectors is considered a programming error; results are not defined. 3) For hard disks, the upper 2 bits of the 10-bit cylinder number are placed in the upper 2 bits of register CL. 4) For floppy drives, an error may result from the drive motor being off at the time of the request. The BIOS does not automatically wait for the drive to come up to speed before attempting the read operation. The calling program should reset the floppy disk system with function 00h and retry the operation three times before assuming that the error results from some other cause. Function 04h Verify - verify that a write operation was successful entry AH 04h AL number of sectors to verify (1-8) CH track number (for hard disk, bits 8,9 in high bits of CL) CL beginning sector number DH head number DL drive number (0-7) DL drive number (0-7) 00h-7Fh floppy disk 80h-FF0h hard disk ES:BX address of buffer for data return CF set on error AH status (see 01h above) AL number of sectors verified note 1) With IBM PC, XT, and AT with ROM BIOS earlier than 11/15/85, ES:BX should point to a valid buffer. 2) For hard disks, the upper 2 bits of the 10-bit cylinder number are placed in the upper 2 bits of register CL. 3) This function can be used to test whether a readable media is in a floppy drive. An error may result from the drive motor being off at the time of the request since the BIOS does not automatically wait for the drive to come up to speed before attempting the verify operation. The requesting program should reset the floppy disk system with function 00h and retry the operation three times before assuming that a readable disk is not present. Function 05h Format Track - write sector ID bytes for 1 track (floppy disk) entry AH 05h AL number of sectors to create on this track interleave (for XT hard disk only) CH track (or cylinder) number (bits 8,9 in high bits of CL) CL sector number DH head number (0, 1) DL drive number (0-3) 00h-7Fh floppy disk 80h-0FFh hard disk ES:BX pointer to 4-byte address field (C-H-R-N) (except XT hard disk) byte 1 = (C) cylinder or track byte 2 = (H) head byte 3 = (R) sector byte 4 = (N) bytes/sector (0 = 128, 1 = 256, 2 = 512, 3 = 1024) return CF set if error occurred AH status code (see 01h above) note 1) Not valid for ESDI hard disks on PS/2. 2) For floppy disks, the number of sectors per track is taken from the BIOS floppy disk parameter table whose address is stored in the vector for int 1Eh. 3) When this function is used for floppies on ATs or the PS/2, it should be preceded by a call to int 13h/fn 17h to select the type of media to format. 4) For hard disks, the upper 2 bits of the 10-bit cylinder number are placed in the upper 2 bits of CL. 5) On the XT/286, AT, and PS/2 hard disks, ES:BX points to a 512-byte buffer containing byte pairs for each physical disk sector as follows: Byte Contents 0 00h good sector 80h bad sector 1 sector number For example, to format a track with 17 sectors and an interleave of two, ES:BX would point to the following 34-byte array at the beginning of a 512-byte buffer: db 00h, 01h, 00h, 0Ah, 00h, 02h, 00h, 0Bh, 00h, 03h, 00h, 0Ch db 00h, 04h, 00h, 0Dh, 00h, 05h, 00h, 0Eh, 00h, 06h, 00h, 0Fh db 00h, 07h, 00h, 10h, 00h, 08h, 00h, 11h, 00h, 09h Function 06h Hard Disk - format track and set bad sector flags (PC2, PC-XT, and Portable) entry AH 06h AL interleave value (XT only) CH cylinder number (bits 8,9 in high bits of CL) CL sector number DH head DL drive (80h-0FFh for hard disk) ES:BX 512 byte format buffer the first 2*(sectors/track) bytes contain f,n for each sector f 00h good sector 80h bad sector n sector number return CF error AH status code (see 01h above) Function 07h Hard Disk - format the drive starting at the desired track (PC2, PC-XT and Portable) entry AH 07h AL interleave value (XT only) (01h-10h) CH cylinder number (bits 8,9 in high bits of CL) (00h-03FFh) CL sector number DH head number (0-7) DL drive number (80h-0FFh, 80h=C, 81h=D,...) ES:BX format buffer, size = 512 bytes the first 2*(sectors/track) bytes contain f,n for each sector f 00h good sector 80h bad sector n sector number return CF set on error AH status code (see 01h above) note Award AT BIOS routines are extended to handle more than 1024 cylinders. AL number of sectors CH cylinder numberm low 8 bits CL sector number bits 0-5, bits 6-7 are high 2 cylinder bits DH head number (bits 0-5) bits 6-7 are extended high cyls (>1024) DL drive number (0-1 for diskette, 80h-81h for hard disk) ES:BX transfer address Function 08h Read Drive Parameters (except PC, Jr) entry AH 08h DL drive number 00h-7Fh floppy disk 80h-0FFh hard disk return CF set on error AH status code (see above) BL drive type (AT/PS2 floppies only) 01h if 360 Kb, 40 track, 5¼" 02h if 1.2 Mb, 80 track, 5¼" 03h if 720 Kb, 80 track, 3½" 04h if 1.44 Mb, 80 track, 3½" CH low 8 bits of maximum useable value for cylinder number CL bits 6-7 high-order 2 bits of maximum cylinder number 0-5 maximum sector number DH maximum usable value for head number DL number of consecutive acknowledging drives (0-2) ES:DI pointer to drive parameter table note 1) On the PC and PC/XT, this function is supported on hard disks only. 2) The Columbia MPC supports functions 6-14 for its hard disk. It returns drive information, same as int 13 function 8, except that the BL and ES:DI values are omitted and AL <- burst length. Function 09h Initialize Two Fixed Disk Base Tables (XT, AT, XT/286, PS/2) (install nonstandard drive) entry AH 09h DL 80h-0FFh hard disk number return CF set on error AH status code (see 01h above) For PC, XT hard disks, the disk parameter block format is: 00h-01h maximum number of cylinders 02h maximum number of heads 03h-04h starting reduced write current cylinder 05h-06h starting write precompensation cylinder 07h maximum ECC burst length 08h drive options bits 7 1 disable disk access retries 6 1 disable ECC retries 3-5 set to 0 0-2 drive option 09h standard timeout value 0Ah timeout value for format drive 0Bh timeout value for check drive 0Ch-0Fh reserved For AT and PS/2 hard disks: 00h-01h maximum number of cylinders 02h maximum number of heads 03h-04h reserved 05h-06h starting write precompensation cylinder 07h maximum ECC burst length 08h drive options byte bits 6-7 nonzero (10, 01, or 11) if retries disabled 5 1 if manufacturer's defect map present at maximum cylinder + 1 4 not used 3 1 if more than 8 heads 0-2 not used 09h-0Bh reserved 0Ch-0Dh landing zone cylinder 0Eh sectors per track 0Fh reserved note 1) For the XT, int 41h must point to the Disk Parameter Block. 2) For the AT and PS/2, int 41h points to table for drive 0 and int 46h points to table for drive 1. 3) Initializes the hard disk controller for subsequent I/O operations using the values found in the BIOS disk parameter block(s). 4) This function is supported on hard disks only. Function 0Ah Read Long (Hard disk) (XT, AT, XT/286, PS/2) entry AH 0Ah CH cylinder number (bits 8,9 in high bits of CL) CL sector number (upper 2 bits of cyl # in upper 2 bits of CL) DH head number DL drive ID (80h-0FFh hard disk) ES:BX pointer to buffer to fill return CF set on error AH status code (see 01h above) AL number of sectors actually transferred note 1) A "long" sector includes a 4 byte EEC (Extended Error Correction) code. 2) Used for diagnostics only on PS/2 systems. 3) This function is supported on fixed disks only. 4) Unlike the normal Read Sector (02h) function, ECC errors are not automatically corrected. Multisector transfers are terminated after any sector with a read error. Function 0Bh Write Long (XT, AT, XT/286, PS/2) entry AH 0Bh AL number of sectors CH cylinder (bits 8,9 in high bits of CL) CL sector number DH head number DL drive ID (80h-0FFh hard disk) ES:BX pointer to buffer containing data return CF set on error AH status code (see 01h above) AL number of sectors actually transferred note 1) A "long" sector includes a 4 byte EEC (Extended Error Correction) code. 2) Used for diagnostics only on PS/2 systems. 3) Valid for hard disks only. Function 0Ch Seek To Cylinder (except PC, PCjr) entry AH 0Ch CH lower 8 bits of cylinder CL upper 2 bits of cylinder in bits 6-7 DH head number DL drive number (0 or 1) (80h-0FFh for hard disk) return CF set on error AH status code (see 01h above) note 1) Positions heads over a particular cylinder, but does not move any data. 2) This function is supported on hard disks only. 3) The upper 2 bits of the 10-bit cylinder number are placed in the upper 2 bits of CL. 4) The Read Sector, Read Sector Long, Write Sector, and Write Sector Long functions include an implied seek operation and need not be preceded by an explicit call to this function. Function 0Dh Alternate Hard Disk Reset (except PC, PCjr) entry AH 0Dh DL hard drive number (80h-0FFh hard disk) return CF set on error AH status code (see 01h above) note 1) Not for PS/2 ESDI hard disks. 2) Resets the hard disk controller, recalibrates attached drives (moves the read/write arm to cylinder 0), and prepares for subsequent disk I/O. 3) This function is for hard disks only. It differs from fn 00h by not resetting the floppy disk controller. Function 0Eh Read Sector Buffer (XT, Portable, PS/2) entry AH 0Eh ES:BX pointer to buffer return CF set on error AH status code (see 01h above) AL number of sectors actually transferred note 1) Transfers controller's sector buffer. No data is read from the drive. 2) Used for diagnostics only on PS/2 systems. 3) This fn is supported by the XT's hard disk adapter only. It is "not defined" for hard disk adapters on the AT or PS/2. Function 0Fh Write sector buffer (XT, Portable) entry AH 0Fh ES:BX pointer to buffer return CF set if error AH status code (see 01h above) AL number of sectors actually transferred note 1) Should be called before formatting to initialize the controller's sector buffer. 2) Used for diagnostics only on PS/2 systems. 3) Transfers data from system RAM to the hard disk adapter's internal sector buffer. 4) No data is written to the physical disk drive. 5) This fn is for the XT hard disk controller only. It is "not defined" for AT or PS/2 controllers. Function 10h Test For Drive Ready (XT, AT, XT/286, PS/2) entry AH 10h DL hard drive number 0 or 1 (80h-0FFh) return CF set on error AH status code (see 01h above) note 1) Tests whether the specified hard disk drive is operational and returns the drive's status. 2) This function is supported on hard disks only. 3) Perstor and Novell controllers allow more than one controller. Does not work for multiple Perstor controllers. (reports first two drives only). 4) Does not work with network drives. Function 11h Recalibrate Drive (XT, AT, XT/286, PS/2) entry AH 11h DL hard drive number (80h-0FFh hard disk) return CF set on error AH status code (see 01h above) note 1) Causes the HD controller to recalibrate itself for the specified drive, positioning the read/arm to cylinder 0, and returns the drive's status. 2) This function is for hard disks only. Function 12h Controller RAM Diagnostics (XT, Portable, PS/2) entry AH 12h return CF set on error AH status code (see fn 01h above) note 1) Used for diagnostics only on PS/2 systems. 2) Makes the hard disk controller carry out a built-in diagnostic test on its internal sector buffer. Function 13h Controller Drive Diagnostic (XT, Portable, PS/2) entry AH 13h return CF set on error AH status code (see 01h above) note 1) Used for diagnostics only on PS/2 systems. 2) Causes HD controller to run internal diagnostic tests of the attached drive, indicating whether the test was passed by the returned status. 3) This function is supported on XT HDs only. Function 14h Controller Internal Diagnostic (AT, XT/286) entry AH 14h return CF set on error AH status code (see 01h above) note 1) OEM is Western Digital 1003-WA2 hard/floppy combination controller in AT and XT/286. 2) Used for diagnostics only in PS/2 systems. 3) Causes HD controller to do a built-in diagnostic self-test, indicating whether the test was passed by the returned status. 4) This function is supported on hard disks only. Function 15h Get Disk Type (except PC and XT) entry AH 15h DL drive ID 00h-7Fh floppy disk 80h-0FFh fixed disk return CF set on error AH error code (see 01h above) AH disk type 00h no drive is present 01h diskette, no change detection present 02h diskette, change detection present 03h hard disk CX:DX number of 512-byte sectors note 1) Returns a code indicating the type of disk referenced by the specified drive code. 2) This function is not supported on the PC or XT. Function 16h Get Disk Change Status (diskette) (except PC, XT, & Jr) entry AH 16h DL drive to check return CF set on error AH disk change status 00h no disk change 01h disk changed DL drive that had disk change (00h-07Fh floppy disk) note Returns the status of the change line, indicating whether the disk in the drive may have been replaced since the last disk access. If this function returns with CF set, the disk has not necessarily been changed; the change line can be activated by simply unlocking and relocking the disk drive door without removing the floppy disk. Function 17h Set Disk Type for Format (diskette) (except PC and XT) entry AH 17h AL 00h not used 01h 160, 180, 320, or 360Kb diskette in 360kb drive 02h 360Kb diskette in 1.2Mb drive 03h 1.2Mb diskette in 1.2Mb drive 04h 720Kb diskette in 720Kb drive DL drive number (0-7) return CF set on error AH status of operation (see 01h above) note 1) This function is probably enhanced for the PS/2 series to detect 1.44 in 1.44 and 720k in 1.44. 2) This function is not supported for floppy disks on the PC or XT. 3) If the change line is active for the specified drive, it is reset. 4) The BIOS sets the data rate for the specified drive and media type. The rate is 250k/sec for double-density media and 500k/sec for high density media. The proper hardware is required. Function 18h Set Media Type For Format (diskette) (AT, XT2, XT/286, PS/2) entry AH 18h CH lower 8 bits of number of tracks CL high 2 bits of number of tracks (6,7) sectors per track (bits 0-5) DL drive number (0-7) return CF clear no errors AH 00h if requested combination supported 01h if function not available 0Ch if not suppported or drive type unknown 80h if there is no media in the drive ES:DI pointer to 11-byte disk parameter table for media type CF set error code (see 01h above) note 1) A floppy disk must be present in the drive. 2) This function should be called prior to formatting a disk with Int 13h Fn 05h so the BIOS can set the correct data rate for the media. 3) If the change line is active for the specified drive, it is reset. Function 19h Park Hard Disk Heads (PS/2) entry AH 19h DL drive number (80h-0FFh) return CF set on error AH error code (see fn 01h) note This function is defined for PS/2 fixed disks only. Function 1Ah ESDI Hard Disk - Low Level Format (PS/2) entry AH 1Ah AL Relative Block Address (RBA) defect table count 00h no errors on disk 01h+ number of disk errors CL format modifiers byte bits 0 ignore primary defect map 1 ignore secondary defect map 2 update secondary defect map 3 perform extended surface analysis 4 generate periodic interrupt after each cylinder format 5 reserved - must be 0 6 reserved - must be 0 7 reserved - must be 0 DL drive (80h-0FFh) ES:BX pointer to RBA defect table return CF set on error AH error code (see fn 01h above) note 1) Initializes disk sector and track address fields on a drive attached to the IBM "ESDI Fixed Disk Drive Adapter/A." 2) If periodic interrupt selected, int 15h/fn 0Fh is called after each cylinder is formatted 3) If bit 4 of CL is set, Int 15h, AH=0Fh, AL=phase code after each cylinder is formatted or analyzed. The phase code is defined as: 0 reserved 1 surface analysis 2 formatting 4) If bit 2 of CL is set, the drive's secondary defect map is updated to reflect errors found during surface analysis. If both bit 2 and bit 1 are set, the secondary defect map is replaced. 5) For an extended surface analysis, the disk should first be formatted by calling this function with bit 3 cleared and then analyzed by calling this function with bit 3 set. Function 1Bh ESDI Hard Disk - Get Manufacturing Header (PS/2) entry AH 1Bh AL number of record DL drive ES:BX pointer to buffer for manufacturing header (defect list) return CF set on error AH status note Manufacturing header format (Defect Map Record format) can be found in the "IBM 70Mb, 115Mb Fixed Disk Drives Technical Reference." Function 1Ch ESDI Hard Disk - Get Configuration (PS/2) entry AH 1Ch AL 0Ah Get Device Configuration DL drive ES:BX pointer to buffer for device configuration (drive physical parameter) 0Bh Get Adapter Configuration ES:BX pointer to buffer for adapter configuration 0Ch Get POS Information ES:BX pointer to POS information 0Dh unknown 0Eh Translate RBA to ABA CH low 8 bits of cylinder number CL sector number, high two bits of cylinder number in bits 6 and 7 DH head number DL drive number ES:BX pointer to ABA number return CF set on error AH status (see 01h) note 1) Device configuration format can be found in IBM ESDI Fixed Disk Drive Adapter/A Technical Reference. 2) ABA (absolute block address) format can be found in IBM ESDI Adapter Technical Reference by using its Device Configuration Status Block. Function 1Dh IBMCACHE.SYS (PS/2 50+) entry AH 1Dh other parameters unknown note IBMCACHE.SYS comes on the setup disk for MCA-bus PS/2 machines. Function 20h Western Digital HD SuperBIOS entry AH 20h other parameters unknown note SuperBIOS may be purchased separately from Western Digital and added to standard HD controllers. SuperBIOS contains additional setup tables and parameters.