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Text File | 1993-04-13 | 14.3 KB | 439 lines

- ASMVLA01 - File I/O - 04/14/93 - Lately we have been quite busy with school, so this second issue is a little behind schedule. But that's life... This little issue will quickly show off the DOS file functions: read, write, open, close, create & others. They are all pretty much the same, so there isn't a whole lot to go over. But, as a bonus, I'm going to throw in a bit about how to do a subroutine. Let's do the subroutine stuff first. `Procedures' as they are called, are declared like this: ──────────────────────────────────────────────────────────────────────────── PROC TheProcedure ... ;do whatever.. ret ;MUST have a RET statement! ENDP TheProcedure ──────────────────────────────────────────────────────────────────────────── In the procedure, you can do basically anything you want, just at the end of it, you say ret. You can also specify how to call the PROC by putting a NEAR or FAR after the procedure name. This tells the compiler whether to change segment AND offset, or just offset when the procedure is called. Note that if you don't specify, it compiles into whatever the default is for the current .MODEL (small = near, large = far) ──────────────────────────────────────────────────────────────────────────── PROC TheProc NEAR ... ret ;this compiles to `retn' (return near- pops offset off ENDP TheProc ; stack only) OR PROC TheProc FAR ... ret ;compiles to `retf' pops both offset & segment off stack ENDP TheProc ; pops offset first ──────────────────────────────────────────────────────────────────────────── That's basically all there is to that. Note that if you REALLY wanted to be tricky, you could do a far jump by doing this: ──────────────────────────────────────────────────────────────────────────── push seg TheProc push offset TheProc retf ──────────────────────────────────────────────────────────────────────────── This would "return" you to the beginning of the procedure "TheProc"... This code is just to illustrate a point. If you actually did something like this and compiled and executed it, it would bomb. Know why? What happens when it hits the `ret' in the PROC? Well it pops off the offset and puts it in IP and then pops the segment and puts it in CS. Who knows what was on the stack... will return to an unknown address and probably crash. (It DEFINATELY will not continue executing your code.) Of course, the only stack operations are PUSH and POP. All they do is push or pop off the stack a word sized or a Dword sized piece of data. NEVER under ANY circumstance try to push a byte sized piece of data! The results are unpredictable. Well, not really, but just don't do it, ok? There are also two commands that'll save you some time and code space: PUSHA and POPA (push all and Pop all) PUSHA pushes the general registers in this order: AX, CX, DX, BX, SP, BP, SI, DI POPA pops the general registers in this order: DI, SI, BP, (sp), BX, DX, CX, AX SP is different because popa does NOT restore the value of SP. It merely pops it off and throws it away. For the 386+, pushad and popad push and pop all extended registers in the same order. You don't need to memorize the order, because you don't need to know the order until you go and get tricky. (hint: the location of AX on the stack is [sp + 14] - useful if you want to change what AX returns, but you did a pusha cause you wanted to save all the registers (except AX) Then you'd do a popa, and AX= whatever value you put in there. ──── Alright, now a slightly different topic: memory management Ok, this isn't true by-the-book memory management, but you need to know one thing: Upon execution of a program, DOS gives it ALL memory up to the address A000:0000. This happens to be the beginning of the VGA buffer... Another thing you must know is that, if you used DOSSEG at the top of your file, the segment is the last piece of your program. The size of the segment is derived from the little command `STACK 200h' or whatever the value was that you put up there. The 200h is the number of bytes in the stack. To get the number of paragraphs, you'd divide by 16. Here's an example of how I can get a pointer to the first valid available segment that I can use for data: ──────────────────────────────────────────────────────────────────────────── mov ax,ss ;grab the stack segment add ax,200h/16 ;add the size of the stack 200h/16 = 20h ;AX now contains the value of the first available segment the you can ; use. ──────────────────────────────────────────────────────────────────────────── This is very nice, because you can just plop your data right there and you have a 64k buffer you can use for anything you want. Ok, say you want to find out how much memory is available to use. This would be done like this: (no suprises, I hope.) ──────────────────────────────────────────────────────────────────────────── mov ax,ss ;grab the stack segment add ax,200h/16 ;add the size of the stack 200h/16 = 20h mov bx,0A000h ;upper limit of the free memory sub bx,ax ;bx= # of paragraphs available ──────────────────────────────────────────────────────────────────────────── Pretty darn simple. That's enough of the overhead that you must know to understand the included ANSI viewer (asm3.asm) Now to the FILE I/O stuff... Files can be opened, read from, written to, created, and closed. To open a file, all you need to do is give the DOS interrupt a name & path. All references to that file are done through what's known as a file handle. A file handle is simply a 16bit integer that DOS uses to identify the file. It's used more or less like an index into chart of pointers that point to a big structure that holds all the info about the file- like current position in the file, file type, etc.. all the data needed to maintain a file. The `FILES= 20' thing in your autoexec simply tells DOS how much memory to grab for those structures. ( Files=20 grabs enough room for 20 open files. ) ANYway, here's each of the important function calls and a rundown on what they do and how to work them. ──────────────────────────────────────────────────────────────────────────── FILE OPEN: Function 3Dh IN: ah= 3Dh al= open mode bits 7-3: Stuff that doesn't matter to us bits 2-0: Access code 000 read only access 001 write only access 010 read and write access DS:DX= pointer to the ASCIIZ filename ASCIIZ means that its an ASCII string with a Zero on the end. Returns: CF=1 error occured AX= error code- don't worry about what they are, if the carry is set, you didn't open the file. CF=0 no error AX= File Handle ;you need to keep this- it's your only way to ; reference your file! ──── EXAMPLE ──── [...] ;header stuff .CODE ;this stuff is used for all the examples FileName db "TextFile.TXT",0 FileHandle dw 0 Buffer db 300 dup (0) BytesRead dw 0 FileSize dd 0 [...] ;more stuff mov ax,3d00h ; open file for read only mov ax,cs mov ds,ax ;we use CS, cause it's pointing to the CODE segment ; and our file name is in the code segment mov dx,offset FileName int 21h jc FileError_Open mov [FileHandle],ax [...] ;etc... ──────────────────────────────────────────────────────────────────────────── FILE CLOSE: Function 3Eh IN: AH= 3Eh BX= File Handle RETURN: CF=1 error occured, but who cares? ──── EXAMPLE ──── mov bx,[FileHandle] mov ah,3eh int 21h ──────────────────────────────────────────────────────────────────────────── FILE READ: Function 3Fh IN: AH= 3Fh BX= File Handle CX= Number of bytes to read DS:DX= where to put data that is read from the file (in memory) RETURN: AX= number of bytes actually read- if 0, then you tried to read from the end of the file. ──── EXAMPLE ──── mov bx,[FileHandle] mov ax,cs mov ds,ax mov dx,offset buffer mov ah,3Fh mov cx,300 int 21h mov [BytesRead],ax ──────────────────────────────────────────────────────────────────────────── FILE WRITE: Function 40h IN: AH= 40h BX= File Handle CX= Number of bytes to write DS:DX= where to read data from (in memory) to put on disk RETURN: AX= number of bytes actually written- if not equal to the number of bytes that you wanted to write, you have an error. ──── EXAMPLE ──── mov bx,[FileHandle] mov ax,cs mov ds,ax mov dx,offset buffer mov ah,40h mov cx,[BytesRead] int 21h cmp cx,ax jne FileError_Write ──────────────────────────────────────────────────────────────────────────── FILE CREATE: Function 3Ch IN: ah= 3Ch cl= file attribute bit 0: read-only bit 1: hidden bit 2: system bit 3: volume label bit 4: sub directory bit 5: Archive bit 6&7: reserved DS:DX= pointer to the ASCIIZ filename ASCIIZ means that its an ASCII string with a Zero on the end. Returns: CF=1 error occured AX= error code- don't worry about what they are, if CF is set, you didn't create the file. CF=0 no error AX= File Handle ;you need to keep this- it's your only way to ; reference your file! ──── EXAMPLE ──── mov ah,3ch mov ax,cs mov ds,ax ;we use CS, cause it's pointing to the CODE segment ; and our file name is in the code segment mov dx,offset FileName mov cx,0 ;no attributes int 21h jc FileError_Create mov [FileHandle],ax ──────────────────────────────────────────────────────────────────────────── FILE DELETE: Function 41h IN: ah= 41h DS:DX= pointer to the ASCIIZ filename Returns: CF=1 error occured AX= error code- 2= file not found, 3= path not found 5= access denied CF=0 no error ──── EXAMPLE ──── mov ah,41h ;kill the sucker mov ax,cs mov ds,ax mov dx,offset FileName int 21h jc FileError_Delete ──────────────────────────────────────────────────────────────────────────── FILE MOVE POINTER: Function 42h IN: ah= 42h BX= File Handle CX:DX= 32 bit pointer to location in file to move to AL= 0 offset from beginning of file = 1 offset from curent position = 2 offset from the end of the file Returns: CF=1 error occured AX= error code- no move occured CF=0 no error DX:AX 32 bit pointer to indicate current location in file ──── EXAMPLE ──── mov ah,42h ;find out the size of the file mov bx,[FileHandle] xor cx,cx xor dx,dx mov al,2 int 21h mov [word low FileSize],ax mov [word high FileSize],dx ;load data into filesize (or in MASM mode, mov word ptr [FileSize],ax mov word ptr [FileSize+2],dx need I say why I like Ideal mode? ) ──────────────────────────────────────────────────────────────────────────── FILE CHANGE MODE: Function 43h IN: ah= 43h DS:DX= pointer to the ASCIIZ filename al= 0 returns file attributes in CX al= 1 sets file attributes to what's in CX Returns: CF=1 error occured AX= error code- 2= file not found, 3= path not found. 5= access denied CF=0 no error ──── EXAMPLE ──── Lets erase a hidden file in your root directory... FileName db "C:\msdos.sys",0 [...] mov ah,43h ;change attribute to that of a normal file mov ax,cs mov ds,ax mov dx,offset FileName mov al,1 ;set to whats in CX mov cx,0 ;attribute = 0 int 21h mov ah,41h ;Nuke it with the delete command int 21h ──────────────────────────────────────────────────────────────────────────── Well, that's all for now. I hope this info is enough for you to do some SERIOUS damage... :) I just don't want to see any 'bombs' running around erasing the hidden files in the root directory, ok? Anyway, go take a look at asm3.asm- it's a SIMPLE ansi/text displayer. It just opens the file, reads it all into a "buffer" that was "allocated" immediatly after the stack & reads in the entire file (if it's < 64k) and prints out the file character by character via DOS's print char (fn# 2). Very simple and very slow. You'd need a better print routine to go faster... The quickest display programs would decode the ANSI on its own... But that's kinda a chore... Oh, well. Enjoy. Draeden/VLA Suggested projects: 1) Write a program that will try to open a file, but if it does not find it, the program creates the file and fills it with a simple text message. 2) Write a program that will input your keystrokes and write them directly to a text file. 3) The write & read routines actually can be used for a file or device. Try to figure out what the FileHandle for the text screen is by writing to the device with various file handles. This same channel, when read from, takes it's data from the keyboard. Try to read data from the keyboard. Maybe read like 20 characters... CTRL-Z is the end of file marker. 4) Try to use a file as `virtual memory'- open it for read/write access and write stuff to it and then read it back again after moving the cursor position.