Shareware Grab Bag

home *** CD-ROM | disk | FTP | other *** search

/ Shareware Grab Bag / Shareware Grab Bag.iso / 007 / moredasm.arc / MOREDASM.NOT

Wrap

Text File | 1987-01-26 | 20.0 KB | 513 lines

.lh 6 .cw 7 .mt 4 .mb 11 .pl 88 .op APPLICATIONS: Using ASMGEN to disassemble MORE.COM .he ASMGEN disassembly of MORE.COM Page # The primary justification for writing programs in assembly language is execution speed. Few would argue in favor of such direct machine contact if there were easier ways to achieve the performance improvements that are possible in comparison with high-level programs like BASIC and Pascal. The best way to achieve some level of literacy in assembly language programming is to examine the programs written by others. You can incorporate segments of their code in your programs--or modify their programs to serve your own application. Programs available in the public domain are ideal for this purpose, since the sheer variety of programs raises the likelihood that you will be able to find something close to what you need. Unfortunately, they do not all come with the source code. For modifying programs for which you do not have the source code, there is a disassembler available (again, in the public domain) which you can use to reconstruct the basic program structure. Named ASMGEN, it was written by Jack Gersbach and J. Damke. (It turns out he is also the co-author of one of the utilities packages now being marketed by IBM in its Personally Developed Software series.) It available on Windmill, Thousand Oaks, Simi, RIMS and most other public domain bulletin boards. Look for version 2.01 or later. We've chosen for an example the DOS filter MORE.COM. This modest little program at 384 bytes is the smallest of the three filters supplied with DOS. The other two, FIND.COM and SORT.COM, are considerably more complex. MORE.COM provides in its small space assembly language examples of the following operations (in rough order of appearance in the program): . Checking for DOS version number (2.00 or higher) . Activating the required input and standard error devices . Reading standard input into a buffer . Using DOS file handles instead of File Control Blocks . Using a System BIOS call for checking the width of the current screen . Managing a "shadow screen" to keep track of the cursor . Checking input for end of file, enter, line feed, backspace, tab and bell (beep) characters . Correctly adjusting screen display for 8-space tab . Wrapping the screen display if the line width exceeds the screen width, whether it is 40 or 80 columns wide . Pausing the display at each full screen, and waiting for a keypress to proceed To do this sort of disassembly operation properly, you need a number of tools. A complete list comprises the following items: . Disassembler: ASMGEN, V2.01 or later by Gersbach/Damke (Public Domain) . File compare: CMP10, V1.10 or later by Gersbach (Public Domain) . Concurrent text editor and utility: The Notepad and the Calculator features of SideKick, V1.10B from Borland International . Main text editor: WordStar 3.3 in non-document mode from MicroPro International . Macro Assembler V1.00, from IBM/Microsoft .cp 5 . Manuals: Disk Operating System Reference Manual Disk Operating System Technical Reference Manual Technical Reference Personal Computer XT and Portable Personal Computer A reference manual on the Intel 8086/8088 instruction set There is no point in attempting a disassembly of any size without the DOS Technical Reference Manual. IBM and Microsoft have moved all the required DOS function call information into a separate manual starting with version 2.10. It is possible to get by with the integrated DOS version 2.00 manual for the function calls, but only as a last resort. The hardware manual is another absolute requirement because it is only in the System BIOS program listing that you will find what the BIOS function calls mean. Even a program as nominally trivial as MORE.COM uses a BIOS call to determine the width of the current display screen. You will not be able to decode the system interrupts without this manual. The Macro Assembler is also required. Unlike CP/M, DOS comes with no assembler. The only facility it has to assemble programs is the "A" command in DEBUG, which will assemble into memory the instructions you enter via the keyboard. This is on the same approximate level as the Apple II monitor ROM assembler. Since you must have the macro assembler, you might as well use its macro facility. As you will see in the reconstructed program listing that follows, all the BIOS and DOS function calls have been recoded as macros. This allows you to remove yourself from the requirement of remembering the purpose of each of the hexadecimal functions, and shows more clearly the underlying flow of the program in a language that you can define yourself that makes most sense to you. This is how you can build up your DOS function library. Unfortunately, ASMGEN does not take its interpretation this far; it is up to you to replace the INT 21 forms with the appropriate macro call. Equally unfortunately, the DOS Technical Reference Manual is mainly organized along the lines of the hexadecimal function number, so you may find yourself referring back to the macro definition to find the hex value of the call so that you can locate its reference in the DOS manual. Sigh... It would be difficult to imagine undertaking this sort of exercise without SideKick. SideKick has a "notepad", which can function as a concurrent text editor. SideKick has all the features and command syntax of WordStar with the exception of word wrap (not needed in the context of programming). It remains resident so that by invoking it with the <shift><shift> keys (holding down both <shift> keys simultaneously) you can edit the .SEQ file required by ASMGEN to interpret the .COM file without the delay of reloading and then unloading the main text editor. A superior utility. We use WordStar for word processing and text editing. There are other word processors available. For this type of work, where you are doing programming and debugging, you need windows. We use SideKick for that purpose. You might also try XyWrite II Plus (two windows), or FinalWord (multiple windows). Pmate also has multiple edit buffers, but is thoroughly inscrutable. Microsoft Word supports up to eight windows, but the entry/exit overhead is too much and the internal format must be translated to ASCII before it can be used. XyWrite takes a some setup time as well, but it provides a facility whereby you can drop down to DOS and back again very quickly. Finally, the file compare program CMP10, also by Jack Gersbach. This too is a public domain program. The absolute first objective of a disassembly is the reconstruction of the original file--byte for byte. The program listing that follows, with all its macro definitions, will assemble, link, EXE2BIN and REN into a .COM file that is absolutely identical to the source .COM file. It is only after you have achieved this objective that you should feel comfortable about modifying the reconstructed .ASM file for your own purposes. DOS provides a file compare program (COMP.COM), but unfortunately if the files are of different lengths the utility stops. This is of little use, since in most cases almost all efforts at reassembly except for the very last will result in .COM files of different lengths. CMP10 will compare files of different lengths and show the differences as well--in hex and ASCII format similar to a DEBUG display listing. Indispensable. The reassembled and reconstituted source file for MORE.COM follows. The source code shown here will generate a .COM file that is absolutely identical, byte for byte, with the original DOS 2.00 version from which it was constructed. It has been checked against DOS Version 3.00. With the exception of a slightly shorter buffer tail at the end of the program, DOS 3.00 is identical byte-for-byte to the DOS 2.00 program. R. Zider San Francisco, CA 11.18.84 Copyright 1984. All right reserved. Released to the public domain for personal use only. .pa REASSEMBLY OF MORE.COM Exhibit 1 Listing of MOR.ASM ------------------ TITLE MOR 3-8-83 [11-17-84] ; ; Reassembly of IBM PC DOS 2.00 MORE.COM filter ; ; R. Zider Zider Brothers, San Francisco ; ; .RADIX 16 BEL EQU 07H BKSP EQU 08H TAB EQU 09H LF EQU 0AH CR EQU 0DH EOF EQU 1AH ; ; MACRO DEFINITIONS ; ;SYSTEM BIOS MACROS ; VIDEO_BIOSCALL MACRO ;;Invoke System BIOS video I/O function call ;;On entry: ;;(AH) = Function number ;;See page 5-68ff Technical Reference Personal Computer XT and ;; Portable Personal Computer INT 10H ENDM ; GET_CURRENT_VIDEO_STATE MACRO ;;Get screen width using a video BIOS call ;;On entry: ;;If (AH) = 15 (0FH), then function returns current video state ;;On exit: ;;"(AL) = Mode currently set (see AH=0 for explanation)" ;;"(AH) = Number of character columns on screen" ;;"(BH) = Current active display page" ;; Quoted from TRM page 5-69 MOV AH,0FH VIDEO_BIOSCALL ENDM ; ; ;DOS MACROS ; DOS_EXIT MACRO INT 20H ENDM ; DOSCALL MACRO ;;Standard DOS function call ;;On entry: ;;(AH) = Function call number INT 21H ENDM CLEAR_KB_INPUT MACRO ;;Clear keyboard buffer, read char from std input device (keyboard) ;;Character returned in (AL) MOV AH,0CH ;;Clear KB buffer (plus function in AL) MOV AL,1H ;;Read char from KB function DOSCALL ENDM ; .cp 5 DOS_DISPLAY_OUT MACRO ;;(DL) has the character to be displayed MOV AH,2H DOSCALL ENDM ; DOS_PRINT_STRING MACRO STRING_LABEL ;Output a "$"-terminated string MOV DX,OFFSET STRING_LABEL MOV AH,9H DOSCALL ENDM ; GET_DOS_VERSION MACRO ;;Version number returned in two pieces: ;;(AL) = Major version number (as "02" or "03") ;;(AH) = Minor version number (as "00" or "01") MOV AH,30H DOSCALL ENDM ; CLOSE_FILE_HANDLE MACRO ;;On entry: ;;(BX) = File handle returned by "open" or "create" ;;On exit: ;;(AX) = Error codes (NONE if carry not set) MOV AH,3EH DOSCALL ENDM ; READ_FROM_FILE MACRO HANDLE, BYTES, BUFFER ;;Transfer bytes from a file into a buffer ;;On entry: ;;(BX) = File handle of source file ;;(CX) = Number of bytes to be read (max--may be less than this) ;;(DX) = Location of destination buffer ;;On exit: ;;(AX) = Number of bytes actually read; or else error code if carry set MOV DX, OFFSET BUFFER MOV CX, BYTES MOV BX, HANDLE MOV AH,3FH DOSCALL ENDM ; DUP_FILE_HANDLE MACRO ;;HANDLE argument not yet implemented ;;This function duplicates a file handle ;;On entry: ;;(BX) = File handle to be duplicated ;;On exit: ;;(AX) = New file handle, referring to same file at same position ;; (or else error codes if carry set) ;; MOV BX, HANDLE ;;Need conditional assembly if arg missing MOV AH,45H DOSCALL ENDM ; ; PROGRAM STARTS HERE ; ; ;INITIAL VALUES : CS:IP 0000:0100 ; SS:SP 0000:FFFF S0000 SEGMENT ASSUME DS:S0000, SS:S0000 ,CS:S0000 ,ES:S0000 ORG $+0100H START: GET_DOS_VERSION ;Check DOS version first XCHG AH,AL ;Put major version in AH, minor in AL CMP AX,200 ;Must be 2.00 or higher JNB OK_VER_200 DOS_PRINT_STRING WRONG_DOS ;Nope.. DOS_EXIT ;..bye .cp 5 OK_VER_200: MOV BYTE PTR SCREEN_HEIGHT,19 ;Screen height 25 lines GET_CURRENT_VIDEO_STATE ;Not sure of width, though--get it MOV SCREEN_WIDTH,AH ;OK, save current width DOS_PRINT_STRING CR_LF ;Flush screen with a CR-LF XOR BX,BX ;Zero BX to get a duplicate.. DUP_FILE_HANDLE ;..file handle of std input (handle 0) MOV BP,AX ;New file handle to BP CLOSE_FILE_HANDLE ;Close handle 0 (std input)--use dup MOV BX,2 ;Default handle 2 is std error output.. DUP_FILE_HANDLE ;..device--activate it READ_STD_INPUT_INTO_BUFFER: CLD ;Clear direction ; MOV DX,OFFSET BUFFER ;Destination buffer ; MOV CX,1000 ;Max of 1000H bytes (4KB) ; MOV BX,BP ;This has new std input file handle READ_FROM_FILE BP 1000 BUFFER ;OK, read on.. ;..and ignore any error return OR AX,AX ;Check for non-zero bytes read JNZ OK_READ ;"OK" if non-zero EXIT: DOS_EXIT ;Empty std input file--bye OK_READ: MOV CX,AX ;Number of bytes read to CX MOV SI,DX ;Buffer location to SI (now source) LOAD_NEXT_BYTE: LODSB ;Start processing bytes.. CMP AL,EOF ;..looking for an end-of-file (1AH).. JZ EXIT ;..and exit if found CMP AL,CR ;Check if it is a carriage return.. JNZ CK_LINEFEED ;..and if not, continue on with check MOV BYTE PTR COLUMN_COUNT,1 ;CR, so reset to screen column 1 and.. JMP SHORT DISPLAY_OUT ;..output the CR to the screen CK_LINEFEED: CMP AL,LF ;Check if it is a line feed (OAH).. JNZ CK_BKSPACE ;..and if not, go to next char check INC BYTE PTR LINE_COUNT ;LF, so bump the line count and.. JMP SHORT DISPLAY_OUT ;..output the LF to the screen CK_BKSPACE: CMP AL,BKSP ;Check for backspace char (08H).. JNZ CK_TAB ;..and if not, continue on to next CMP BYTE PTR COLUMN_COUNT,1 ;Is backspace, check if at column 1.. JZ DISPLAY_OUT ;..and if so, just output the char DEC BYTE PTR COLUMN_COUNT ;Not at column 1, so dec the column.. JMP SHORT DISPLAY_OUT ;..and output the backspace CK_TAB: CMP AL,TAB ;Check for tab char (09H).. JNZ CK_BEL ;..and if not, press on MOV AH,COLUMN_COUNT ;Is tab, so get the current column.. ADD AH,7 ;..add 7 to it.. AND AH,0F8 ;..and mask for MOD 8.. INC AH ;..adding one.. MOV COLUMN_COUNT,AH ;..and make this the new column.. JMP SHORT DISPLAY_OUT ;..and output it to the display CK_BEL: CMP AL,BEL ;Check for BEL (beep) char (07) JZ DISPLAY_OUT ;Is a BEL, so let console handle it INC BYTE PTR COLUMN_COUNT ;None of these, so bump the column.. MOV AH,COLUMN_COUNT ;..and check to see if we're at the CMP AH,SCREEN_WIDTH ;..edge of the screen.. JBE DISPLAY_OUT ;..and if not, output the character INC BYTE PTR LINE_COUNT ;At screen edge, so do a line wrap.. MOV BYTE PTR COLUMN_COUNT,1 ;..and bump the line and reset to col 1 DISPLAY_OUT: MOV DL,AL ;Output the character.. DOS_DISPLAY_OUT ;..to the display via DOS call MOV AH,LINE_COUNT ;Check the line count.. CMP AH,SCREEN_HEIGHT ;..against the screen height.. JB NEXT_BYTE ;..and continue if no overflow DOS_PRINT_STRING MORE_LINE ;"--More--" message.. CLEAR_KB_INPUT ;..and pause for keyboard input DOS_PRINT_STRING CR_LF ;OK, continue and clear More line.. MOV BYTE PTR COLUMN_COUNT,1 ;..reset screen pointers to Home.. MOV BYTE PTR LINE_COUNT,1 ;..at top left position (1,1) DEC SI ;Back up one byte in buffer so that.. INC CX ;..the byte can be re-sent NEXT_BYTE: DEC CX ;One less byte to do, and.. JZ RD_NEXT_4K ;..if now zero, read another 4KB JMP LOAD_NEXT_BYTE ;A computer's work is never done RD_NEXT_4K: JMP READ_STD_INPUT_INTO_BUFFER ;Heigh Ho, Heigh Ho... SCREEN_HEIGHT EQU $ DB 18 ;24 lines, but gets overwritten SCREEN_WIDTH EQU $ DB 50 ;Here, 80 columns wide (overwtritten) LINE_COUNT EQU $ DB 1 ;Starts at 1 COLUMN_COUNT EQU $ DB 1,5 DUP(0) ;Starts at 1 MORE_LINE EQU $ DB CR,'-- More --$' ;Displayed on 25th line WRONG_DOS EQU $ DB 'MORE: Incorrect DOS version' CR_LF EQU $ DB CR,LF,'$' BUFFER EQU $ DB 66 DUP(0) ;Read buffer only 66H bytes here S0000 ENDS ; END START .pa MORE.ASM Exhibit 2 Assembly and link to .COM file ------------------------------ This is what you will have to do to get a .COM file that you can compare with the original MORE.COM program in DOS 2.00. The following listing has been captured from the screen display with FSPOOL.COM. B>dir Volume in drive B is PCDISASM Directory of B:\mor . <DIR> 11-04-84 2:17p .. <DIR> 11-04-84 2:17p M OBJ 536 11-17-84 7:24p MOR ASM 2468 11-17-84 9:10a M LST 15785 11-17-84 7:24p M COM 384 11-17-84 7:21p MOR OBJ 501 11-17-84 11:53a MOR SEQ 617 11-17-84 9:07a M EXE 1152 11-17-84 7:24p MORE300 COM 320 8-14-84 8:00a MORE COM 384 3-08-83 12:00p M BIN 384 11-17-84 7:24p M ASM 7168 11-17-84 4:39p MOR COM 384 11-17-84 11:54a N LST 15785 11-17-84 7:14p NOTES 3525 11-17-84 7:04p T PRN 512 11-17-84 7:44p 17 File(s) 181248 bytes free B>masm m Bad command or file name B>a:ini B>path=a:\ B>masm m The IBM Personal Computer MACRO Assembler Version 1.00 (C)Copyright IBM Corp 1981 Object filename [M.OBJ]: Source listing [NUL.LST]: m.lst Cross reference [NUL.CRF]: Warning Severe Errors Errors 0 0 B>link m IBM Personal Computer Linker Version 2.00 (C)Copyright IBM Corp 1981, 1982, 1983 Run File [M.EXE]: List File [NUL.MAP]: Libraries [.LIB]: Warning: No STACK segment There was 1 error detected. B>exe2bin m B>cmp10 m.bin more.com Files are identical .cp 15 B>dir m.* Volume in drive B is PCDISASM Directory of B:\mor M OBJ 536 11-17-84 7:46p M LST 15785 11-17-84 7:46p M COM 384 11-17-84 7:21p M EXE 1152 11-17-84 7:47p M BIN 384 11-17-84 7:47p M ASM 7168 11-17-84 4:39p 6 File(s) 180224 bytes free B>fspool