xiaolabaDIY 的部落格

Experiment for a .COM file.
XP, DOS BOX
the CPU has CS = DS = ES = SS = PSP

Result -
0x0080h = length of the string for our command line argument, 1 byte
0x0081h =  string of the input from command line, include the SPACE [0x20h] and last byte is CR [0x0d]

This is the way our program can  obtains the command line argument. the standard approach would be
a) check the length of argument string
b) count the SPACE character [0x20] from this string, one [0x20] = start of one argument
c) preserve the individual argument for user's purpose

screen capture for 1 argument


screen capture for 2 arguments






Test purpose
procedure -
a) assemble the source test.asm to be test.com
b) create a file myfile.txt, can be empty
c) load the test.com, test if the message will be displayed




Reference for DOS interrupt service -
http://www.emu8086.com/assembly_language_tutorial_assembler_reference/8086_bios_and_dos_interrupts.html#int21h_09h

The Art of assembly
http://www.arl.wustl.edu/~lockwood/class/cs306/books/artofasm/toc.html
http://www.skynet.ie/~darkstar/assembler/tut5.html
http://eji.com/a86/
http://www.masm32.com/

http://home.educities.edu.tw/wanker742126/index.html

http://heim.ifi.uio.no/~stanisls/helppc/

DEBUG and the specific application
http://www.geocities.com/thestarman3/asm/2bytejumps.htm


Test result
DOS int 21, ah=9, the message can printed to screen when the myfile.txt is existed.



Enviroment : XP DOS BOX
Assembler : A86.com

listing of test.asm
-----------------------------
    org 100h
myDATA:
    jmp my_code
    db 400h dup 90h
   
   
    org 180h
my_code:
   
    call OpenFile
    call ReadFile
    call CloseFile
    mov dx, offset msg_ok
    call print_msg
    ret


   
;-----------------------------------
;INT 21h / AH= 3Dh - open existing file.
;
;Entry:
;
;AL = access and sharing modes:
;
;mov al, 0   ; read
;mov al, 1   ; write
;mov al, 2   ; read/write
;
;DS:DX -> ASCIZ filename.
;
;Return:
;
;CF clear if successful, AX = file handle.
;CF set on error AX = error code.
;
;note 1: file pointer is set to start of file.
;note 2: file must exist.
;-----------------------------------
openFile:   
    mov al, 2
    mov dx, offset filename
    mov ah, 3dh
    int 21h
    jc err
    mov handle, ax    ;save file handle
    ret



;-----------------------------------
;INT 21h / AH= 3Eh - close file.
;
;Entry: BX = file handle
;
;Return:
;
;CF clear if successful, AX destroyed.
;CF set on error, AX = error code (06h)
;-----------------------------------   
CloseFile:
    mov ah, 3eh
    mov bx, handle
    int 21h
    jc err
    ret


err:
    mov dx, offset msg_err
    call print_msg
    ret    ;exit the program

print_msg:
    ;mov dx, offset msg_open
    mov ah, 9
    int 21h
    ret


;-----------------------------------
;INT 21h / AH= 3Fh - read from file.
;Entry:
;BX = file handle.
;CX = number of bytes to read.
;DS:DX -> buffer for data.

;Return:
;CF is clear if successful - AX = number of bytes actually read;
;0 if at EOF (end of file) before call.
;CF is set on error AX = error code.

;Note: data is read beginning at current file position, and the file position is
;updated after a successful read the returned AX may be smaller than the request
;in CX if a partial read occurred.

readFile:
    mov bx, handle
    mov cx, 80h
    mov dx, 100h    ;load the file data to 0x100, override this COM file
    mov ah, 3fh
    int 21h
    jc err
    ret
;-----------------------------------                    

handle        dw ?                                    
msg_err        db "myfile.txt, open or read or close, it was not OK $"
msg_ok        db "myfile.txt, open / read / close, ok $"
filename    db "myfile.txt", 0
        db 256 dup 0h




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