Category : Files from Magazines
Archive   : PCTV1N1.ZIP
Filename : L3.ASM

;
; *** Listing 3 ***
;
; Assembler subroutine to perform a 16-bit checksum on the file
; opened on the passed-in handle. Stores the result in the
; passed-in checksum variable. Returns 1 for success, 0 for error.
;
; Call as:
; int ChecksumFile(unsigned int Handle, unsigned int *Checksum);
;
; where:
; Handle = handle # under which file to checksum is open
; Checksum = pointer to unsigned int variable checksum is
; to be stored in
;
; Parameter structure:
;
Parms struc
dw ? ;pushed BP
dw ? ;return address
Handle dw ?
Checksum dw ?
Parms ends
;
.model small
.data
TempWord label word
TempByte db ? ;each byte read by DOS will be
; stored here
db 0 ;high byte of TempWord is always 0
; for 16-bit adds
;
.code
public _ChecksumFile
_ChecksumFile proc near
push bp
mov bp,sp
push si ;save C's register variable
;
mov bx,[bp+Handle] ;get file handle
sub si,si ;zero the checksum accumulator
mov cx,1 ;request one byte on each read
mov dx,offset TempByte ;point DX to the byte in which
; DOS should store each byte read
ChecksumLoop:
mov ah,3fh ;DOS read file function #
int 21h ;read the byte
jc ErrorEnd ;an error occurred
and ax,ax ;any bytes read?
jz Success ;no-end of file reached-we're done
add si,[TempWord] ;add the byte into the checksum total
jmp ChecksumLoop
ErrorEnd:
sub ax,ax ;error
jmp short Done
Success:
mov bx,[bp+Checksum] ;point to the checksum variable
mov [bx],si ;save the new checksum
mov ax,1 ;success
;
Done:
pop si ;restore C's register variable
pop bp
ret
_ChecksumFile endp
end