Category : Files from Magazines
Archive   : PCTV1N1.ZIP
Filename : L7.ASM

;
; *** Listing 7 ***
;
; Assembler subroutine to perform a 16-bit checksum on a block of
; bytes 1 to 64Kb in size. Adds checksum for block into passed-in
; checksum.
;
; Call as:
; void ChecksumChunk(unsigned char *Buffer,
; unsigned int BufferLength, unsigned int *Checksum);
;
; where:
; Buffer = pointer to start of block of bytes to checksum
; BufferLength = # of bytes to checksum (0 means 64K, not 0)
; Checksum = pointer to unsigned int variable checksum is
; stored in
;
; Parameter structure:
;
Parms struc
dw ? ;pushed BP
dw ? ;return address
Buffer dw ?
BufferLength dw ?
Checksum dw ?
Parms ends
;
.model small
.code
public _ChecksumChunk
_ChecksumChunk proc near
push bp
mov bp,sp
push si ;save C's register variable
;
cld ;make LODSB increment SI
mov si,[bp+Buffer] ;point to buffer
mov cx,[bp+BufferLength] ;get buffer length
mov bx,[bp+Checksum] ;point to checksum variable
mov dx,[bx] ;get the current checksum
sub ah,ah ;so AX will be a 16-bit value
; after LODSB
ChecksumLoop:
lodsb ;get the next byte
add dx,ax ;add it into the checksum total
loop ChecksumLoop ;continue for all bytes in block
mov [bx],dx ;save the new checksum
;
pop si ;restore C's register variable
pop bp
ret
_ChecksumChunk endp
end