home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Graphics Programming Black Book (Special Edition)
/
BlackBook.bin
/
disk1
/
source
/
chapter21
/
l21-2.asm
< prev
next >
Wrap
Assembly Source File
|
1997-06-18
|
1KB
|
28 lines
; Calculates TCP/IP (16-bit carry-wrapping) checksum for buffer
; starting at ESI, of length ECX words.
; Returns checksum in AX.
; High word of EAX, DX, ECX and ESI destroyed.
; All cycle counts assume 32-bit protected mode.
; Assumes buffer length > 0.
sub eax,eax ;initialize the checksum
mov dx,[esi] ;first word to checksum
dec ecx ;we'll do 1 checksum outside the loop
jz short ckloopend ;only 1 checksum to do
add esi,2 ;point to the next word to checksum
ckloop:
add al,dl ;cycle 1 U-pipe
mov dl,[esi] ;cycle 1 V-pipe
adc ah,dh ;cycle 2 U-pipe
mov dh,[esi+1] ;cycle 2 V-pipe
adc eax,0 ;cycle 3 U-pipe
add esi,2 ;cycle 3 V-pipe
dec ecx ;cycle 4 U-pipe
jnz ckloop ;cycle 4 V-pipe
ckloopend:
add ax,dx ;checksum the last word
adc eax,0