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James Gregory 2013-12-31 11:38:48 +11:00
commit f658bfbb40
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21-03.md
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@ -40,34 +40,36 @@ right?
**LISTING 21.1 L21-1.ASM**
; Calculates TCP/IP (16-bit carry-wrapping) checksum for buffer
; starting at ESI, of length ECX words.
; Returns checksum in AX.
; ECX and ESI destroyed.
; All cycle counts assume 32-bit protected mode.
; Assumes buffer length > 0.
; Note that timing indicates that the pipe sequence and
; cycle counts shown (based on documented execution rules)
; differ from the actual execution sequence and cycle counts;
; this loop has been measured to execute in 5 cycles; apparently,
; the 1st half of ADD somehow pairs with the prefix byte, or the
; refix byte gets executed ahead of time.
```nasm
; Calculates TCP/IP (16-bit carry-wrapping) checksum for buffer
; starting at ESI, of length ECX words.
; Returns checksum in AX.
; ECX and ESI destroyed.
; All cycle counts assume 32-bit protected mode.
; Assumes buffer length > 0.
; Note that timing indicates that the pipe sequence and
; cycle counts shown (based on documented execution rules)
; differ from the actual execution sequence and cycle counts;
; this loop has been measured to execute in 5 cycles; apparently,
; the 1st half of ADD somehow pairs with the prefix byte, or the
; refix byte gets executed ahead of time.
sub ax,ax ;initialize the checksum
sub ax,ax ;initialize the checksum
ckloop:
add ax,[esi] ;cycle 1 U-pipe prefix byte
;cycle 1 V-pipe idle (no pairing w/prefix)
;cycle 2 U-pipe 1st half of ADD
;cycle 2 V-pipe idle (register contention)
;cycle 3 U-pipe 2nd half of ADD
;cycle 3 V-pipe idle (register contention)
adc ax,0 ;cycle 4 U-pipe prefix byte
;cycle 4 V-pipe idle (no pairing w/prefix)
;cycle 5 U-pipe ADC AX,0
add esi,2 ;cycle 5 V-pipe
dec ecx ;cycle 6 U-pipe
jnz ckloop ;cycle 6 V-pipe
ckloop:
add ax,[esi] ;cycle 1 U-pipe prefix byte
;cycle 1 V-pipe idle (no pairing w/prefix)
;cycle 2 U-pipe 1st half of ADD
;cycle 2 V-pipe idle (register contention)
;cycle 3 U-pipe 2nd half of ADD
;cycle 3 V-pipe idle (register contention)
adc ax,0 ;cycle 4 U-pipe prefix byte
;cycle 4 V-pipe idle (no pairing w/prefix)
;cycle 5 U-pipe ADC AX,0
add esi,2 ;cycle 5 V-pipe
dec ecx ;cycle 6 U-pipe
jnz ckloop ;cycle 6 V-pipe
```
Wrong, wrong, wrong! As detailed in Listing 21.1, this loop should take
6 cycles per checksummed word in 32-bit protected mode, a ridiculously