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James Gregory 2013-12-31 11:38:48 +11:00
commit f658bfbb40
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22
21-01.md
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@ -65,7 +65,9 @@ areas of AGIs and register dependencies.
AGIs are *Address Generation Interlocks*, a fancy way of saying that if
a register is used to address memory, as is EBX in this instruction
mov [ebx],eax
```nasm
mov [ebx],eax
```
and the value of the register is not set far enough ahead for the
Pentium to perform the addressing calculations before the instruction
@ -101,14 +103,16 @@ As an example of a sort of AGI that's new to the Pentium, consider the
following test for a NULL pointer, followed by the use of the pointer if
it's not NULL:
push ebx ;U-pipe cycle 1
mov ebx,[Ptr] ;V-pipe cycle 1
and ebx,ebx ;U-pipe cycle 2
jz short IsNull ;V-pipe cycle 2
mov eax,[ebx] ;U-pipe cycle 3 AGI stall
mov edx,[ebp-8] ;V-pipe cycle 3 lockstep idle
;U-pipe cycle 4 mov eax,[ebx]
;V-pipe cycle 4 mov edx,[ebp-8]
```nasm
push ebx ;U-pipe cycle 1
mov ebx,[Ptr] ;V-pipe cycle 1
and ebx,ebx ;U-pipe cycle 2
jz short IsNull ;V-pipe cycle 2
mov eax,[ebx] ;U-pipe cycle 3 AGI stall
mov edx,[ebp-8] ;V-pipe cycle 3 lockstep idle
;U-pipe cycle 4 mov eax,[ebx]
;V-pipe cycle 4 mov edx,[ebp-8]
```
This commonplace code loses a U-pipe cycle to the AGI caused by **AND
EBX,EBX**, followed by the attempt two instructions later to use EBX to