Avoid Partial Register Reads And Writes - AMD Athlon Processor x86 Optimization Manual

22007E/0—November 1999

Avoid Partial Register Reads and Writes

Avoid Partial Register Reads and Writes
Example 2 (Preferred):
05 78 56 34 12 add eax, 12345678h
83 C3 FB add ebx, -5
74 05 jz
In order to handle partial register writes, the AMD Athlon
processor execution core implements a data-merging scheme.
In the execution unit, an instruction writing a partial register
merges the modified portion with the current state of the
remainder of the register. Therefore, the dependency hardware
can potentially force a false dependency on the most recent
instruction that writes to any part of the register.
Example 1 (Avoid):
MOV AL, 10
MOV AH, 12
In addition, an instruction that has a read dependency on any
part of a given architectural register has a read dependency on
the most recent instruction that modifies any part of the same
architectural register.
Example 2 (Avoid):
MOV BX, 12h
MOV BL, DL
MOV BH, CL
MOV AL, BL
AMD Athlon™ Processor x86 Code Optimization
;uses single byte
; opcode form
;uses 8-bit sign
; extended immediate
$label1 ;uses 1-byte opcode,
; 8-bit immediate
;inst 1
;inst 2 has a false dependency on
; inst 1
;inst 2 merges new AH with current
; EAX register value forwarded
; by inst 1
;inst 1
;inst 2, false dependency on
; completion of inst 1
;inst 3, false dependency on
; completion of inst 2
;inst 4, depends on completion of
; inst 2
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