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66 General 64-Bit Optimizations Chapter 3
25112 Rev. 3.06 September 2005
Software Optimization Guide for AMD64 Processors
To improve performance and substantially reduce code size, the following procedure performs the
same 64-bit integer multiplication using 64-bit instead of 32-bit operations:
; 64bitalu_64x64(int *a, int *b, int *c);
;
; TO ASSEMBLE INTO *.obj DO THE FOLLOWING:
; ml64.exe -c 64bitalu_64x64.asm
;
_TEXT SEGMENT
64bitalu_64x64 PROC NEAR
;==============================================================================
; Parameters passed into routine:
; rcx = ->a
; rdx = ->b
; r8 = ->c
;==============================================================================
mov rax, [rcx] ; RAX = [a0]
mul [rdx] ; Multiply [a0] by [b0] such that
; RDX:RAX = [c1]:[c0].
mov [r8], rax ; Store 128-bit product of a and b.
mov [r8+8], rdx
;==============================================================================
ret
64bitalu_64x64 ENDP
END