Author: Gerd Isenberg
Date: 04:34:36 05/04/05
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>Targeting 16-bit ax leaves the upper word of eax unchanged (but clears high rax >as well - iirc). wrong! 24592—Rev. 3.10—March 2005 AMD64 Architecture Programmer’s Manual Volume 1: Application Programming 3.4.5 High 32 Bits In 64-bit mode, the following rules apply to extension of results into the high 32 bits when results smaller than 64 bits are written: �� Zero-Extension of 32-Bit Results: 32-bit results are zeroextended into the high 32 bits of 64-bit GPR destination registers. �� No Extension of 8-Bit and 16-Bit Results: 8-bit and 16-bit results leave the high 56 or 48 bits, respectively, of 64-bit GPR destination registers unchanged.
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