Using amd64 assembly, whats the best way to zero out the top 32 bits of a 64-bit register, e.g. zero out the bits of rax that are not covered by eax? It appears that I cannot and the whole register against a 64-bit constant.
movl %eax, %eax or mov eax, eax, depending on the assembler in use.
see: Intel® 64 and IA-32 Architectures - Software Developer’s Manual, Volume 1, 3.4.1.1 : General-Purpose Registers in 64-Bit Mode.
32-bit operands generate a 32-bit result, zero-extended to a 64-bit result in the destination general-purpose register.
I should also add, in regards to @HansPassant's comment: 7.3.1.7 :
The MOVSXD instruction operates on 64-bit data. It sign-extends a 32-bit value to 64 bits.
来源：https://stackoverflow.com/questions/21439289/zero-out-top-32-bits-of-64-bit-register