I copied some lines from Intel processor software manuals:
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Intel 64 and IA-32
Architectures
Software Developers Manual
Volume 2B:
Instruction Set Reference, N-Z
Order Number: 253667-034US
March 2010
In 64-bit mode, the instructions default operation size is 64 bits. In a push, the
64-bit RSP register (stack pointer) is decremented by 8. A 66H override causes
16-bit operation.
Intel 64 and IA-32 Architectures
Software Developers Manual
Volume 1:
Basic Architecture
Order Number: 253665-034US
March 2010
In 64-bit mode, all of the above actions are forced to 64 bits regardless of operand
size prefixes (operand size prefixes are silently ignored).
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In one manual it says, 66H , means operand size override prefix is effective in 64 bit mode, then it mentions that this prefix is ignored!!
