64-BIT mode does not support PUSH and POP instructions

The general idea is that you usually push and pad full registers, i.e. 64-bit registers in 64-bit mode. push

the default operand size is 64-bit, and 32-bit operand size is not available. Does each PUSH instruction do multiples of 8 bytes on x64? (yes, unless you specifically use 16-bit push, but 32-bit is not available).

You cannot push a 32-bit register in 64-bit mode; instead, you can push and put the entire 64 bit register that contains the 32 bit value you need to push rax

instead push eax

. The same is true for memory references - you can push qword ptr[rax]

, but not push dword ptr[rax]

.

But: even in 64 bit mode, you can still press:

• 8 or 32 bit signs sign extended to 64; this is usually handled by your assembler as an optimization (if you do push 1
  
  , it will encode it with the most compact encoding there will be 6A01
  
  , that is, with the operand imm8). It will always be a 64-bit push unless you explicitly specify itpush word 1
  
  , no matter what width is immediately picked by the assembler.

• segment registers fs
  
  , and gs
  
  , but not the registers cs
  
  , ds
  
  , es
  
  , ss
  
  (which is not important in 64-bit mode, and can be read mov
  
  , but not push
  
  releasing the push / pop code for potential use in the future).

  As an exception, segment registers are either zero-extended or pushed onto the stack with a 16-bit move (that is, the remaining 48 bits in the stack remain unchanged); this is not a big problem as pop fs
  
  it pop gs
  
  just throws those extra bits away.

You can emulate push imm64

with push low32

/ mov dword [rsp+4], high32

. Or with mov r64, imm64

/ push r64

; mov

for registration (not memory) is the only x86-64 instruction that can accept 64-bit immediate.

With a 16-bit operand size (prefix 66h

), you can do a 16-bit push that sets the RSP to 2 instead of 8. But usually don't do that because it will shift the stack until you do a 16-bit pop or otherwise correct it.

• 16-bit registers ( push ax
  
  ) and memory references ( push word ptr[rax]
  
  );
• 8-bit extended character or 16-bit.

8-bit registers cannot be pressed in any mode (except as part of a wider register), and 32-bit is not available in 64-bit mode, with a prefixREX.W=0

.