X86 Assembly/Other Instructions

Stack Instructions

push arg

This instruction decrements the stack pointer and loads the data specified as the argument into the location pointed to by the stack pointer.

pop arg

This instruction loads the data stored in the location pointed to by the stack pointer into the argument specified and then increments the stack pointer. For example:

mov eax, 5 mov ebx, 6
push eax	The stack is now: [5]
push ebx	The stack is now: [6] [5]
pop eax	The topmost item (which is 6) is now stored in eax. The stack is now: [5]
pop ebx	ebx is now equal to 5. The stack is now empty.

pushf

This instruction decrements the stack pointer and then loads the location pointed to by the stack pointer with the contents of the flag register.

popf

This intruction loads the flag register with the contents of the memory location pointed to by the stack pointer and then increments the contents of the stack pointer.

pusha

This instruction pushes all the general purpose registers onto the stack in the following order: EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI. The value of ESP pushed is the value before the instruction is executed. It is useful for saving state before an operation that could potential change these registers.

popa

This instruction pops all the general purpose registers off the stack in the reverse order of PUSHA. That is, EDI, ESI, EBP, ESP, EBX, EDX, ECX, EAX. Used to restore state after a call to PUSHA.

Flags instructions

While the flags register is used to report on results of executed instructions (overflow, carry, etc.), it also contains flags that affect the operation of the processor. These flags are set and cleared with special instructions.

Interrupt Flag

The IF flag tells a processor if it should accept hardware interrupts. It should be kept set under normal execution. In fact, in protected mode, neither of these instructions can be executed by user-level programs.

sti

Sets the interrupt flag. If set, the processor can accept interrupts from peripheral hardware.

cli

Clears the interrupt flag. Hardware interrupts cannot interrupt execution. Programs can still generate interrupts, called software interrupts, and change the flow of execution. Non-maskable interrupts (NMI) cannot be blocked using this instruction.

Direction Flag

The DF flag tells the processor which way to read data when when using string instructions. That is, whether to decrement or increment the esi and edi registers after a movs instruction.

std

Sets the direction flag. Registers will decrement, reading backwards.

cld

Clears the direction flag. Registers will increment, reading forwards.

Carry Flag

The CF flag is often modified after arithmetic instructions, but it can be set or cleared manually as well.

stc

Sets the carry flag.

clc

Clears the carry flag.

cmc

Complements (inverts) the carry flag.

Other

sahf

Stores the content of AH register into the lower byte of the flag register.

lahf

Loads the AH register with the contents of the lower byte of the flag register.

I/O Instructions

in src, dest	GAS Syntax
in dest, src	Intel syntax

The IN instruction almost always has the operands AX and DX (or EAX and EDX) associated with it. DX (src) frequently holds the port address to read, and AX (dest) receives the data from the port. In Protected Mode operating systems, the IN instruction is frequently locked, and normal users can't use it in their programs.

out src, dest	GAS Syntax
out dest, src	Intel syntax

The OUT instruction is very similar to the IN instruction. OUT outputs data from a given register (src) to a given output port (dest). In protected mode, the OUT instruction is frequently locked so normal users can't use it.

System Instructions

These instructions were added with the Pentium II.

sysenter

This instruction causes the processor to enter protected system mode (supervisor mode or "kernel mode").

sysexit

This instruction causes the processor to leave protected system mode, and enter user mode.