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PUSH -- Push Operand onto the Stack

Opcode     Instruction   Clocks   Description

FF   /6    PUSH m16      5        Push memory word
FF   /6    PUSH m32      5        Push memory dword
50 + /r    PUSH r16      2        Push register word
50 + /r    PUSH r32      2        Push register dword
6A         PUSH imm8     2        Push immediate byte
68         PUSH imm16    2        Push immediate word
68         PUSH imm32    2        Push immediate dword
0E         PUSH CS       2        Push CS
16         PUSH SS       2        Push SS
1E         PUSH DS       2        Push DS
06         PUSH ES       2        Push ES
0F   A0    PUSH FS       2        Push FS
OF   A8    PUSH GS       2        Push GS

Operation

IF StackAddrSize = 16
THEN
   IF OperandSize = 16 THEN
      SP := SP - 2;
      (SS:SP) := (SOURCE); (* word assignment *)
   ELSE
      SP := SP - 4;
      (SS:SP) := (SOURCE); (* dword assignment *)
   FI;
ELSE (* StackAddrSize = 32 *)
   IF OperandSize = 16
   THEN
      ESP := ESP - 2;
      (SS:ESP) := (SOURCE); (* word assignment *)
   ELSE
      ESP := ESP - 4;
      (SS:ESP) := (SOURCE); (* dword assignment *)
   FI;
FI;

Description

PUSH decrements the stack pointer by 2 if the operand-size attribute of the instruction is 16 bits; otherwise, it decrements the stack pointer by 4. PUSH then places the operand on the new top of stack, which is pointed to by the stack pointer.

The 80386 PUSH eSP instruction pushes the value of eSP as it existed before the instruction. This differs from the 8086, where PUSH SP pushes the new value (decremented by 2).

Flags Affected

None

Protected Mode Exceptions

#SS(0) if the new value of SP or ESP is outside the stack segment limit; #GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

None; if SP or ESP is 1, the 80386 shuts down due to a lack of stack space

Virtual 8086 Mode Exceptions

Same exceptions as in real-address mode; #PF(fault-code) for a page fault

up: Chapter 17 -- 80386 Instruction Set
prev: POPF/POPFD Pop Stack into FLAGS or EFLAGS Register
next: PUSHA/PUSHAD Push all General Registers