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80386 Programmer's Reference Manual -- Opcode PUSH

PUSH -- Push Operand onto the Stack

Opcode	Instruction	Clocks	Description	Example
FF /6	pushw m16	5	Push memory word	`pushw m16`
FF /6	pushl m32	5	Push memory dword	`pushl m32`
50 + /r	pushw r16	2	Push register word	`pushw %bx`
50 + /r	pushl r32	2	Push register dword	`pushl %ebx`
68	pushw imm16	2	Push immediate word	`pushw $0x7fff`
68	pushl imm32	2	Push immediate dword	`pushl $0x7fffffff`
0E	pushw cs	2	Push CS	`pushw %cs`
16	pushw ss	2	Push SS	`pushw %ss`
1E	pushw ds	2	Push DS	`pushw %ds`
06	pushw es	2	Push ES	`pushw %es`
0F A0	pushw fs	2	Push FS	`pushw %fs`
OF A8	pushw gs	2	Push GS	`pushw %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

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