FF /6         PUSH r/m16         Push r/m16

FF /6         PUSH r/m32         Push r/m32

50+ rw         PUSH r16         Push r16

50+ rd         PUSH r32         Push r32

6A         PUSH imm8         Push imm8

68         PUSH imm16         Push imm16

68         PUSH imm32         Push imm32

0E         PUSH CS                 Push CS

16         PUSH SS                 Push SS

1E         PUSH DS                 Push DS

06         PUSH ES                 Push ES

0F A0         PUSH FS                 Push FS

0F A8         PUSH GS         Push GS


Description

Decrements the stack pointer and then stores the source operand on the top of the stack. The address-size attribute of the stack segment determines the stack pointer size (16 bits or 32 bits), and the operand-size attribute of the current code segment determines the amount the stack pointer is decremented (2 bytes or 4 bytes). For example, if these address- and operand-size attributes are 32, the 32-bit ESP register (stack pointer) is decremented by 4 and, if they are 16, the 16-bit SP register is decremented by 2.(The B flag in the stack segment's segment descriptor determines the stack's address-size attribute, and the D flag in the current code segment's segment descriptor, along with prefixes, determines the operand-size attribute and also the address-size attribute of the source operand.) Pushing a 16-bit operand when the stack address-size attribute is 32 can result in a misaligned the stack pointer (that is, the stack pointer is not aligned on a doubleword boundary).


The PUSH ESP instruction pushes the value of the ESP register as it existed before the instruction was executed. Thus, if a PUSH instruction uses a memory operand in which the ESP register is used as a base register for computing the operand address, the effective address of the operand is computed before the ESP register is decremented.


In the real-address mode, if the ESP or SP register is 1 when the PUSH instruction is executed, the processor shuts down due to a lack of stack space. No exception is generated to indicate this condition.


Operands        Bytes                Clocks

reg                 1                    1   UV

mem              2+d(0-2)         2   NP

seg                 1                   1   NP

imm                     1+i(1,2)         1   NP

FS/GS               2                 1   NP


Flags

None.

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