Skip to content

Stack register

A stack register is a computer central processor register whose purpose is to keep track of a call stack. On an accumulator-based architecture machine, this may be a dedicated register such as SP on an Intel x86 machine.

Stack registers in x86

In 8086, the main stack register is called stack pointer - SP. The stack segment register (SS) is usually used to store information about the memory segment that stores the call stack of currently executed program. SP points to current stack top. By default, the stack grows downward in memory, so newer values are placed at lower memory addresses. To push a value to the stack, the PUSH instruction is used. To pop a value from the stack, the POP instruction is used.

NOTE: 对于 8086 ， SP和SS需要一起才能够工作。

Example: Assuming that SS = 1000h and SP = 0xF820. This means that current stack top is the physical address 0x1F820 (this is due to memory segmentation in 8086). The next two machine instructions of the program are:

PUSH AX
PUSH BX
  • These first instruction shall push the value stored in AX (16-bit register) to the stack. This is done by subtracting a value of 2 (2 bytes) from SP.
  • The new value of SP becomes 0xF81E. The CPU then copies the value of AX to the memory word whose physical address is 0x1F81E.
  • When "PUSH BX" is executed, SP is set to 0xF81C and BX is copied to 0x1F81C.

NOTE: 上面描述的过程就相当于函数在执行的时候，给自动变量分配内存空间。

This illustrates how PUSH works. Usually, the running program pushes registers to the stack to make use of the registers for other purposes, like to call a routine that may change the current values of registers. To restore the values stored at the stack, the program shall contain machine instructions like this:

POP BX
POP AX
  • POP BX copies the word at 0x1F81C (which is the old value of BX) to BX, then increases SP by 2. SP now is 0xF81E.
  • POP AX copies the word at 0x1F81E to AX, then sets SP to 0xF820.

NOTE: The program above pops BX first, that's because it was pushed last.

NOTE: In 8086, PUSH & POP instructions can only work with 16-bit elements.