The Stack

Remarks#

The stack of computers is like a stack of books. PUSH adds one to the top and POP takes the uppermost away. Like in real life the stack cannot be endless, so it has maximum size. The stack can be used for sorting algorithms, to handle a bigger amount of data or to safe values of registers while doing another operation.

Zilog Z80 Stack

The register sp is used as stack pointer, pointing to the last stored value into stack (“top” of stack). So EX (sp),hl will exchange value of hl with the value on top of stack.

Contrary to “top” word, the stack grows in memory by decreasing the sp, and releases (“pops”) values by increasing sp.

For sp = $4844 with values 1, 2, 3 stored on stack (the 3 being pushed onto stack as last value, so being at top of it), the memory will look like this:

|    address | value bytes | comment (btw, all numbers are in hexadecimal)
| ---------- | ----------- | ---------------------------------
|       4840 | ?? ??       | free stack spaces to be used by next push/call
|       4842 | ?? ??       | or by interrupt call! (don't expect values to stay here)
| sp -> 4844 | 03 00       | 16 bit value "3" on top of stack
|       4846 | 02 00       | 16 bit value "2"
|       4848 | 01 00       | 16 bit value "1"
|       484A | ?? ??       | Other values in stack (up to it's origin)
|       484C | ?? ??       | like for example return address for RET instruction

Examples, how instructions work with stack:

    LD   hl,$0506
    EX   (sp),hl           ; $0003 into hl, "06 05" bytes at $4844
    POP  bc                ; like: LD c,(sp); INC sp; LD b,(sp); INC sp
                           ; so bc is now $0506, and sp is $4846
    XOR  a                 ; a = 0, sets zero and parity flags
    PUSH af                ; like: DEC sp; LD (sp),a; DEC sp; LD (sp),f
                           ; so at $4844 is $0044 (44 = z+p flags), sp is $4844
    CALL $8000             ; sp is $4842, with address of next ins at top of stack
                           ; pc = $8000 (jumping to sub-routine)
                           ; after RET will return here, the sp will be $4844 again
    LD   (L1+1),sp         ; stores current sp into LD sp,nn instruction (self modification)
    DEC  sp                ; sp is $4843
L1  LD   sp,$1234          ; restores sp to $4844 ($1234 was modified)
    POP  de                ; de = $0044, sp = $4846
    POP  ix                ; ix = $0002, sp = $4848
    ...

    ...
    ORG  $8000
    RET                    ; LD pc,(sp); INC sp; INC sp
                           ; jumps to address at top of stack, "returning" to caller

Summary: PUSH will store value on top of stack, POP will fetch value from top of stack, it’s a LIFO (last in, first out) queue. CALL is same as JP, but it also pushes address of next instruction after CALL at top of stack. RET is similar to JP also, popping the address from stack and jumping to it.

Warning: when interrupts are enabled, the sp must be valid during interrupt signal, with enough free space reserved for interrupt handler routine, as the interrupt signal will store the return address (actual pc) before calling handler routine, which may store further data on stack as well. Any value ahead of sp may be thus modified “unexpectedly”, if interrupt happens.

Advanced trick: on Z80 with PUSH taking 11 clock cycles (11t) and POP taking 10t, the unrolled POP/PUSH trough all registers, including EXX for shadow variants, was the fastest way to copy block of memory, even faster than unrolled LDI. But you had to time the copy in between interrupt signals to avoid memory corruption. Also unrolled PUSH was the fastest way to fill memory with particular value on ZX Spectrum (again with the risk of corruption by Interrupt, if not timed properly, or done under DI).