Msx Assembly 101: 04 Stack

Stack in cartridges

According to MSX Technical Handbook, a cartridge software should initialize stack pointer before using the stack. The common practice is to put 0xF380 into sp register (see this) and remember that stack grows downwards.

It means that any cartridge program, which uses the stack, should near its beginning do something like ld sp, 0f380h.

Stack in CP/M programs

When a CP/M program starts execution, sp points to the stack of 16 bytes. Since any subroutine call puts a return address on the stack, you can do no more than 7 nested calls if you do not use stack and do not call BDOS routines (2 bytes are already consumed by the CP/M return address, used when your program does final ret). Due to this, it is a good practice to use own stack space in CP/M program. Moreover, CP/M expects that the program preserves stack pointer. It all means that the following should be done at the start of a program:

  1. sp must be preserved
  2. sp should be initialized to point to some own space

and in the end sp should be restored.

In assembly it means the following:

At the start

        ld      [SavedStack], sp
        ld      sp, Stack

The memory for saving the old sp value (2 bytes) and for the new stack area (whatever) should be reserved. The following reserves 32 bytes for the program stack:

SavedStack:
        ds 2
        ds 31
Stack:
        ds 1

Remember that stack grows downwards, so the corresponding label points to the last reserved byte in stack area.

At the end, sp should be restored: ld sp, SavedStack.

Putting it all together

We will modify our framework which can compile both cartridge software and CP/M program from the same source introduced in YAZE post to add stack handling to it. Now the program should look like this:

.INCLUDE "format.asm"

IFDEF CARTRIDGE
.PAGE 1
.ROM
.BIOS
ENDIF

IFDEF CPM
.MSXDOS
ENDIF

; this adds macro, initializing the stack pointer
Start: MACRO
IFDEF CARTRIDGE
        ld      sp, 0f380h
ENDIF
IFDEF CPM
        ld      [SavedStack], sp
        ld      sp, Stack
ENDIF
ENDM

Stop: MACRO
IFDEF CARTRIDGE
@@end:
        jr      @@end
ENDIF
; if we are compiling CP/M program, it restores the stack pointer to the
; original value and reserves the space after program end to store `sp`,
; and also area for the new stack
IFDEF CPM
        ld      sp, [SavedStack]
        ret
SavedStack:
        ds 2
        ds 31
Stack:
        ds 1
ENDIF
ENDM

        Start
        ; program body
        Stop
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