Updating variables when compiling as ROM

Gilbert François Duivesteijn

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When your program is compiled for cartridge as ROM, you cannot change values of variables in this memory area. For example:

xxxxxxxxxx
; ==[ Constants ]===============================================

ORGADR  equ $4000
RomSize equ $4000

; ==[ Header ]==================================================

    ; Place header inside the binary.
    org ORGADR
    ; ROM header
    db "AB"
    dw Main
    dw 0, 0, 0, 0, 0, 0

; ==[ Program ]=================================================

Main:
    ; increase the value of MyVar
    ld hl, MyVar  ; Load the address of MyVar in HL
    ld a, (hl)    ; Load the value of MyVar in a
    inc a         ; Increase a
    ld (hl), a    ; Write the updated value in MyVar

    ; finish
    di
    halt
    
MyVar:
    db $00
    
; ==[ ROM Padding ]=============================================

ProgEnd:
    ds $4000 + RomSize - ProgEnd, 255

Compile with VASM

$ vasmz80_oldstyle hello_screen2.asm -chklabels -nocase -Dvasm=1 -Fbin -L out.sym -o out.rom

or with Glass (you can choose)

$ java -jar Glass.jar hello_screen2.asm -L out.sym out.rom

This example won't work. The memory of MyVar lies inside the ROM. Let's look at the debugger and see what is happening. Pay special attention to memory location $4018, where MyVar is located.

In the section Memory layout of the debugger, we can see that page 0 and 1 are reserved for the ROM:

Our ORG address is $4000 and MyVar is located at $4018. Since this is still in ROM area, the memory content is read-only. The solution is to store the variables or anything that needs to be manipulated in memory, in RAM area. Page 2 and 3 are RAM. There are several ways to do that in code. However, most methods are assembler specific. The standard syntax that works with all assembers is by simply specifying the address as constant, e.g:MyVarRam equ $c000. The new listing is:

xxxxxxxxxx
; ==[ Constants ]===============================================

ORGADR   equ $4000
RomSize  equ $4000
MyVarRam equ $c000

; ==[ Header ]==================================================

    ; Place header inside the binary.
    org ORGADR
    ; ROM header
    db "AB"
    dw Main
    dw 0, 0, 0, 0, 0, 0

; ==[ Program ]=================================================

Main:
    ; copy initial MyVar value to RAM.
    ; Assuming you have more variables,
    ; usr ldir for fast copy.
    ld hl, MyVar      ; source addr
    ld de, MyVarRam   ; destination addr
    ld bc, 1          ; block size
    ldir              ; copy

    ; increase the value of MyVarRam
    ld hl, MyVarRam
    ld a, (hl)
    inc a
    ld (hl), a

    di
    halt

MyVar:
    db $00

; ==[ ROM Padding ]=============================================

ProgEnd:
    ds $4000 + RomSize - ProgEnd, 255

First, we copy the initial values from ROM to RAM (when needed). Then we continue our original program, and can successfully change values in MyVarRam. You can see in the debugger that the increase of the value is successful.

Final words....

When you have many variables, lists or other data, you can declare them as follows:

MyOneByteVarRam    equ $c000
My10ByteListRam    equ MyOneByteVar + 1
My2ByteVarRam      equ My10ByteList + 10
MyEndRamBlock      equ My2ByteVarRam + 2

TotalRamSize       equ MyEndRamBlock - MyOneByteVarRam