[Coco] Color BASIC BREAK CHECK RAM hook ($17c)

[Allen Huffman]

Hi, 6809 ROM folks… I started looking at the RAM hooks in Color BASIC to see what they did. I wrote a simple program that hijacks the CONSOLE OUT hook so it jumps to my routine, then returns to handle the normal stuff. I will be posting an article about this soon.

consoleout.asm:
————
* lwasm consout.asm -fbasic -oconsout.bas --map

RVEC equ $167      console out RAM hook

    org $3f00

init
    lda RVEC        get op code
    sta savedrvec   save it
    ldx RVEC+1      get address
    stx savedrvec   save it

    lda #$7e        op code for JMP
    sta RVEC        store it in RAM hook
    ldx #newcode    address of newcodee
    stx RVEC+1      store it in RAM hook

    rts             done

newcode
    inc $400        inc top left byte of 32-col screen
savedrvec rmb 3     3 bytes to save original RVEC entry


I then looked at the BREAK CHECK hook. It is similar, but it processes the break check after calling the RAM hooks. I thought it would be easy to make it skip that check by doing something like Disk BASIC does when if it doesn’t want to break (disk file I/O). It manipulates the stack pointer so when it returns, it returns to the original jsr caller and does not finish the rest of the Color BASIC code:

Disk BASIC RAM hook code:
————
* BREAK CHECK RAM HOOK
DEVC11 TST DEVNUM
    BLE LC8AF * IF NOT A DISK FILE
    LEAS $02,S * PURGE RETURN ADDRESS OFF THE STACK
   LC8AF RTS

In my code, I save whatever RAM hook is there, then point it to my new code. For the break check, I call the saved vector routine first, and then do this stack trick. It still ends up running the code, so I don’t quite grasp what I am doing with the stack.

Any clue what I should be looking at?

nobreak.asm
————
* lwasm consout.asm -fbasic -oconsout.bas --map

RVEC equ $167      console out RAM hook

    org $3f00

init
    lda RVEC        get op code
    sta savedrvec   save it
    ldx RVEC+1      get address
    stx savedrvec   save it

    lda #$7e        op code for JMP
    sta RVEC        store it in RAM hook
    ldx #newcode    address of newcodee
    stx RVEC+1      store it in RAM hook

    rts             done

newcode
    inc $400        inc top left byte of 32-col screen
savedrvec rmb 3     3 bytes to save original RVEC entry

Thanks, much. I sure wish I had these Unravelled books back then.

--
Allen Huffman - PO Box 7634 - Urbandale IA 50323 - 515-999-0227 (vmail/TXT only)
http://www.subethasoftware.com - https://www.facebook.com/subethasoftware
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