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

[Arthur Flexser]

Did you change RVEC from $167 to the break key hook?  Your listing shows it
as $167 still.

Art

On Thu, Jun 16, 2022 at 11:20 AM Allen Huffman via Coco <
coco at maltedmedia.com> wrote:

> 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
> http://www.PayPal.me/AllenHuffman.
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>
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