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

[Allen Huffman]

Thanks Art and Chris. Still no go... 

> On Jun 18, 2022, at 3:21 PM, Christopher R. Hawks <chawks at dls.net> wrote:
> 
>    I have to differ, Art. If the saved vector starts with JMP, the
> code will NOT come back to the LEAS 2,s. JMP does not save a return
> address on the stack. So the code will RTS back to the code that called
> JSR $017f (and placed a return address on the stack).
> 
>    Allen's code needs to change the JMP to a JSR. (And do the right
> thing if DECB is missing, $17f contains an RTS.)


Color BASIC ROM:

LA549 JSR RVEC11   (HOOK INTO RAM)
   LDA DEVNUM    (GET DEVICE NUMBER)
   INCA   (CHECK FOR TAPE)
   BEQ LA5A1   (RETURN IF TAPE)
…
LA5A1 RTS

First it jumps out to the 3 RAM hook bytes, which is code expected to do something and then RTS back.

Then it sees if device is not -1 (if inc devnum makes 0) and branches to an RTS if so, skipping break check.

If RVEC11 is:

JMP NEWCODE

and NEWCODE is:

NEWCODE RTS

…that would just return back and finish the break check code.

The other ROMs add checking for DLOAD (#-3), Disk Devices, etc. and then want to bypass the break check. I see that they do it like this:

DVEC11 TST DEVNUM
   BLE LC85F
   LEAS $02, S
LC85 RTS


If the device number is negative (? I guess ?) it pops a JSR return value off the stack, then does an RTS. This makes it return to whatever did the JSR LA549 to the Color BASIC check code, rather than returning to the check code.

That is what I was trying to do, but no luck yet.

For normal RAM hooks, I’d want to still service the regular stuff, but for this one, I don’t really need to, and it “should” just work like this:

* lwasm nobreak.asm -fbasic -onobreak.bas --map

RVEC11 equ $17f      break check RAM hook

    org $3f00

init
    lda #$7e        op code for JMP
    sta RVEC11      store it in RAM hook
    ldx #newcode    address of new code
    stx RVEC11+1    store it in RAM hook
    rts             done

newcode
    inc $400        inc top left byte of 32-col screen
    leas $02,s
    rts

I thought that would be the same as what I see in the Disk BASIC ROM, just without the TST DEVNUM being in the mix.

somewhere in BASIC:

JSR LA549   *Go to a break check

…gets to...

LA549 JSR RVEC11
—> JMP NEWCODE
——> inc $400
——> leas $02,s
——> rts (should go back to the “somewhere in BASIC” JSR, yes?)
…this never executes:
   LDA DEVNUM 
   INCA
   BEQ LA5A1
…

I feel like I’m missing something very simple I just do not understand.

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