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

[Christopher R. Hawks]

On Sat, 18 Jun 2022 14:48:01 -0400
Arthur Flexser via Coco <coco at maltedmedia.com> wrote:

> Looks to me like what's being bypassed isn't what you intend.
> 
> Your code does the INC $400, then the regular break check processing
> (JMP SAVEDVEC), which ends in an RTS, taking you back to the LEAS 2,S
> that you patched in..  Then you purge the next deeper RTS from the
> stack, bypassing some entirely different ROM code.  If you wanted it
> to INC $400 and then return immediately, bypassing normal break key
> processing, you should just put an RTS after the INC $400 and not
> bother saving the old vector.
> 
> Art.
> 
> On Sat, Jun 18, 2022 at 12:43 PM Allen Huffman via Coco <
> coco at maltedmedia.com> wrote:
> 
> > > On Jun 17, 2022, at 12:00 AM, Arthur Flexser via Coco <
> > coco at maltedmedia.com> wrote:
> > >
> > > Did you change RVEC from $167 to the break key hook?  Your
> > > listing shows
> > it
> > > as $167 still.
> > >
> > > Art
> >
> >
> > Oops - looks like I pasted the same thing twice.  Here is my
> > NOBREAK.ASM attempt. If it’s something super obvious like that, I’d
> > be quite happy.
> >
> >
> > * lwasm nobreak.asm -fbasic -onobreak.bas --map
> >
> > RVEC11 equ $17f      break check RAM hook
> >
> >     org $3f00
> >
> > init
> >     lda RVEC11      get op code
> >     sta savedrvec   save it
> >     ldx RVEC11+1    get address
> >     stx savedrvec+1 save it
> >
> >     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
> > savedrvec rmb 3     call regular RAM hook
> >     leas $02,S      remove PC from stack
> >     rts             return back, skipping.
> >
> >
> > --
> > Allen Huffman - PO Box 7634 - Urbandale IA 50323 - 515-999-0227
> > (vmail/TXT only)
> > http://www.subethasoftware.com -
> > https://www.facebook.com/subethasoftware
> >
> >
> >
> > --
> > Coco mailing list
> > Coco at maltedmedia.com
> > https://pairlist5.pair.net/mailman/listinfo/coco
> >
> 

    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.)



Christopher R. Hawks
HAWKSoft
-- 
"The leader of Jamestown was "John Smith" (not his real name), under
whose direction the colony engaged in a number of activities, primarily
related to starving.		-- Dave Barry, "Dave Barry Slept
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