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vic20/cputc: Fix incorrect CRAM_PTR at startup when using conio

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To do this we add a constructor call to UPDCRAMPTR, which is the ROM
routine that fixes up CRAM_PTR to match the screen location pointed to
by SCREEN_PTR.

This adds two additional bytes to programs using cputc() or other
routines that call it. These are in theory recoverable, but the VIC-20
does not yet free space used by constructors after the constructors
have been called.

Thanks to <greg.king5@verizon.net> (GitHub: greg-king5) for
investigating the difference in the VIC-20 KERNAL from the C64 and
proposing this solution to the problem.[1]

[1]: https://github.com/cc65/cc65/issues/946#issuecomment-538502820
This commit is contained in:
Curt J. Sampson 2019-10-09 21:05:00 +09:00 committed by greg-king5
parent 128991d868
commit 1d39863a60
1 changed files with 21 additions and 0 deletions
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21
libsrc/vic20/cputc.s
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@ -10,8 +10,29 @@
.import gotoxy
.import PLOT
.scope KERNAL
.include "cbm_kernal.inc"
.endscope
.include "vic20.inc"
; VIC-20 KERNAL routines (such as PLOT) do not always leave the color RAM
; pointer CRAM_PTR pointing at the color RAM location matching the screen
; RAM pointer SCREEN_PTR. Instead they update it when they need it to be
; correct by calling UPDCRAMPTR.
;
; We make things more efficient by having conio always update CRAM_PTR when
; we move the screen pointer to avoid extra calls to ensure it's updated
; before doing screen output. (Among other things, We replace the ROM
; version of PLOT with our own in libsrc/vic20/kplot.s to ensure this
; precondition.)
;
; However, this means that CRAM_PTR may be (and is, after a cold boot)
; incorrect for us at program startup, causing cputc() not to work. We fix
; this with a constructor that ensures CRAM_PTR matches SCREEN_PTR.
;
UPDCRAMPTR := KERNAL::UPDCRAMPTR ; .constructor doesn't understand namespaces
.constructor UPDCRAMPTR
_cputcxy:
pha ; Save C