From 1d39863a602a728c83aa600a7929b76ce9d1bb06 Mon Sep 17 00:00:00 2001
From: "Curt J. Sampson" <cjs@cynic.net>
Date: Wed, 9 Oct 2019 21:05:00 +0900
Subject: [PATCH 081/170] vic20/cputc: Fix incorrect CRAM_PTR at startup when
using conio
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
---
libsrc/vic20/cputc.s | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
diff --git a/libsrc/vic20/cputc.s b/libsrc/vic20/cputc.s
index 43aacdae..1db81854 100644
--- a/libsrc/vic20/cputc.s
+++ b/libsrc/vic20/cputc.s
@@ -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
--
2.26.0