From 1aeb3aeb13bc2cf64a630808b4f343401467aacd Mon Sep 17 00:00:00 2001
From: mc78 <mc78@outlook.de>
Date: Mon, 26 Aug 2024 10:25:02 +0200
Subject: [PATCH] Reject setting screen at $0000 due to overlap with zeropage
 and kernal vars.

---
 libsrc/cbm/workingscreenaddr.s | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/libsrc/cbm/workingscreenaddr.s b/libsrc/cbm/workingscreenaddr.s
index a964455ac..849cc0691 100644
--- a/libsrc/cbm/workingscreenaddr.s
+++ b/libsrc/cbm/workingscreenaddr.s
@@ -11,6 +11,7 @@
 
 _cbm_set_working_screen:
         tax
+        beq @error  ; Reject setting screen at zero page location
         and #3
         bne @error
         cpx #$10    ; Screen addr must not be set to $1000-$1fff nor $9000-$9fff
@@ -33,6 +34,6 @@ _cbm_set_working_screen:
         tax
         rts
     @error:
-        lda #$ff
-        tax
+        lda #$01
+        ldx #$00
         rts