From 7c2671be2a8c365f4254abec8682928ff72651de Mon Sep 17 00:00:00 2001
From: Oliver Schmidt <ol.sc@web.de>
Date: Wed, 12 Mar 2025 21:33:13 +0100
Subject: [PATCH] Disable interrupts during aux memory access.

Interrupt handlers rather likely access text screen holes. Especially MSLOT is obligatory for every interrupt handler that requires access to an extension ROM ($C800-$CFFE) in order to be able to re-enable the extension ROM that was enabled when the interrupt occured. Those text screen holes only hold valid values in main memory so interrupts must be disabled while the aux memory text screen is mapped.
---
 libsrc/apple2/cgetc.s  |  5 +++--
 libsrc/apple2/cpeekc.s | 11 ++++++++---
 libsrc/apple2/cputc.s  | 16 +++++++++++-----
 3 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/libsrc/apple2/cgetc.s b/libsrc/apple2/cgetc.s
index ecce9f9de..d2ebe5db8 100644
--- a/libsrc/apple2/cgetc.s
+++ b/libsrc/apple2/cgetc.s
@@ -9,6 +9,7 @@
         .export         _cgetc
         .import         cursor, putchardirect
 
+        .include        "zeropage.inc"
         .include        "apple2.inc"
 
 _cgetc:
@@ -22,7 +23,7 @@ _cgetc:
         .else
         lda     #' ' | $40      ; Blank, flashing
         .endif
-        jsr     putchardirect   ; Returns old character in X
+        jsr     putchardirect   ; Saves old character in tmp3
 
         ; Wait for keyboard strobe.
 :       inc     RNDL            ; Increment random counter low
@@ -37,7 +38,7 @@ _cgetc:
 
         ; Restore old character.
         pha
-        txa
+        lda     tmp3
         jsr     putchardirect
         pla
 
diff --git a/libsrc/apple2/cpeekc.s b/libsrc/apple2/cpeekc.s
index a547f7867..200206177 100644
--- a/libsrc/apple2/cpeekc.s
+++ b/libsrc/apple2/cpeekc.s
@@ -11,18 +11,23 @@
 _cpeekc:
         ldy     CH
         .ifdef  __APPLE2ENH__
+        sec                     ; Assume main memory
         bit     RD80VID         ; In 80 column mode?
         bpl     peek            ; No, just go ahead
         tya
         lsr                     ; Div by 2
         tay
         bcs     peek            ; Odd cols are in main memory
+        php
+        sei                     ; No valid MSLOT et al. in aux memory
         bit     HISCR           ; Assume SET80COL
         .endif
 peek:   lda     (BASL),Y        ; Get character
         .ifdef  __APPLE2ENH__
-        bit     LOWSCR          ; Doesn't hurt in 40 column mode
-        .endif
+        bcs     :+              ; In main memory
+        bit     LOWSCR
+        plp
+:       .endif
         eor     #$80            ; Invert high bit
-        ldx     #$00
+        ldx     #>$0000
         rts
diff --git a/libsrc/apple2/cputc.s b/libsrc/apple2/cputc.s
index 0a27abacd..2dcfa1e00 100644
--- a/libsrc/apple2/cputc.s
+++ b/libsrc/apple2/cputc.s
@@ -15,6 +15,7 @@
         .import         uppercasemask
         .endif
 
+        .include        "zeropage.inc"
         .include        "apple2.inc"
 
         .macpack        cpu
@@ -87,23 +88,28 @@ putchar:
 mask:   and     INVFLG          ; Apply normal, inverse, flash
 
 putchardirect:
-        pha
+        tax
         .ifdef  __APPLE2ENH__
         lda     CH
+        sec                     ; Assume main memory
         bit     RD80VID         ; In 80 column mode?
         bpl     put             ; No, just go ahead
         lsr                     ; Div by 2
         bcs     put             ; Odd cols go in main memory
+        php
+        sei                     ; No valid MSLOT et al. in aux memory
         bit     HISCR           ; Assume SET80COL
 put:    tay
         .else
         ldy     CH
         .endif
         lda     (BASL),Y        ; Get current character
-        tax                     ; Return old character for _cgetc
-        pla
+        sta     tmp3            ; Save old character for _cgetc
+        txa
         sta     (BASL),Y
         .ifdef  __APPLE2ENH__
-        bit     LOWSCR          ; Doesn't hurt in 40 column mode
-        .endif
+        bcs     :+              ; In main memory
+        bit     LOWSCR
+        plp
+:       .endif
         rts