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/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