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/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