Apple2: Safely check for 80-columns card

Check the Pascal ID bytes for an 80-columns card
to avoid blindly jumping into a $C300 that could be
totally unrelated.

libsrc/apple2/detect80cols.s

@@ -0,0 +1,56 @@
+;
+; Colin Leroy-Mira, 27/05/2025
+;
+; Verify the presence of a 80 columns card in slot 3,
+; and publish a flag accordingly.
+;
+        .export         aux80col
+
+        .ifndef __APPLE2ENH__
+        .import         machinetype
+        .endif
+
+        .constructor    detect80cols
+
+        .include        "apple2.inc"
+
+        .data
+
+aux80col:        .byte 0
+
+        .segment        "ONCE"
+
+IdOfsTable:                     ; Table of bytes positions, used to check four
+                                ; specific bytes on the slot's firmware to make
+                                ; sure this is a serial card.
+        .byte   $05             ; Pascal 1.0 ID byte
+        .byte   $07             ; Pascal 1.0 ID byte
+        .byte   $0B             ; Pascal 1.1 generic signature byte
+        .byte   $0C             ; Device signature byte
+
+IdValTable:                     ; Table of expected values for the four checked
+                                ; bytes
+        .byte   $38             ; ID Byte 0 (from Pascal 1.0), fixed
+        .byte   $18             ; ID Byte 1 (from Pascal 1.0), fixed
+        .byte   $01             ; Generic signature for Pascal 1.1, fixed
+        .byte   $88             ; Device signature byte (80 columns card)
+
+IdTableLen      = * - IdValTable
+
+detect80cols:
+        .ifndef __APPLE2ENH__
+        lda     machinetype     ; Check we're on a //e at least, otherwise we
+        bpl     NoDev           ; handle no 80cols hardware (like Videx)
+        .endif
+
+        ldx     #IdTableLen-1
+:       ldy     IdOfsTable,x    ; Check Pascal 1.1 Firmware Protocol ID bytes
+        lda     IdValTable,x
+        cmp     $C300,y
+        bne     NoDev
+        dex
+        bpl     :-
+
+        dec     aux80col   ; We have an 80-columns card! Set flag to $FF
+
+NoDev:  rts

libsrc/apple2/exec.s

@@ -6,6 +6,7 @@
 
         .export         _exec
         .import         mli_file_info_direct
+        .import         aux80col
         .import         pushname, popname, popax, done, _exit
 
         .include        "zeropage.inc"
@@ -115,7 +116,8 @@ setbuf: lda     #$00            ; Low byte
         sta     io_buffer
         stx     io_buffer+1
 
-        .ifdef  __APPLE2ENH__
+        bit     aux80col
+        bpl     :+
         ; Calling the 80 column firmware needs the ROM switched
         ; in, otherwise it copies the F8 ROM to the LC (@ $CEF4)
         bit     $C082
@@ -128,9 +130,8 @@ setbuf: lda     #$00            ; Low byte
 
         ; Switch in LC bank 2 for R/O
         bit     $C080
-        .endif
 
-        ; Reset stack as we already passed
+:       ; Reset stack as we already passed
         ; the point of no return anyway
         ldx     #$FF
         txs

libsrc/apple2/videomode.s

@@ -5,14 +5,10 @@
 ;
         .export         _videomode
 
-        .ifndef __APPLE2ENH__
-        .import         machinetype
-        .endif
-
+        .import         aux80col
         .import         returnFFFF
 
         .include        "apple2.inc"
-        .include        "mli.inc"
 
 
 VIDEOMODE_40x24 = $15
@@ -21,19 +17,14 @@ VIDEOMODE_80x24 = $00
         .segment        "LOWCODE"
 
 _videomode:
-        ; Functionally equivalent to previous assumption that
-        ; __APPLE2ENH__ == 80 columns hardware present. Will be
-        ; correctly checked in the very near future.
-        .ifndef __APPLE2ENH__
-        bit     machinetype
-        bvs     set_mode
+        bit     aux80col
+        bmi     set_mode
 
         ; No 80 column card, return error if requested mode is 80cols
         cmp     #VIDEOMODE_40x24
         beq     out
         jmp     returnFFFF
 set_mode:
-        .endif
 
         ; Get and save current videomode flag
         bit     RD80VID