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.
2025-05-27 22:42:07 +02:00
parent 816666615b
commit e444f4c40e
3 changed files with 63 additions and 15 deletions
--- a/libsrc/apple2/detect80cols.s
+++ b/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