NSC: Probe for Z80 card before probing for NSC to avoid hang

The NSC driver installer starts off by checking each slot, bit-banging
into the card's memory space to try and detect an NSC installed. If a
Z80 card is encountered before the NSC is hit, the Z80 card is
activated, causing the 6502 to halt.

Borrow the Z80 detection routine from A2D's This Apple and verify that
there isn't a Z80 in the slot before we start bit-banging.

Fixes #15

clocks/ns.clock/ns.clock.system.s

@@ -71,6 +71,11 @@
         lda     #3              ; treat slot 0 as slot 3
 
 sloop:  ora     #$C0            ; A=$Cs
+        pha
+        jsr     DetectZ80
+        pla                     ; A=$Cs
+        bcs     next            ; Z80 present, skip this slot
+
         sta     st1+2
 rloop:  sta     ld1+2
         sta     ld2+2
@@ -203,6 +208,68 @@ loop:   lda     driver,y
         rts                     ; done!
 .endproc
 
+;;; ------------------------------------------------------------
+;;; Detect Z80
+;;; ------------------------------------------------------------
+
+;;; This routine gets swapped into $0FFD for execution
+.proc Z80Routine
+        target := $0FFD
+        ;; .org $FFFD
+        patch := *+2
+        .byte   $32, $00, $e0   ; ld ($Es00),a   ; s=slot being probed turn off Z80, next PC is $0000
+        .byte   $3e, $01        ; ld a,$01
+        .byte   $32, $08, $00   ; ld (flag),a
+        .byte   $c3, $fd, $ff   ; jp $FFFD
+        flag := *
+        .byte   $00             ; flag: .db $00
+.endproc
+        .assert Z80Routine > Z80Routine::target + .sizeof(Z80Routine), error, "Z80 collision"
+
+;;; Input: A = $Cn where n = slot number
+;;; Output: C=1 if Z80 found in slot
+.proc DetectZ80
+        ;; Location to poke to invoke Z80
+        sta     store+1
+
+        ;; Convert $Cn to $En, update Z80 code
+        ora     #$E0
+        sta     Z80Routine::patch
+
+        ;; Clear detection flag
+        copy    #0, Z80Routine::flag
+
+        ;; Put routine in place
+        jsr     SwapRoutine
+
+        ;; Try to invoke Z80
+        php
+        sei
+        store := *+1
+        sta     $C000           ; self-modified
+        plp
+
+        ;; Restore memory
+        jsr     SwapRoutine
+
+        ;; Flag will be set to 1 by routine if Z80 was present.
+        lda     Z80Routine::flag
+        ror                     ; move flag into carry
+        rts
+
+.proc SwapRoutine
+        ldx     #.sizeof(Z80Routine)-1
+:       ldy     Z80Routine::target,x
+        lda     Z80Routine,x
+        sta     Z80Routine::target,x
+        tya
+        sta     Z80Routine,x
+        dex
+        bpl     :-
+        rts
+.endproc
+.endproc
+
 ;;; ============================================================
 ;;; NSC driver - modified as needed and copied into ProDOS
 ;;; ============================================================