1 files changed, 53 insertions, 0 deletions
diff --git a/src/keyclick.s b/src/keyclick.s
new file mode 100644
index 0000000..bbe613c
--- /dev/null
+++ b/src/keyclick.s
@@ -0,0 +1,53 @@
+ .include "atari.inc"
+ .export _keyclick
+
+; keyclick.s - just what you think it is.
+
+; How do you make it possible to disable the keyclick on the 400/800?
+; Don't use the OS K: handler.
+
+ ;.ifdef REAL_KEYCLICK
+ .if 1
+
+; Copied from OS ROM source, 12 bytes of code. Because of the repeated
+; stx WSYNC, you don't want to use this if you're using DLIs.
+; Preserves A and Y, returns with X=$ff, N set, Z clear.
+_keyclick:
+ ldx #$7f
+@rc1:
+ stx CONSOL
+ stx WSYNC
+ dex
+ bpl @rc1
+ rts
+
+ .else
+
+; Fake keyclick. 25 bytes of code. Sounds just like the real one, but
+; it won't mess up your DLIs because it's interruptible (though DLIs
+; might mess it up). Loop timing assumes a GR.0 type display list
+; with playfield DMA enabled and P/M DMA disabled.
+; Preserves A, returns with X=$ff, Y=0, N set, Z clear.
+_keyclick:
+@wcount:
+ ldx VCOUNT   ; To make the timing consistent, wait until we're sure
+ cpx #$10     ; that ANTIC is doing DMA.
+ bne @wcount
+
+ ldx #$7f
+@click:     ; I tuned this by ear, then cycle-counted afterward.
+ stx CONSOL ; 4
+ ldy #$07   ; 2
+ ;lda #0     ; 2
+ nop
+@c1:
+ dey        ; inner: 2
+ nop        ;        2
+ bne @c1    ;        3
+            ;        7 cycles/loop, *7 = 49, -1 for failed BNE
+			   ; 48
+ dex        ; 2
+ bpl @click ; 3 = 61
+ rts
+
+ .endif