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View topic - Flicky tries to write $73A3-$74A2

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  • Joined: 25 Feb 2013
  • Posts: 234
  • Location: Osaka
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Flicky tries to write $73A3-$74A2
Post Posted: Fri May 21, 2021 9:01 am
If you let the demo play, and then start the game, flicky for sg-1000 tries to write in the range $73A3-$74A2 (at $040D).
Did anyone ever disassembly it and know what kind of bug/removed feature it is?
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  • Joined: 08 Dec 2005
  • Posts: 485
  • Location: Melbourne, Australia
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Post Posted: Sat May 22, 2021 8:04 am
It's a bug. It's present in both Flicky v0 and v1, but these addresses refer to v1.

After the demo, these game makes these calls:

046a:           call 7395
046d:           call 0408
0470:           call 0173

The function at 7395 silences the PSG by writing $9f, $bf, $df, $ff to it:

silence_psg:    ld hl,739Fh
                ld c,7Fh
                ld b,04h
                otir
                ret

The second call is intended to clear the copy of the Sprite Attribute Table kept in RAM, before the third call uses that table:

clear_sat:      ld hl,C0ECh
                bit 5,(hl)
                ret nz
                ld hl,C1A1h
                ld b,80h
clear_sat_loop: ld a,b
                and 03h
                jr nz,+
                ld (hl),E0h
                jr ++
+:              ld (hl),00h
++:             inc hl
                djnz clear_sat_loop
                ret

However, $0408 isn't the address of `clear_sat`, it's the address of `clear_sat_loop`. So the loop runs but without first initializing `hl` and `b`, which retain their values from the end of `silence_psg` - $73a3 and $00 respectively. Therefore instead of clearing $c1a1 - $c220, the loop tries to clear $73a3 - $74a2.
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  • Joined: 25 Feb 2013
  • Posts: 234
  • Location: Osaka
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Post Posted: Sun May 23, 2021 11:38 pm
Thank you so much for this wonderful analysis!
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