nesdev.com
http://forums.nesdev.com/

VS Excitebike [solved]
http://forums.nesdev.com/viewtopic.php?f=3&t=16063
Page 1 of 1

Author:  Zepper [ Sat Jun 10, 2017 8:31 pm ]
Post subject:  VS Excitebike [solved]

In my emulator, the game freezes on startup, waiting for an NMI (jmp $85F4, same of PC register). If I press RESET or HARD RESET, the game works.

Any clue what I'm missing on my CPU/PPU startup?

Author:  lidnariq [ Sat Jun 10, 2017 9:10 pm ]
Post subject:  Re: VS Excitebike

It looks like Vs Excitebike explicitly chooses to go into that infinite loop if P2 start (i.e. "3 player" ?) is pressed on power-up:
Code:
  lda $14                 
  ora $15                 
  and #$10                 
  beq $85F7               
  jmp $85F4               

Author:  Zepper [ Sun Jun 11, 2017 5:43 pm ]
Post subject:  Re: VS Excitebike

I believe it's another game that relies on the startup contents of RAM. Filling up with zeroes won't boot, but will do if filled up with $FF.

Author:  lidnariq [ Sun Jun 11, 2017 5:57 pm ]
Post subject:  Re: VS Excitebike [solved]

Er.

The first 22 instructions in the reset vector unambiguously include a loop that clears all internal RAM:
Code:
_Reset  $85AF  78:          sei
        $85B0  D8:          cld
        $85B1  A9 00:       lda #$00
        $85B3  8D 00 20:    sta $2000
        $85B6  A2 FF:       ldx #$FF
        $85B8  9A:          txs
-       $85B9  AD 02 20:    lda $2002
        $85BC  29 80:       and #$80
        $85BE  F0 F9:       beq -               ; $85B9
-       $85C0  AD 02 20:    lda $2002
        $85C3  29 80:       and #$80
        $85C5  F0 F9:       beq -               ; $85C0
        $85C7  A0 07:       ldy #$07 ; loop that clears memory starts here, with address 0x700
        $85C9  84 01:       sty $01
        $85CB  A0 00:       ldy #$00
        $85CD  84 00:       sty $00
        $85CF  98:          tya
-       $85D0  91 00:       sta ($00),y
        $85D2  88:          dey ; counting down, so 700 7FF 7FE &c
        $85D3  D0 FB:       bne -               ; $85D0
        $85D5  C6 01:       dec $01 ; counting down, so 700 600 500 &c
        $85D7  10 F7:       bpl -               ; $85D0

Author:  Zepper [ Sun Jun 11, 2017 6:17 pm ]
Post subject:  Re: VS Excitebike [solved]

Odd. I don't know what to say... it should work, but it isn't.

EDIT: argh, you're right. What could be avoiding its proper boot?
EDIT2: it seems that holding START affects the boot, since I press "ENTER" mapped to START.

Page 1 of 1 All times are UTC - 7 hours
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/