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Notes:Star Battle

From The Cutting Room Floor
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This page contains notes for the game Star Battle.

Restoring the Developer Credit

Documenting the developer credit easter egg, adapted from Commodore enthusiast Robin Harbron (aka 8-Bit Show and Tell)'s video in which he restored it.

At 0xA50E is some code which checks memory at 0x1A33. If it does not equal zero, the program branches to 0xA514 to begin the credits routine. Otherwise, it simply returns and continues as normal.

0xA50E    AD 33 1A    lda $1A33
0xA511    D0 01       bne $A514
0xA513    60          rts

However, there is no longer anywhere in the code which sets this address to a nonzero value, only code which clears it. Still, almost all of the code which originally would have done this remains in the game. At 0xA7EC, after the title screen's input routine has checked for an F1 (one player) or F3 (two player) key press, it continues checking for the Commodore, LeftShift and RightShift keys pressed at the same time.

0xA7EC    A2 02       ldx #$02
0xA7EE    BD 07 A8    lda $A807,X    ; loads in one keycode row from 0xA807 + rX
0xA7F1    8D 20 91    sta $9120
0xA7F4    AD 21 91    lda $9121      ; read back the pressed key
0xA7F7    CD 21 91    cmp $9121
0xA7FA    D0 F8       bne $A7F4
0xA7FC    3D 0A A8    and $A80A,X    ; bitwise and with the keycode column at 0xA80A + rX
0xA7FF    D0 05       bne $A806      ; b to rts if there's no secret key match, or ...
0xA801    CA          dex            ; decrement rX and ...
0xA802    10 EA       bpl $A7EE      ; loop back up to load in a different key code
0xA804    A9 01       lda #$01       ; load 01 into rA
0xA806    60          rts            ; return to subroutine

Upon returning, the game does nothing with that 01 in rA, it is never written out to memory and is immediately replaced by another lda. Harbron suspects that the lda #$01 was originally followed by a sta $1A33, writing the value out to the secret flag and enabling the credits screen, which was commented out in the final build.

Harbron's patch makes the following adjustments to re-enable the credits screen. Uncommented lines are unchanged from their original code.

0xA7FF    D0 8E       bne $A78F      ; b to earlier rts since original at 0xA806 is gone
0xA801    CA          dex
0xA802    10 EA       bpl $A7EE
0xA804    4C F0 BD    jmp $BDFO      ; jump to unused ROM area after graphics data

...

0xBDF0    EE 33 1A    inc $1A33      ; increment the secret flag
0xBDF3    60          rts            ; return