Display the Press Start Button text

(Original guide by Quickman)

I've finally isolated precisely why the PRESS START BUTTON (PSB) text doesn't appear on the Sonic 1 title screen, and given the nature I suspect it to be a bug in Sonic Team's original code. (You may now be shocked.)

Now, the PSB object is initialised in object RAM slot $D080 on line 3237 of the disassembly I'm currently using (modified by Pu7o to work on Linux; it's just under the label Title_ClrObjRam2, in case it's not the same line in an unmodified Sonic 1 disassembly). That same chunk of RAM is previously used for the SONIC TEAM PRESENTS (STP) object earlier in the TitleScreen routine (line 3162, adjust as necessary).

The reason the PSB text doesn't appear is because the STP object scribbles in object RAM, but the TitleScreen routine only clears the top $1C bytes. Any data between $D09C and $D0A0 previously touched by the STB object is unmodified. This, I suspect, is why the PSB text fails to appear, and it's a surprisingly difficult bug to track down without really scrutinising the code and a few quite remarkable deductions.

The correction is simple enough; delete the object RAM for the object at $D080 more thoroughly. Here's the replacement I made. (Lines are prefixed diff-style - a minus indicates a deleted line, a plus indicates an inserted line.)

<asm> lea ($FFFFD080).w,a1 - moveq #0,d0 - move.w #7,d1

-Title_ClrObjRam2: - move.l d0,(a1)+ - dbf d1,Title_ClrObjRam2

+ jsr DeleteObject2 move.b #$E,($FFFFD040).w; load big Sonic object</asm>