SCHG How-to:Load chunks from ROM in Sonic 1

(Original guide by FraGag)

In Sonic 1, the chunks (metatiles) are larger than in the sequels: 256x256 instead of 128x128. This makes them take up a lot more space and there is a limit to how much you can put in RAM. The game limits itself to $52 chunks (plus one empty chunk), and this takes up more than half of the RAM already. If you were to have $80 chunks, you would take up the whole RAM! If you desperately need more than $52 chunks, you can load them from ROM directly. However, the chunks data is usually compressed in the ROM, so to use the chunks data from ROM, you will need to store them uncompressed, which takes a lot more space (6 KB to 10 KB compressed, compared to at least 41 KB uncompressed — 512 bytes per chunk).

This guide will allow you to continue using chunks loaded to RAM for some zones, while also allowing chunks to be read from ROM for other zones.

Using the uncompressed chunk mappings

Since the game normally stores compressed data, we need to change it to store uncompressed data instead. For example, for Green Hill Zone, replace this: <asm>Blk256_GHZ: incbin map256\ghz.bin even</asm> with this: <asm>Blk256_GHZ: incbin map256_u\ghz.bin even</asm> Note that SonED2 will only change the compressed chunk mappings. You will need to update the uncompressed chunk mappings every time you change them. Use the following command at the command prompt or in a batch (.bat) file (which can be build.bat, right after the @echo off line):

derecmp.exe kd map256 map256_u

Skipping loading chunks to RAM (title screen)

If you decide to read the chunks from ROM for Green Hill Zone and you still use the Green Hill Zone data for your title screen, you should skip decompressing the chunks to avoid overwriting other data in RAM. Go to Title_LoadText and find the following lines: <asm> lea (Blk256_GHZ).l,a0 ; load GHZ 256x256 mappings lea ($FF0000).l,a1 bsr.w KosDec</asm> Comment them out or delete them.

Skipping loading chunks to RAM (levels and ending sequence)

In MainLoadBlockLoad, the chunks data, amongst others, is decompressed to RAM. We are going to disable this. Find the following lines: <asm> lea ($FF0000).l,a1 ; RAM address for 256x256 mappings bsr.w KosDec</asm> and replace them with the following: <asm> tst.b ($FFFFFE10).w ; are we in Green Hill Zone? beq.s @no_dec ; if yes, branch cmpi.b #6,($FFFFFE10).w ; are we in the ending sequence? beq.s @no_dec ; if yes, branch lea ($FF0000).l,a1 ; RAM address for 256x256 mappings bsr.w KosDec

@no_dec:</asm>

Fixing art

The chunks are composed of blocks, which contain art tiles. Since the chunks are not in RAM anymore, you will get empty levels. Go to sub_6BD6 (loc_712C if you followed this guide) and replace the code from <asm> moveq #-1,d3</asm> to <asm> rts </asm> with the following: <asm> tst.b ($FFFFFE10).w ; are we in Green Hill Zone? beq.s @ghz ; if yes, branch cmpi.b #6,($FFFFFE10).w ; are we in the ending sequence? beq.s @ghz ; if yes, branch moveq #-1,d3 ; load chunks from RAM bsr.s LocateBlock bra.s @continue

@ghz: moveq #0,d3 bsr.s LocateBlock add.l #Blk256_GHZ,d3

@continue: movea.l d3,a0 move.w (a0),d3 andi.w #$3FF,d3 lsl.w #3,d3 adda.w d3,a1 rts

---------------------------------------------------------------------------

LocateBlock: move.b (a4,d0.w),d3 ; load chunk ID in d3 beq.s LocateBlock_EmptyChunk subq.b #1,d3 andi.w #$7F,d3 ; '?' ror.w #7,d3 add.w d4,d4 andi.w #$1E0,d4 andi.w #$1E,d5 add.w d4,d3 add.w d5,d3 rts

---------------------------------------------------------------------------

LocateBlock_EmptyChunk: addq.w #4,sp ; pop a stack frame to leave a1 pointing at the first tile rts</asm>

Fixing collision

The chunks also contain collision information, so now we need to tell the game that the data is loaded elsewhere. Find the Floor_ChkTile subroutine and replace it with the following code: <asm>Floor_ChkTile_LocateBlock: lea ($FFFFA400).w,a1 move.b (a1,d0.w),d1 beq.s Floor_ChkTile_EmptyChunk ; if the chunk ID is 0 (empty chunk), branch bmi.s loc_1499A subq.b #1,d1 ; the empty chunk is not included in the chunk mappings, subtract 1 to read the correct data ext.w d1 ror.w #7,d1 move.w d2,d0 add.w d0,d0 andi.w #$1E0,d0 add.w d0,d1 move.w d3,d0 lsr.w #3,d0 andi.w #$1E,d0 add.w d0,d1 rts

---------------------------------------------------------------------------

loc_1499A: andi.w #$7F,d1 btst #6,1(a0) beq.s loc_149B2 addq.w #1,d1 cmpi.w #$29,d1 bne.s loc_149B2 move.w #$51,d1

loc_149B2: subq.b #1,d1 ror.w #7,d1 move.w d2,d0 add.w d0,d0 andi.w #$1E0,d0 add.w d0,d1 move.w d3,d0 lsr.w #3,d0 andi.w #$1E,d0 add.w d0,d1 rts

---------------------------------------------------------------------------

Floor_ChkTile_EmptyChunk: lea ($FFFFFF00).w,a1 ; override a1 addq.w #4,sp ; pop a stack frame to avoid adding the address of the chunk mappings to a1 rts

---------------------------------------------------------------------------

Subroutine to find which tile the object is standing on

---------------------------------------------------------------------------

||||||||||||||| S U B R O U T I N E |||||||||||||||||||||||||||||||||||||||

Floor_ChkTile: ; XREF: FindFloor; et al move.w d2,d0 lsr.w #1,d0 andi.w #$380,d0 move.w d3,d1 lsr.w #8,d1 andi.w #$7F,d1 add.w d1,d0 tst.b ($FFFFFE10).w ; are we in Green Hill Zone? beq.s @ghz ; if yes, branch cmpi.b #6,($FFFFFE10).w ; are we in the ending sequence? beq.s @ghz ; if yes, branch moveq #-1,d1 bsr.w Floor_ChkTile_LocateBlock movea.l d1,a1 rts

---------------------------------------------------------------------------

@ghz: moveq #0,d1 bsr.w Floor_ChkTile_LocateBlock add.l #Blk256_GHZ,d1 movea.l d1,a1 rts

End of function Floor_ChkTile</asm>