Mode 0

What is Mode 0?

Mode 0 is the first mode on the SNES, characterized by its offer of four layers, as opposed to the normal three Mode 1 offers you. However, there are two significant drawbacks: you only have four colors (3 actual colors plus transparency) per palette, and the layer 3 tilemap is halved to make room for layer 4. However, the latter drawback can be circumvented by sacrificing GFX slots.

Applications of Mode 0

When it comes to the SNES library of games, only a few have used Layer 4, such as Super Mario Kart, Super Mario World 2 - Yoshi's Island, S.O.S., and Earthbound.

To make things more interesting, in the field of Super Mario World hacking, a few hacks and contest levels have used it, such as ASMWCP, LMPuny's work-in-progress SUPER MARIO BROS. Returns, idol and Katrina's Chocolate Level Design Contest 2018 entry lightest, and AnasMario130's Power Mario Contest entry Pyro-Blue's Fort.

Palette and Tilemap Distribution

As mentioned in the previous heading, Mode 0 can only offer you four colors per palette, including transparency. Each layer takes up 8 four-color palettes distributed in two rows each. Layer 1 takes up rows 0-1, layer 2 rows 2-3, layer 3 rows 4-5, and layer 4 rows 6-7.

As for the tilemap, it is 2bpp, but the size must be 4 KB. However, the arrangement for the tilemap is doubled, meaning that in a graphics editor like YY-Chr, you can have eight rows' worth of graphics instead of the usual four.

How to Use Mode 0 in Super Mario World

First of all, please use one of the given codes in a level using UberASM Tool before proceeding:

Stationary Layer 4, by LMPuny

init:
LDA #$51 : STA $2109
LDA #$59 : STA $210A
LDA #$44 : STA $210C
STZ $3E

LDA #$11 : STA $212C : STA $212E
LDA #$0E : STA $212D : STA $212F
    
REP #$20 : LDA $20 : CLC : ADC #$0080 : SEP #$20
STA $2112 : XBA : STA $2112
    
REP #$20 : LDA $24
CLC : ADC #$0070 : SEP #$20
STA $2114 : XBA : STA $2114

RTL

Automatically-Scrolling Layer 4, by LMPuny

!base1 = $0000
!base2 = $0000

if read1($00FFD5) == $23
sa1rom
!base1 = $3000
!base2 = $6000
endif

!FreeRAM = $60

init:
stz !FreeRAM
stz !FreeRAM+1

LDA #$51 : STA $2109
LDA #$59 : STA $210A
LDA #$44 : STA $210C
STZ $3E

LDA #$11 : STA $212C : STA $212E
LDA #$0E : STA $212D : STA $212F
    
REP #$20 : LDA $20 : CLC : ADC #$0080 : SEP #$20
STA $2112 : XBA : STA $2112
    
REP #$20 : LDA $24
CLC : ADC #$0070 : SEP #$20
STA $2114 : XBA : STA $2114
RTL

Camera-Based Scrolling Layer 4, by LMPuny; scrolling code by MolSno from his Layer 4 code; however the init: code is broken in that layer 4 doesn't show up at all

init:

LDA #$51 : STA $2109
LDA #$59 : STA $210A
LDA #$44 : STA $210C
STZ $3E

LDA #$11 : STA $212C : STA $212E
LDA #$0E : STA $212D : STA $212F
    
REP #$20 : LDA $20 : CLC : ADC #$0080 : SEP #$20
STA $2112 : XBA : STA $2112
    
REP #$20 : LDA $24
CLC : ADC #$0070 : SEP #$20
STA $2114 : XBA : STA $2114


main:
; Layer 4 H-Scroll = Layer 3 H-Scroll / 4
REP #$20 : LDA $22 : LSR A : LSR A : SEP #$20
STA $2113 : XBA : STA $2113
	
; Layer 4 V-Scroll = Layer 3 V-Scroll
REP #$20 : LDA $24 : SEP #$20
STA $2114 : XBA : STA $2114
RTL

Bugs so far:

1. Layer 2/3 translucency doesn't work when set in LM.
2. The 2nd layer 4 code doesn't seem to initialize the layer 4 BG's position at all, so you will notice that the position of layer 4 in a section of your level will be carried off from the previous section's. A workaround to this would be swapping their priorities around in LM's Layer 3 Tilemap Editor using the Priority -- tab. More on that later.

Please make a fix to these two bugs and cross them out respectively if possible.