Super Mario World 2: Yoshi's Island (SNES) Disassembly

by Raidenthequick and TheGreekBrit

Full Disassembly including:

• 65816 code
• Super FX code
• SPC-700 code
• data sections

Disassembly is 100% finished, assembles under asar and produces a fully clean ROM. Do asar/asar disassembly/assemble.asm yi.sfc to assemble the full ROM.

Not all code is documented, however, so we would welcome any efforts in documentation including naming labels and comments, etc.

Please make sure, before you submit any changes, corrections, or documentation, that the code still assembles correctly and still produces a clean ROM (please run a diff or hash compare with a clean U 1.0 ROM to find out).

Target version: V1.0 (NTSC-US)

Consult the Yoshi's Island Technical Information Wiki, ROM Map, RAM Map, and SRAM Map for technical information about the game that was found using this disassembly.

Naming conventions:

Labels

• Please use all_lowercase_and_underscore_delimiting for label names.
• Maximum label / variable name length is currently 32 characters. This is because of current spacing and addressing modes to take into account.
• Put parent labels at the top of every major piece of code, such as a subroutine, interrupt routine, entity or operation.
• Try to be somewhat brief in naming parent labels.
• Use .sublabels for minor labels within the parent label's code (i.e. before hitting the next parent label).
• Feel free to use ..nested_sublabels for better organization when beneficial.

Example:

init_hookbill:
  ...

.fog_fade
  ...

..ret
  RTS

• For "long" entry points of subroutines (often these just change the data bank to the current bank), create an additional parent label above the major one but add suffix _l.

Example:

main_loop_0F_l:
  PHB
  PHK
  PLB

main_loop_0F:
  ...

• For other entry points within a parent label, use .sublabels as you would for any minor label, and simply call it via label_sublabel from calling code.
• Use .ret for when a piece of code is primarily meant to return, including RTS, RTL, and any stack or other cleanup.
• If there are multiple return points that essentially have differing forms of cleanup, use a distinct suffix for the type of cleanup. However, plain .ret should still be used on the last one regardless.

Example:

.ret_sep
  SEP #$30

.ret
  PLB
  RTL

• Use a parent or sublabel for most data tables. Exception: There are cases when data is found amongst code, like calls to $00BE39 and the like. These take data from the caller's address as parameters, so they are not referenced directly and hence do not need labels; comments should serve nicely.
• ..nested_sublabels should not be used for data.
• Feel free to replace $hex addresses in code with data labels, but please first check if you need to do any label arithmetic to make it correct. Often the tables will be referenced a bit off from where they actually are, due to code like PHA RTS or the nature of $00 sometimes not being a valid index. Example: JSR (item_use_ptr-2,x)
• Prefix conventions:
  • init_ should be applied when a piece of code is meant to set something up and only runs one time for the lifespan of the operation / entity.
  • main_ should be applied when a piece of code is meant to run every frame for a given operation / entity; it's what is driving it.
  • check_ should be applied when a piece of code is primarily meant to check and see whether a certain operation / entity needs to be initialized, updated, or destroyed.
  • gsu_ should be applied for all Super FX routines.
• Suffix conventions:
  • _ptr for pointer tables
  • _state_ptr or .state_ptr specifically for state pointer tables

Variables

• Please use !all_lowercase_and_underscore_delimiting for variable names.

• Prefix conventions:

  • reg_ for SNES hardware registers
  • gsu_ for Super FX registers
  • r_ for RAM, meaning $0000-$1FFF when data bank is $00-$3F, and $7E0000-$7FFFFF
  • s_ for SRAM, meaning $6000-$7FFF when data bank is $00-$3F, and $700000-$71FFFF
  • s_spr_ specifically for sprite tables in SRAM

• Suffix conventions:

  • _l for long (24-bit) addresses

Constants

Use ALL_CAPS_UNDERSCORE for constants, example:

HOOKBILL_HEALTH = #$0003

Bugs

If you come across a bug in the original code, please mark it in the comments with a bug name that begins with BUG_. Then, somewhere above the section or routine it's found in, put the same BUG_xxxxx name and describe it in more detail there. Example:

; BUG_wavy_column_bounds
; this should have checked > 1FC1 because the check
; aims to prevent corruption of anything outside of
; 1FC2-1FE1, but instead it checks >= 1FC1, so it
; DOES corrupt 1FC1, which is part of a different table
.wavy_column_loop
  dec   r11                                 ; $089E50 |\  wavy_column_loop exit condition:
  bmi .wavy_loop_continue                   ; $089E51 | | wavy_width_counter has
  nop                                       ; $089E53 |/  exceeded width
  iwt   r0,#$1FC1                           ; $089E54 |\  BUG_wavy_column_bounds
  cmp   r5                                  ; $089E57 | | screen bounds check for wavy column table
  beq .flag_on_1FC2                         ; $089E59 | | if wavy_column_address <= $1FC1
  nop                                       ; $089E5B | | then don't corrupt non-1FC2~1FE1 memory
  bcs .check_end_of_1FC2                    ; $089E5C | | so ignore flagging on
  nop                                       ; $089E5E |/  else flag on