REL (File Format)

REL files are relocatable object files used by Nintendo. Mario Kart Wii features one such file: StaticR.rel. In general these files allow extra code to be loaded as needed by games, especially when the memory footprint becomes a concern. In Mario Kart, the REL code contains much of the Mario Kart specific logic, such as gameplay, and, once loaded during the health and safety screen, remains loaded for the entire run of the application. Information about specific alteration that can be made to the Mario Kart Wii file can be found here.

File Format

Like many object formats, the file is divided into sections, grouped by like access. For example, all executable code is placed in one section, read only data in another, etc. The file begins with a header, followed by the section info table, followed by the section data, followed by the relocation information.

File Header

The file begins with a header of up to 0x4C bytes:

Offset	Size	Description
0x00	4	id: Arbitrary identification number for game use.
0x04	8	padding? ignored by MKW
0x0c	4	numSections: Number of sections in the file.
0x10	4	sectionInfoOffset: Offset to the start of the section table.
0x14	4	nameOffset: Offset to ASCII string containing name of module. May be NULL. Unclear what it is relative to.
0x18	4	nameSize: Size of the module name in bytes.
0x1c	4	version: Version number of the REL file format.
0x20	4	bssSize: Size of the 'bss' section.
0x24	4	relOffset: Offset to the relocation table.
0x28	4	impOffset: Offset to imp(?).
0x2c	4	impSize: Size of imp(?).
0x30	4	flags of some sort? ignored by MKW
0x34	4	prolog: Offset into first executable section of the _prolog function.
0x38	4	epilog: Offset into first executable section of the _epilog function.
0x3c	4	unresolved: Offset into first executable section of the _unresolved function.
0x40	4	Version ≥ 2 only. align: Alignment constraint on all sections.
0x44	4	Version ≥ 2 only. bssAlign: Alignment constraint on all 'bss' section.
0x48	4	Version ≥ 3 only. fixSize: ?
0x4c or 0x48 or 0x40	End of header

Section Info Table

The section info table comprises of numSections entries each 0x8 bytes long, of the following form:

Offset	Size	Description
0x0	4	offset: Location in file of the section information. The last bit determines if this section is executable or not. If this is zero, the section is an uninitialized section.
0x4	4	length: Length in bytes of the section.
0x8	Next entry

imp

The imp information may be a table. It seems to contain pointers into the relocation information, potentially to allow it to be done in phases. If so, the table contains 0x8 byte entries of the form:

Offset	Size	Description
0x0	4	type: Type of the relocation entries. 1 for internal, 0 for external.
0x4	4	offset: Offset from the beginning of the REL to the relocation data.
0x8	Next entry

Relocation Data

The relocation data seems to be heavily based on the ELF relocation format. It is a list of 0x8 byte structures, and must be parsed in order. Each entry has the form:

Offset	Size	Description
0x0	2	offset: Number of bytes to skip before this relocation.
0x2	1	type: The relocation type. Described below.
0x3	1	section: The section of the symbol to locate.
0x4	4	address: Address of the symbol in the section.
0x8	Next entry

The followng relocation types are known^[1]:

Type	Name	Description
0	R_PPC_NONE	Do nothing. Skip this entry.
1	R_PPC_ADDR32	Write the 32 bit address of the symbol.
2	R_PPC_ADDR24	Write the 24 bit address of the symbol divided by four shifted up 2 bits to the 32 bit value (for relocating branch instructions). Fail if address won't fit.
3	R_PPC_ADDR16	Write the 16 bit address of the symbol. Fail if address more than 16 bits.
4	R_PPC_ADDR16_LO	Write the low 16 bits of the address of the symbol.
5	R_PPC_ADDR16_HI	Write the high 16 bits of the address of the symbol.
6	R_PPC_ADDR16_HA	Write the high 16 bits of the address of the symbol plus 0x8000.
7 - 9	R_PPC_ADDR14	Write the 14 bits of the address of the symbol divided by four shifted up 2 bits to the 32 bit value (for relocating conditional branch instructions). Fail if address won't fit.
10	R_PPC_REL24	Write the 24 bit address of the symbol minus the address of the relocation divided by four shifted up 2 bits to the 32 bit value (for relocating relative branch instructions). Fail if address won't fit.
11 - 13	R_PPC_REL14	Write the 14 bit address of the symbol minus the address of the relocation divided by four shifted up 2 bits to the 32 bit value (for relocating conditional relative branch instructions). Fail if address won't fit.
201	R_RVL_NONE	Do nothing. Skip this entry. Carry the address of the symbol to the next entry.
202	R_RVL_SECT	Change which section relocations are being applied to. Set the offset into the section to 0.
203	R_RVL_STOP	Stop parsing the relocation table.

The loader walks this table obeying each command until it encounters R_RVL_STOP. In the case of internal relocations, the symbol address is calculated by taking the load address of the section section and adding the offset address. For external relocation, the symbol address is just address. The purpose of section in such instances is unknown.

Tools

The following tools can handle REL files:

• BrawlBox, by Kryal & BlackJax
• wstrt (see Wiimms SZS Tools) is able to patch data in the Mario Kart Wii file StaticR.rel.
• StaticR.rel Tool, by Atlas, can edit the region number, plus HUD, region line, Mii suit and layout buttons colors from the same file.