REL (File Format)

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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:

REL file header
Offset Size Field Name Description
0x00 4 id Arbitrary identification number. Must be unique amongst all RELs used by a game.
0x04 4 Next module Pointer to next module, filled at runtime.
0x08 4 Previous module Pointer to previous module, filled at runtime.
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 table.
0x2c 4 impSize Size of imp table in bytes.
0x30 1 prologSection Index into section table which prolog is relative to. Skip if this field is 0.
0x31 1 epilogSection Index into section table which epilog is relative to. Skip if this field is 0.
0x32 1 unresolvedSection Index into section table which unresolved is relative to. Skip if this field is 0.
0x33 1 bssSection Index into section table which bss is relative to. Filled at runtime!
0x34 4 prolog Offset into specified section of the _prolog function.
0x38 4 epilog Offset into specified section of the _epilog function.
0x3c 4 unresolved Offset into specified section of the _unresolved function.
0x40 4 align Version ≥ 2 only. Alignment constraint on all sections, expressed as power of 2.
0x44 4 bssAlign Version ≥ 2 only. Alignment constraint on all '.bss' section, expressed as power of 2.
0x48 4 fixSize Version ≥ 3 only. Unknown.
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:

Section table entry
Offset Size Description
0x0 30 bits Offset from the beginning of the REL to the section. If this is zero, the section is an uninitialized section (i.e. .bss).
0x3.6 1 bit Unknown.
0x3.7 1 bit Executable flag; if this is 1 the section is executable.
0x4 4 Length in bytes of the section. If this is zero, this entry is skipped.
0x8 Next entry

imp

The imp acts as a directory for the relocation information. It controls which entries in the relocation table refer to. Each entry has the form:

imp table entry
Offset Size Description
0x0 4 Module number (id) that these relocations refer to. 0 means these refer to main.dol instead.
0x4 4 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 one or more lists of 0x8 byte structures, and must be parsed in order. The imp table gives the start of each list. The end of each list is marked by the special type code 203. Each entry has the form:

Relocation table entry
Offset Size Description
0x0 2 Offset in bytes from the previous relocation to this one. If this is the first relocation in the section, this is relative to the section start.
0x2 1 The relocation type. Described below.
0x3 1 The section of the symbol to relocate against. For the special relocation type 202, this is instead the number of the section in this file which the following relocation entries apply to.
0x4 4 Offset in bytes of the symbol to relocate against, relative to the start of its section. This is an absolute address instead for relocations against main.dol.
0x8 Next entry

The following relocation types are known[1]:

Relocation entry type
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.
14 Unknown Undefined, handled like 203 R_RVL_STOP by the Nintendo SDK.
201 R_RVL_NONE
R_DOLPHIN_NOP
Do nothing. Skip this entry. Carry the address of the symbol to the next entry.
202 R_RVL_SECT
R_DOLPHIN_SECTION
Change which section relocations are being applied to. Set the offset into the section to 0.
203 R_RVL_STOP
R_DOLPHIN_END
Stop parsing the relocation table.
204 R_DOLPHIN_MRKREF Unknown.

The loader walks this table obeying each command until it encounters R_RVL_STOP. When relocation against a REL file (including against this file itself), the symbol address is calculated by taking the load address of the section section and adding the offset address. For relocation again main.dol, the symbol address is just address with no adjustment.

Tools

The following tools can handle REL files: