RGBDS(5)

NAME
rgbds - object file format documentation

DESCRIPTION
This is the description of the object files used by rgbasm(1) and rgblink(1). Please note that the specification is not stable yet. RGBDS is still in active development, and some new features require adding more information to the object file, or modifying some fields, both of which break compatibility with older versions.

FILE STRUCTURE
The following types are used:

LONG is a 32-bit integer stored in little-endian format. BYTE is an 8-bit integer. STRING is a 0-terminated string of BYTE. Brackets after a type (e.g. LONG[n]) indicate n consecutive elements (here, LONGs). All items are contiguous, with no padding anywhere—this also means that they may not be aligned in the file!

REPT n indicates that the fields between the REPT and corresponding ENDR are repeated n times.

All IDs refer to objects within the file; for example, symbol ID $0001 refers to the second symbol defined in this object file's Symbols array. The only exception is the Source file info nodes, whose IDs are backwards, i.e. source node ID $0000 refers to the last node in the array, not the first one. References to other object files are made by imports (symbols), by name (sections), etc.—but never by ID.

  Header
BYTE Magic[4]
    "RGB9"
LONG RevisionNumber
    The format's revision number this file uses. (This is always in the same place in all revisions.)
LONG NumberOfSymbols
    How many symbols are defined in this object file.
LONG NumberOfSections
    How many sections are defined in this object file.

  Source file info
LONG NumberOfNodes
    The number of source context nodes contained in this file.
REPT NumberOfNodes
    LONG ParentID
        ID of the parent node, -1 meaning that this is the root node.

        Important: the nodes are actually written in reverse order, meaning the node with ID 0 is the last one in the list!
    LONG ParentLineNo
        Line at which the parent node's context was exited; meaningless for the root node.
    BYTE Type
        Value | Meaning
        0 | REPT node
        1 | File node
        2 | Macro node
    IF Type ≠ 0
        If the node is not a REPT node...

        STRING Name
            The node's name: either a file name, or the macro's name prefixes by its definition's file name (e.g. ‘src/includes/defines.asm::error’).
    ELSE
        If the node is a REPT, it also contains the iteration counter of all parent REPTs.

        LONG Depth
        LONG Iter[Depth]
            The number of REPT iterations, by increasing depth.
    ENDC
ENDR

  Symbols
REPT NumberOfSymbols
    STRING Name
        This symbol's name. Local symbols are stored as their full name (‘Scope.symbol’).
    BYTE Type
        Value | Meaning
        0 | Local symbol only used in this file.
        1 | Import of an exported symbol (by name) from another object file.
        2 | Exported symbol visible from other object files.
    IF Type ≠ 1
        If the symbol is defined in this object file...

        LONG NodeID
            Context in which the symbol was defined.
        LONG LineNo
            Line number in the context at which the symbol was defined.
        LONG SectionID
            The ID of the section in which the symbol is defined. If the symbol doesn't belong to any specific section (i.e. it's a constant), this field contains -1.
        LONG Value
            The symbol's value. If the symbol belongs to a section, this is the offset within that symbol's section.
    ENDC
ENDR

  Sections
REPT NumberOfSections
    STRING Name
        The section's name.
    LONG Size
        The section's size, in bytes.
    BYTE Type
        Bits 0–2 indicate the section's type:
        Value | Meaning
        0 | WRAM0
        1 | VRAM
        2 | ROMX
        3 | ROM0
        4 | HRAM
        5 | WRAMX
        6 | SRAM
        7 | OAM

        Bit 7 being set means that the section is a "union" (see “Unionized sections” in rgbasm(5)). Bit 6 being set means that the section is a "fragment" (see “Section fragments” in rgbasm(5)). These two bits are mutually exclusive.
    LONG Address
        Address this section must be placed at. This must either be valid for the section's Type (as affected by flags like -t or -d in rgblink(1)), or -1 to indicate that the linker should automatically decide (the section is “floating”).
    LONG Bank
        ID of the bank this section must be placed in. This must either be valid for the section's Type (with the same caveats as for the Address), or -1 to indicate that the linker should automatically decide.
    BYTE Alignment
        How many bits of the section's address should be equal to AlignOfs, starting from the least-significant bit.
    LONG AlignOfs
        Alignment offset. Must be strictly less than ‘1 << Alignment’.
    IF Type 2 || Type 3
        If the section has ROM type, it contains data.

        BYTE Data[Size]
            The section's raw data. Bytes that will be patched over must be present, even though their contents will be overwritten.
        LONG NumberOfPatches
            How many patches must be applied to this section's Data.
        REPT NumberOfPatches
            LONG NodeID
                Context in which the patch was defined.
            LONG LineNo
                Line number in the context at which the patch was defined.
            LONG Offset
                Offset within the section's Data at which the patch should be applied. Must not be greater than the section's Size minus the patch's size (see Type below).
            LONG PCSectionID
                ID of the section in which PC is located. (This is usually the same section within which the patch is applied, except for e.g. ‘LOAD’ blocks, see “RAM code” in rgbasm(5).)
            LONG PCOffset
                Offset of the PC symbol within the section designated by PCSectionID. It is expected that PC points to the instruction's first byte for instruction operands (i.e. ‘jp @’ must be an infinite loop), and to the patch's first byte otherwise ‘(db’, ‘dw’, ‘dl’).
            BYTE Type
                Value | Meaning
                0 | Single-byte patch
                1 | Little-endian two-byte patch
                2 | Little-endian four-byte patch
                3 | Single-byte ‘jr’
                patch; the patch's value will be subtracted to PC + 2 (i.e. ‘jr @’ must be the infinite loop ‘18 FE’).
            LONG RPNSize
                Size of the RPNExpr below.
            BYTE RPNExpr[RPNSize]
                The patch's value, encoded as a RPN expression (see RPN EXPRESSIONS).
        ENDR
    ENDC

  Assertions
LONG NumberOfAssertions
    How many assertions this object file contains.
REPT NumberOfAssertions
    Assertions are essentially patches with a message.

    LONG NodeID
        Context in which the assertions was defined.
    LONG LineNo
        Line number in the context at which the assertion was defined.
    LONG Offset
        Unused leftover from the patch structure.
    LONG PCSectionID
        ID of the section in which PC is located.
    LONG PCOffset
        Offset of the PC symbol within the section designated by PCSectionID.
    BYTE Type
        Describes what should happen if the expression evaluates to a non-zero value.
        Value | Meaning
        0 | Print a warning message, and continue linking normally.
        1 | Print an error message, so linking will fail, but allow other assertions to be evaluated.
        2 | Print a fatal error message, and abort immediately.
    LONG RPNSize
        Size of the RPNExpr below.
    BYTE RPNExpr[RPNSize]
        The patch's value, encoded as a RPN expression (see RPN EXPRESSIONS).
    STRING Message
        The message displayed if the expression evaluates to a non-zero value. If empty, a generic message is displayed instead.
ENDR

  RPN EXPRESSIONS
Expressions in the object file are stored as RPN, or “Reverse Polish Notation”, which is a notation that allows computing arbitrary expressions with just a simple stack. For example, the expression ‘2 5 -’ will first push the value “2” to the stack, then “5”. The ‘-’ operator pops two arguments from the stack, subtracts them, and then pushes back the result (“3”) on the stack. A well-formed RPN expression never tries to pop from an empty stack, and leaves exactly one value in it at the end.

RGBDS encodes RPN expressions as an array of BYTEs. The first byte encodes either an operator, or a literal, which consumes more BYTEs after it.
Value | Meaning
$00 | Addition operator (‘+’)
$01 | Subtraction operator (‘-’)
$02 | Multiplication operator (‘*’)
$03 | Division operator (‘/’)
$04 | Modulo operator (‘%’)
$05 | Negation (unary ‘-’)
$06 | Exponent operator (‘**’)
$10 | Bitwise OR operator (‘|’)
$11 | Bitwise AND operator (‘&’)
$12 | Bitwise XOR operator (‘^’)
$13 | Bitwise complement operator (unary ‘~’)
$21 | Logical AND operator (‘&&’)
$22 | Logical OR operator (‘||’)
$23 | Logical complement operator (unary ‘’)!
$30 | Equality operator (‘==’)
$31 | Non-equality operator (‘!=’)
$32 | Greater-than operator (‘>’)
$33 | Less-than operator (‘<’)
$34 | Greater-than-or-equal operator (‘>=’)
$35 | Less-than-or-equal operator (‘<=’)
$40 | Left shift operator (‘<<’)
$41 | Arithmetic/signed right shift operator (‘>>’)
$42 | Logical/unsigned right shift operator (‘>>>’)
$50 | BANK(symbol),
followed by the symbol's LONG ID.
$51 | BANK(section),
followed by the section's STRING name.
$52 | PC's BANK() (i.e. ‘BANK(@)’).
$53 | SIZEOF(section),
followed by the section's STRING name.
$54 | STARTOF(section),
followed by the section's STRING name.
$60 | ‘ldh’
check. Checks if the value is a valid ‘ldh’ operand (see “Load Instructions” in gbz80(7)), i.e. that it is between either $00 and $FF, or $FF00 and $FFFF, both inclusive. The value is then ANDed with $00FF (‘& $FF’).
$61 | ‘rst’
check. Checks if the value is a valid ‘rst’ (see “RST vec” in gbz80(7)) vector, that is one of $00, $08, $10, $18, $20, $28, $30, or $38. The value is then ORed with $C7 (‘| $C7’).
$80 | Integer literal.
Followed by the LONG integer.
$81 | A symbol's value.
Followed by the symbol's LONG ID.

SEE ALSO
rgbasm(1), rgblink(1), rgbds(7), gbz80(7)

HISTORY
rgbds was originally written by Carsten Sørensen as part of the ASMotor package, and was later packaged in RGBDS by Justin Lloyd. It is now maintained by a number of contributors at https://github.com/gbdev/rgbds.