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diff --git a/rdoff/v1/rdoff.txt b/rdoff/v1/rdoff.txt new file mode 100644 index 00000000..7ee86d66 --- /dev/null +++ b/rdoff/v1/rdoff.txt @@ -0,0 +1,114 @@ +The RDOFF version 1.1 Object File Format +======================================== + +I seem to keep writing this document... I don't know what keeps +happening to it. Anyway, this one will hopefully stay around for a +while. + +RDOFF is a relocatable object file format whose design goals were +mainly to keep it simple, so that an RDOFF object can be loaded and +executed by a very small piece of code (primarily so that it can be +used by the microkernel of an operating system to store system +modules, which can then go on to load and execute more complex object +files, eg ELF, if so desired), yet still be able to be cope with +everything required by the operating system; linkage of multiple +modules together (possibly with automatic loading of new libraries +that are referred to by the object) at load time, allowing static or +dynamic linking as required by the application. + +The overall format of the file is summarised in this table: + +Length (bytes) Description + 6 Contains the string 'RDOFF1' (little-endian targets), + or 'RDOFF' followed by the single byte 0x01 + (big-endian targets). + 4 Length of the header section + ? Header section (see above for length) + 4 Length of code section (.text) + ? Code section + 4 Length of data section (.data) + ? Data section + +Segments are referred to as numbers. Imported labels are implicitly +at offset zero from a segment; each is assigned a segment number when +it is imported. Segments in the object file itself are numbered: + 0 - text segemnt + 1 - data segment + 2 - bss segment + +The header consists of a sequence of records, each of which is +preceded by a byte to represent its type. + +These records are one of the following types: + +1: Relocation Record +-------------------- + + This record points to an address that will need either + relocation or linkage to an external segment when the object + is loaded or linked. + + Length Description + 1 Type identifier (must be 1) + 1 Segment number (0 or 1) plus 64 if the reference is + relative (and thus does not require relocation with + the base of the code, only by the difference between + the start of this segment, and the segment referred to + (see below) + 4 Offset from start of segment of item requiring reloc. + 1 Length of item (1, 2, or 4 bytes...) + 2 Segment number to which reference is made. + +2: Import Symbol Record +----------------------- + + This record defines a segment to start at the location of a + named symbol; this symbol may need to be fetched from an + external library. + + Length Description + 1 Type identifier (must be 2) + 2 Segment number to allocate + ? String containing label (null terminated, max length = + 32 chars) + +3: Export Symbol Record +----------------------- + + This record defines a symbol, to which external modules can + link using the above record type. + + Length Description + 1 Type identifier (must be 3) + 1 Segment containing symbol (0,1 or 2) + 4 Offset of symbol within segment + ? String containing label (null terminated, max length = + 32 chars) + +4: Import Library Record +------------------------ + + This record tells the loader that an extra library should be + loaded and linked to the module at either load- or run-time + (load time is easier, run-time is good, though...) + + Length Description + 1 Type identifier (must be 4) + ? Name of library (null terminated string, max len = 128) + +5: Reserve BSS Bytes +-------------------- + + This record tells the loader how much memory to reserve after + the executable code loaded from the object file for the BSS + segment (referred to as segment number 2). + A loader can safely assume that there will only be one of + these records per module, but the linker probably cannot... + NASM will only output one, but other utilities may be written + that do, and future versions of NASM may output more than one. + + Length Description + 1 Type identifier (must be 5) + 4 Number of bytes to reserve + + |