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63 lines
2.7 KiB
Plaintext
63 lines
2.7 KiB
Plaintext
Differences between RDOFF versions 1 & 2
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========================================
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This document is designed primarily for people maintaining code which
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uses RDOFF version 1, and would like to upgrade that code to work
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with version 2.
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The main changes are summarised here:
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Overall format
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==============
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The overall format has changed somewhat since version 1, in order
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to make RDOFF more flexible. After the file type identifier (which
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has been changed to 'RDOFF2', obviously), there is now a 4 byte
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integer describing the length of the object module. This allows
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multiple objects to be concatenated, while the loader can easily
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build an index of the locations of each object. This isn't as
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pointless as it sounds; I'm using RDOFF in a microkernel operating
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system, and this is the ideal way of loading multiple driver modules
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at boot time.
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There are also no longer a fixed number of segments; instead there
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is a list of segments, immediately following the header.
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Each segment is preceded by a 10 byte header giving information about
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that segment. This header has the following format:
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Length Description
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2 Type
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2 Number
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2 Reserved
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4 Length
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'Type' is a number describing what sort of segment it is (eg text, data,
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comment, debug info). See 'rdoff2.txt' for a list of the segment types.
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'Number' is the number used to refer to the segment in the header records.
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Not all segments will be loaded; it is only intended that one code
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and one data segment will be loaded into memory. It is possible, however,
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for a loaded segment to contain a reference to an unloaded segment.
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This is an error, and should be flagged at load time. Or maybe you should
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load the segment... its up to you, really.
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The segment's data immediately follows the end of the segment header.
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HEADER RECORDS
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==============
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All of the header records have changed in this version, but not
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substantially. Each record type has had a content-length code added,
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a single byte immediately following the type byte. This contains the
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length of the rest of the record (excluding the type and length bytes,
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but including the terminating nulls on any strings in the record).
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There are two new record types, Segment Relocation (6), and FAR import (7).
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The record formats are identical to Relocation (1) and import (2). They are
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only of real use on systems using segmented architectures. Systems using
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a flat model should treat FAR import (7) exactly the same as an import (2),
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and should either flag segment relocation as an error, or attempt to figure
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out whether it is a reference to a code or data symbol, and set the value
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referenced to the according selector value. I am opting for the former
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approach, and would recommend that others working on 32 bit flat systems
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do the same.
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