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For 16-bit and 32-bit x86 code, the size and realsize() always matches as only jumps, calls and loops uses PC relative addressing and the address isn't followed by any other opcode bytes. In 64-bit mode there is RIP relative addressing which means the fixup location can be followed by an immediate value, meaning that size > realsize(). When the CPU is calculating the effective address, it takes the RIP at the end of the instruction and adds the fixed up relative address value to it. The linker's point of reference is the end of the fixup location (which is the end of the instruction for Jcc, CALL, LOOP[cc]). It is calculating distance between the target symbol and the end of the fixup location, and add this to the displacement value we are calculating here and storing at the fixup location. To get the right effect, we need to _reduce_ the displacement value by the number of bytes following the fixup. Example: data at address 0x100; REL4ADR at 0x050, 4 byte immediate, end of fixup at 0x054, end of instruction at 0x058. => size = 8. => realsize() -> 4 => CPU needs a value of: 0x100 - 0x058 = 0x0a8 => linker/loader will add: 0x100 - 0x054 = 0x0ac => We must add an addend of -4. => realsize() - size = -4. The code used to do size - realsize() at least since v0.90, probably because it wasn't needed... Signed-off-by: H. Peter Anvin <hpa@zytor.com>
NASM, the Netwide Assembler.
Many many developers all over the net respect NASM for what it is
- a widespread (thus netwide), portable (thus netwide!), very
flexible and mature assembler tool with support for many output
formats (thus netwide!!).
Now we have good news for you: NASM is licensed under the "simplified"
(2-clause) BSD license. This means its development is open to even
wider society of programmers wishing to improve their lovely
assembler.
The NASM project is now situated at SourceForge.net, the most
popular Open Source development site on the Internet.
Visit our website at http://nasm.sourceforge.net/ and our
SourceForge project at http://sourceforge.net/projects/nasm/
See the file CHANGES for the description of changes between revisions,
and the file AUTHORS for a list of contributors.
With best regards,
NASM crew.