Date: Thu, 28 Sep 2006 19:38:02 +0000 (UTC) From: John Baldwin <jhb@FreeBSD.org> To: src-committers@FreeBSD.org, cvs-src@FreeBSD.org, cvs-all@FreeBSD.org Subject: cvs commit: src/sys/boot/i386/btx/btx btx.S Message-ID: <200609281938.k8SJc238038372@repoman.freebsd.org>
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jhb 2006-09-28 19:38:02 UTC
FreeBSD src repository
Modified files:
sys/boot/i386/btx/btx btx.S
Log:
Optimize the int 15/87 handler for space to shave another 16 bytes off of
BTX (and thus boot2):
- Don't bother saving %eax, %ebx, or %ecx as it is not necessary.
- Use a more compact sequence to load the base value out of a GDT entry
by loading the contiguous low 24 bits into the upper 24 bits of %eax,
loading the high 8 bits into %al, and using a ror to rotate the bits
(2 mov's and a ror) rather than loading the pieces in smaller chunks
(3 mov's and a shl).
- Use movzwl + leal instead of movl + movw + shll + addl.
- Use 'xchgl %eax,%foo' rather than 'movl %eax,%foo' for cases where
it's ok to trash %eax. xchgl %eax, foo is a 1-byte opcode whereas the
mov is a 2-byte opcode.
- Use movzwl rather than xorl + movw.
MFC after: 1 week
Revision Changes Path
1.41 +17 -31 src/sys/boot/i386/btx/btx/btx.S
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