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Update README file.

This commit is contained in:
Kevin O'Connor 2011-10-04 21:12:28 -04:00
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README
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@ -24,24 +24,6 @@ directory for rom images. For example:
qemu -L mybiosdir/ -fda myfdimage.img
The following payloads have been tested:
Freedos - see http://www.freedos.org/ . Useful tests include: booting
from installation cdrom, installing to hard drive and floppy, making
sure hard drive and floppy boots then work. It is also useful to take
the bootable floppy and hard-drive images, write them to an el-torito
bootable cdrom using the Linux mkisofs utility, and then boot those
cdrom images.
Linux - useful hard drive image available from
http://fabrice.bellard.free.fr/qemu/linux-0.2.img.bz2 . It is also
useful to test standard distribution bootup and live cdroms.
NetBSD - useful hard drive image available from
http://nopid.free.fr/small.ffs.bz2 . It is also useful to test
standard distribution installation cdroms.
Overview of files:
The src/ directory contains the bios source code. Several of the
@ -62,20 +44,20 @@ The 16bit code is compiled via gcc to assembler (file out/ccode.16.s).
The gcc "-fwhole-program" and "-ffunction-sections -fdata-sections"
options are used to optimize the process so that gcc can efficiently
compile and discard unneeded code. (In the code, one can use the
macros 'VISIBLE16' and 'VISIBLE32' to instruct a symbol to be
macros 'VISIBLE16' and 'VISIBLE32FLAT' to instruct a symbol to be
outputted in 16bit and 32bit mode respectively.)
This resulting assembler code is pulled into romlayout.S. The gas
option ".code16gcc" is used prior to including the gcc generated
assembler - this option enables gcc to generate valid 16 bit code.
The post code (post.c) is entered, via the function _start(), in 32bit
mode. The 16bit post vector (in romlayout.S) transitions the cpu into
32 bit mode before calling the post.c code.
The post code (post.c) is entered, via the function handle_post(), in
32bit mode. The 16bit post vector (in romlayout.S) transitions the
cpu into 32 bit mode before calling the post.c code.
In the last step of compilation, the 32 bit code is merged into the 16
bit code so that one binary file contains both. Currently, both 16bit
and 32bit code will be located in the 64K block at segment 0xf000.
and 32bit code will be located in the memory at 0xe0000-0xfffff.
GCC 16 bit limitations:
@ -164,9 +146,10 @@ insn). An entry to an ISR handler without args takes 30 bytes (6 + 20
Debugging the bios:
The bios will output information messages to a special debug port.
Under qemu, one can view these messages by enabling the '#define
DEBUG_BIOS' definition in 'qemu/hw/pc.c'. Once this is done (and qemu
is recompiled), one should see status messages on the console.
Under qemu, one can view these messages by adding '-chardev
stdio,id=seabios -device isa-debugcon,iobase=0x402,chardev=seabios' to
the qemu command line. Once this is done, one should see status
messages on the console.
The gdb-server mechanism of qemu is also useful. One can use gdb with
qemu to debug system images. To use this, add '-s -S' to the qemu