compiled using standard gnu tools (eg, gas and gcc).
To build, one should be able to run "make" in the main directory. The
-resulting file "out/rom.bin" contains the processed bios image.
-
-The build requires gcc v4.1 or later. Some buggy versions of gcc have
-issues with the '-combine' compiler option - in particular, recent
-versions of Ubuntu are affected. One can use "make AVOIDCOMBINE=1" to
-get around this.
+resulting file "out/bios.bin" contains the processed bios image.
Testing of images:
To test the bios under bochs, one will need to instruct bochs to use
the new bios image. Use the 'romimage' option - for example:
-bochs -q 'floppya: 1_44=myfdimage.img' 'romimage: file=out/rom.bin'
+bochs -q 'floppya: 1_44=myfdimage.img' 'romimage: file=out/bios.bin'
To test under qemu, one will need to create a directory with all the
bios images and then overwrite the main bios image. For example:
cp /usr/share/qemu/*.bin mybiosdir/
-cp out/rom.bin mybiosdir/bios.bin
+cp out/bios.bin mybiosdir/
Once this is setup, one can instruct qemu to use the newly created
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
files are compiled twice - once for 16bit mode and once for 32bit
-mode. (The gcc compile option '-fwhole-program' is used to remove
-code that is not needed for a particular mode.)
+mode. (The build system will remove code that is not needed for a
+particular mode.)
The tools/ directory contains helper utilities for manipulating and
building the final rom.
Build overview:
-The 16bit code is compiled via gcc to assembler (file out/blob.16.s).
-The gcc "-fwhole-program" option is 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 outputted in 16bit and 32bit mode respectively.)
+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 '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 romlayout.S also defines all the mandatory bios visible memory
-locations.
-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:
below). This is due to the 16bit segment nature of the X86 cpu when
it is in "real mode". The C entry code will set DS and SS to point to
the stack segment. Variables not on the stack need to be accessed via
-an explicit segment register. Global constants (loaded into 0xf000)
-can be accessed via the CS segment register. Any other access
-requires altering one of the other segment registers (usually ES) and
-then accessing the variable via that segment register.
+an explicit segment register. Any other access requires altering one
+of the other segment registers (usually ES) and then accessing the
+variable via that segment register.
There are three low-level ways to access a remote variable:
-GET/SET_VAR, GET/SET_FARVAR, and GET/SET_FARPTR. The first set takes
+GET/SET_VAR, GET/SET_FARVAR, and GET/SET_FLATPTR. The first set takes
an explicit segment descriptor (eg, "CS") and offset. The second set
-will take a segment id and offset, set ES to the segment, and then
+will take a segment id and offset, set ES to the segment id, and then
make the access via the ES segment. The last method is similar to the
-second, except it takes a pointer that would be valid in 32-bit mode
-instead of a segment/offset pair.
-
-Most BIOS variables are stored in the "BDA" or "EBDA" memory areas.
-Because this is common, two sets of helper macros (GET/SET_BDA and
-GET/SET_EBDA) are available to simplify these accesses.
+second, except it takes a pointer that would be valid in 32-bit flat
+mode instead of a segment/offset pair.
+
+Most BIOS variables are stored in global variables, the "BDA", or
+"EBDA" memory areas. Because this is common, three sets of helper
+macros (GET/SET_GLOBAL, GET/SET_BDA, and GET/SET_EBDA) are available
+to simplify these accesses.
+
+Global variables defined in the C code can be read in 16bit mode if
+the variable declaration is marked with VAR16, VAR16VISIBLE,
+VAR16EXPORT, or VAR16FIXED. The GET_GLOBAL macro will then allow read
+access to the variable. Global variables are stored in the 0xf000
+segment, and their values are persistent across soft resets. Because
+the f-segment is marked read-only during run-time, the 16bit code is
+not permitted to change the value of 16bit variables (use of the
+SET_GLOBAL macro from 16bit mode will cause a link error). Code
+running in 32bit mode can not access variables with VAR16, but can
+access variables marked with VAR16VISIBLE, VAR16EXPORT, VAR16FIXED, or
+with no marking at all. The 32bit code can use the GET/SET_GLOBAL
+macros, but they are not required.
GCC 16 bit stack limitations:
observed to call into the bios with less than 150 bytes available.
Note that the post code and boot code (irq 18/19) do not have a stack
-limitation because the entry points for these functions reset the
-stack to a known state. Only the general purpose 16-bit service entry
-points are affected.
+limitation because the entry points for these functions transition the
+cpu to 32bit mode and reset the stack to a known state. Only the
+general purpose 16-bit service entry points are affected.
There are some ways to reduce stack usage: making sure functions are
tail-recursive often helps, reducing the number of parameters passed
to functions often helps, sometimes reordering variable declarations
helps, inlining of functions can sometimes help, and passing of packed
-structures can also help.
+structures can also help. It is also possible to transition to/from
+an extra stack stored in the EBDA using the stack_hop helper function.
Some useful stats: the overhead for the entry to a bios handler that
-takes a 'struct bregs' is 38 bytes of stack space (6 bytes from
-interrupt insn, 28 bytes to store registers, and 4 bytes for call
+takes a 'struct bregs' is 42 bytes of stack space (6 bytes from
+interrupt insn, 32 bytes to store registers, and 4 bytes for call
insn). An entry to an ISR handler without args takes 30 bytes (6 + 20
+ 4).
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
projects / seabios.git / blobdiff