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Kevin O'Connorf076a3e2008-02-25 22:25:15 -05001This code implements an X86 legacy bios. It is intended to be
2compiled using standard gnu tools (eg, gas and gcc).
4To build, one should be able to run "make" in the main directory. The
Kevin O'Connor59fead62008-05-10 15:49:20 -04005resulting file "out/bios.bin" contains the processed bios image.
Kevin O'Connorf076a3e2008-02-25 22:25:15 -05006
Kevin O'Connor838f08f2008-03-30 11:07:42 -04007
8Testing of images:
10To test the bios under bochs, one will need to instruct bochs to use
11the new bios image. Use the 'romimage' option - for example:
Kevin O'Connor59fead62008-05-10 15:49:20 -040013bochs -q 'floppya: 1_44=myfdimage.img' 'romimage: file=out/bios.bin'
Kevin O'Connor838f08f2008-03-30 11:07:42 -040014
15To test under qemu, one will need to create a directory with all the
16bios images and then overwrite the main bios image. For example:
18cp /usr/share/qemu/*.bin mybiosdir/
Kevin O'Connor59fead62008-05-10 15:49:20 -040019cp out/bios.bin mybiosdir/
Kevin O'Connorac467be2013-03-17 10:29:06 -040020cp out/*.aml mybiosdir/
Kevin O'Connor838f08f2008-03-30 11:07:42 -040021
22Once this is setup, one can instruct qemu to use the newly created
23directory for rom images. For example:
25qemu -L mybiosdir/ -fda myfdimage.img
Kevin O'Connorf076a3e2008-02-25 22:25:15 -050028Overview of files:
Kevin O'Connor838f08f2008-03-30 11:07:42 -040030The src/ directory contains the bios source code. Several of the
31files are compiled twice - once for 16bit mode and once for 32bit
Kevin O'Connor0942e7f2009-06-15 22:27:01 -040032mode. (The build system will remove code that is not needed for a
33particular mode.)
Kevin O'Connorf076a3e2008-02-25 22:25:15 -050034
Kevin O'Connor1c787112013-09-13 16:29:11 -040035The vgasrc/ directory contains code for VGA BIOS implementations.
36This code is separate from the main BIOS code in the src/ directory.
37It produces a VGA BIOS rom in out/vgabios.bin. The VGA BIOS code is
38always compiled in 16bit mode.
40The scripts/ directory contains helper utilities for manipulating and
Kevin O'Connorf076a3e2008-02-25 22:25:15 -050041building the final rom.
43The out/ directory is created by the build process - it contains all
44temporary and final files.
47Build overview:
Kevin O'Connor0afee522009-02-05 20:32:41 -050049The 16bit code is compiled via gcc to assembler (file out/ccode.16.s).
Kevin O'Connor0942e7f2009-06-15 22:27:01 -040050The gcc "-fwhole-program" and "-ffunction-sections -fdata-sections"
51options are used to optimize the process so that gcc can efficiently
52compile and discard unneeded code. (In the code, one can use the
Kevin O'Connor0fdf1932011-10-04 21:12:28 -040053macros 'VISIBLE16' and 'VISIBLE32FLAT' to instruct a symbol to be
Kevin O'Connor0942e7f2009-06-15 22:27:01 -040054outputted in 16bit and 32bit mode respectively.)
Kevin O'Connorf076a3e2008-02-25 22:25:15 -050055
56This resulting assembler code is pulled into romlayout.S. The gas
57option ".code16gcc" is used prior to including the gcc generated
Kevin O'Connor838f08f2008-03-30 11:07:42 -040058assembler - this option enables gcc to generate valid 16 bit code.
Kevin O'Connorf076a3e2008-02-25 22:25:15 -050059
Kevin O'Connor0fdf1932011-10-04 21:12:28 -040060The post code (post.c) is entered, via the function handle_post(), in
6132bit mode. The 16bit post vector (in romlayout.S) transitions the
62cpu into 32 bit mode before calling the post.c code.
Kevin O'Connorf076a3e2008-02-25 22:25:15 -050063
Kevin O'Connor838f08f2008-03-30 11:07:42 -040064In the last step of compilation, the 32 bit code is merged into the 16
65bit code so that one binary file contains both. Currently, both 16bit
Kevin O'Connor0fdf1932011-10-04 21:12:28 -040066and 32bit code will be located in the memory at 0xe0000-0xfffff.
Kevin O'Connorf076a3e2008-02-25 22:25:15 -050067
69GCC 16 bit limitations:
71Although the 16bit code is compiled with gcc, developers need to be
72aware of the environment. In particular, global variables _must_ be
73treated specially.
75The code has full access to stack variables and general purpose
76registers. The entry code in romlayout.S will push the original
77registers on the stack before calling the C code and then pop them off
78(including any required changes) before returning from the interrupt.
79Changes to CS, DS, and ES segment registers in C code is also safe.
80Changes to other segment registers (SS, FS, GS) need to be restored
83Stack variables (and pointers to stack variables) work as they
84normally do in standard C code.
86However, variables stored outside the stack need to be accessed via
Kevin O'Connor838f08f2008-03-30 11:07:42 -040087the GET_VAR and SET_VAR macros (or one of the helper macros described
88below). This is due to the 16bit segment nature of the X86 cpu when
89it is in "real mode". The C entry code will set DS and SS to point to
90the stack segment. Variables not on the stack need to be accessed via
Kevin O'Connor0afee522009-02-05 20:32:41 -050091an explicit segment register. Any other access requires altering one
92of the other segment registers (usually ES) and then accessing the
93variable via that segment register.
Kevin O'Connor838f08f2008-03-30 11:07:42 -040094
95There are three low-level ways to access a remote variable:
Kevin O'Connor0afee522009-02-05 20:32:41 -050096GET/SET_VAR, GET/SET_FARVAR, and GET/SET_FLATPTR. The first set takes
Kevin O'Connor838f08f2008-03-30 11:07:42 -040097an explicit segment descriptor (eg, "CS") and offset. The second set
Kevin O'Connor0afee522009-02-05 20:32:41 -050098will take a segment id and offset, set ES to the segment id, and then
Kevin O'Connor838f08f2008-03-30 11:07:42 -040099make the access via the ES segment. The last method is similar to the
Kevin O'Connor0afee522009-02-05 20:32:41 -0500100second, except it takes a pointer that would be valid in 32-bit flat
101mode instead of a segment/offset pair.
Kevin O'Connor838f08f2008-03-30 11:07:42 -0400102
Kevin O'Connor0afee522009-02-05 20:32:41 -0500103Most BIOS variables are stored in global variables, the "BDA", or
104"EBDA" memory areas. Because this is common, three sets of helper
105macros (GET/SET_GLOBAL, GET/SET_BDA, and GET/SET_EBDA) are available
Kevin O'Connor14b255b2013-07-14 14:40:19 -0400106to simplify these accesses. Also, an area in the 0xc0000-0xf0000
107memory range is made available for internal BIOS run-time variables
Kevin O'Connor490797e2013-09-14 12:38:56 -0400108that are marked with the VARLOW attribute. These variables can then
Kevin O'Connor14b255b2013-07-14 14:40:19 -0400109be accessed with the GET/SET_LOW macros.
Kevin O'Connor0afee522009-02-05 20:32:41 -0500110
111Global variables defined in the C code can be read in 16bit mode if
Kevin O'Connor14b255b2013-07-14 14:40:19 -0400112the variable declaration is marked with VAR16, VARFSEG, or VAR16FIXED.
113The GET_GLOBAL macro will then allow read access to the variable.
114Global variables are stored in the 0xf000 segment. Because the
115f-segment is marked read-only during run-time, the 16bit code is not
116permitted to change the value of 16bit variables (use of the
117SET_GLOBAL macro from 16bit mode will cause a link error). Code
118running in 32bit mode can not access variables with VAR16, but can
119access variables marked with VARFSEG, VARLOW, VAR16FIXED, or with no
120marking at all. The 32bit code can use the GET/SET_GLOBAL macros, but
121they are not required.
Kevin O'Connor838f08f2008-03-30 11:07:42 -0400122
124GCC 16 bit stack limitations:
126Another limitation of gcc is its use of 32-bit temporaries. Gcc will
127allocate 32-bits of space for every variable - even if that variable
128is only defined as a 'u8' or 'u16'. If one is not careful, using too
129much stack space can break old DOS applications.
131There does not appear to be explicit documentation on the minimum
132stack space available for bios calls. However, Freedos has been
Kevin O'Connor0bb2a442008-04-01 21:09:05 -0400133observed to call into the bios with less than 150 bytes available.
Kevin O'Connor838f08f2008-03-30 11:07:42 -0400134
135Note that the post code and boot code (irq 18/19) do not have a stack
Kevin O'Connor0afee522009-02-05 20:32:41 -0500136limitation because the entry points for these functions transition the
137cpu to 32bit mode and reset the stack to a known state. Only the
138general purpose 16-bit service entry points are affected.
Kevin O'Connor838f08f2008-03-30 11:07:42 -0400139
140There are some ways to reduce stack usage: making sure functions are
141tail-recursive often helps, reducing the number of parameters passed
142to functions often helps, sometimes reordering variable declarations
143helps, inlining of functions can sometimes help, and passing of packed
Kevin O'Connor0afee522009-02-05 20:32:41 -0500144structures can also help. It is also possible to transition to/from
145an extra stack stored in the EBDA using the stack_hop helper function.
Kevin O'Connor838f08f2008-03-30 11:07:42 -0400146
Kevin O'Connor0bb2a442008-04-01 21:09:05 -0400147Some useful stats: the overhead for the entry to a bios handler that
Kevin O'Connor0942e7f2009-06-15 22:27:01 -0400148takes a 'struct bregs' is 42 bytes of stack space (6 bytes from
149interrupt insn, 32 bytes to store registers, and 4 bytes for call
Kevin O'Connor0bb2a442008-04-01 21:09:05 -0400150insn). An entry to an ISR handler without args takes 30 bytes (6 + 20
151+ 4).
Kevin O'Connor838f08f2008-03-30 11:07:42 -0400153
154Debugging the bios:
156The bios will output information messages to a special debug port.
Kevin O'Connor0fdf1932011-10-04 21:12:28 -0400157Under qemu, one can view these messages by adding '-chardev
158stdio,id=seabios -device isa-debugcon,iobase=0x402,chardev=seabios' to
159the qemu command line. Once this is done, one should see status
160messages on the console.
Kevin O'Connor838f08f2008-03-30 11:07:42 -0400161
162The gdb-server mechanism of qemu is also useful. One can use gdb with
163qemu to debug system images. To use this, add '-s -S' to the qemu
164command line. For example:
166qemu -L mybiosdir/ -fda myfdimage.img -s -S
168Then, in another session, run gdb with either out/rom16.o (to debug
169bios 16bit code) or out/rom32.o (to debug bios 32bit code). For
172gdb out/rom16.o
174Once in gdb, use the command "target remote localhost:1234" to have
175gdb connect to qemu. See the qemu documentation for more information
176on using gdb and qemu in this mode. Note that gdb seems to get
177breakpoints confused when the cpu is in 16-bit real mode. This makes
178stepping through the program difficult (though 'step instruction'
179still works). Also, one may need to set 16bit break points at both
180the cpu address and memory address (eg, break *0x1234 ; break