The README file and README.CSM file have gotten a bit out of date. Instead of maintaining technical information in the README file, point new users to the SeaBIOS wiki.
Signed-off-by: Kevin O'Connor kevin@koconnor.net --- README | 187 ++++--------------------------------------------------------- README.CSM | 22 -------- 2 files changed, 11 insertions(+), 198 deletions(-) delete mode 100644 README.CSM
diff --git a/README b/README index 1e61e2f..e18be16 100644 --- a/README +++ b/README @@ -1,182 +1,17 @@ -This code implements an X86 legacy bios. It is intended to be -compiled using standard gnu tools (eg, gas and gcc). +Welcome to the SeaBIOS project! This project implements an X86 legacy +bios that is built with standard GNU tools.
-To build for QEMU, one should be able to run "make" in the main -directory. The resulting file "out/bios.bin" contains the processed -bios image. To build for coreboot, please see the coreboot wiki. To -build for CSM, please see README.CSM. +Please see build and developer information at:
+ http://seabios.org/Developer_Documentation
-Testing of images: +For the impatient, SeaBIOS is built for QEMU and tested on QEMU with:
-To test the bios under bochs, one will need to instruct bochs to use -the new bios image. Use the 'romimage' option - for example: + make + qemu -bios out/bios.bin
-bochs -q 'floppya: 1_44=myfdimage.img' 'romimage: file=out/bios.bin' +SeaBIOS can be configured with kconfig. To change the default +configuration one can run "make menuconfig" prior to running "make".
-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/bios.bin mybiosdir/ -cp out/*.aml 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 - - -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 build system will remove code that is not needed for a -particular mode.) - -The vgasrc/ directory contains code for VGA BIOS implementations. -This code is separate from the main BIOS code in the src/ directory. -It produces a VGA BIOS rom in out/vgabios.bin. The VGA BIOS code is -always compiled in 16bit mode. - -The scripts/ directory contains helper utilities for manipulating and -building the final rom. - -The out/ directory is created by the build process - it contains all -temporary and final files. - - -Build overview: - -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 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 memory at 0xe0000-0xfffff. - - -GCC 16 bit limitations: - -Although the 16bit code is compiled with gcc, developers need to be -aware of the environment. In particular, global variables _must_ be -treated specially. - -The code has full access to stack variables and general purpose -registers. The entry code in romlayout.S will push the original -registers on the stack before calling the C code and then pop them off -(including any required changes) before returning from the interrupt. -Changes to CS, DS, and ES segment registers in C code is also safe. -Changes to other segment registers (SS, FS, GS) need to be restored -manually. - -Stack variables (and pointers to stack variables) work as they -normally do in standard C code. - -However, variables stored outside the stack need to be accessed via -the GET_VAR and SET_VAR macros (or one of the helper macros described -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. 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_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 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 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_GLOBAL, GET/SET_BDA, and GET/SET_EBDA) are available to -simplify these accesses. Also, an area in the 0xc0000-0xf0000 memory -range is made available for internal BIOS run-time variables that are -marked with the VARLOW attribute. These variables can then be -accessed with the GET/SET_LOW macros. - -Global variables defined in the C code can be read in 16bit mode if -the variable declaration is marked with VAR16, VARFSEG, or VAR16FIXED. -The GET_GLOBAL macro will then allow read access to the variable. -Global variables are stored in the 0xf000 segment. Because the -f-segment is marked read-only during run-time, the 16bit code is not -permitted to change the value of 16bit variables. Code running in -32bit mode can not access variables with VAR16, but can access -variables marked with VARFSEG, VARLOW, VAR16FIXED, or with no marking -at all. The 32bit code can use the GET_GLOBAL macros, but they are -not required. - - -GCC 16 bit stack limitations: - -Another limitation of gcc is its use of 32-bit temporaries. Gcc will -allocate 32-bits of space for every variable - even if that variable -is only defined as a 'u8' or 'u16'. If one is not careful, using too -much stack space can break old DOS applications. - -There does not appear to be explicit documentation on the minimum -stack space available for bios calls. However, Freedos has been -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 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. 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 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 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 -command line. For example: - -qemu -L mybiosdir/ -fda myfdimage.img -s -S - -Then, in another session, run gdb with either out/rom16.o (to debug -bios 16bit code) or out/rom32.o (to debug bios 32bit code). For -example: - -gdb out/rom16.o - -Once in gdb, use the command "target remote localhost:1234" to have -gdb connect to qemu. See the qemu documentation for more information -on using gdb and qemu in this mode. Note that gdb seems to get -breakpoints confused when the cpu is in 16-bit real mode. This makes -stepping through the program difficult (though 'step instruction' -still works). Also, one may need to set 16bit break points at both -the cpu address and memory address (eg, break *0x1234 ; break -*0xf1234). +For other types of builds, and for more detailed developer +documentation, please see the online documentation listed above. diff --git a/README.CSM b/README.CSM deleted file mode 100644 index b904e65..0000000 --- a/README.CSM +++ /dev/null @@ -1,22 +0,0 @@ -Enabling CONFIG_CSM allows SeaBIOS to be built as a Compatibility Support -Module for use with the OMVF/EDK-II UEFI firmware. - -It will provide "legacy" BIOS services for booting non-EFI operating -systems and will also allow OVMF to display on otherwise unsupported -video hardware by using the traditional VGA BIOS. - -Windows 2008r2 is known to use INT 10h BIOS calls even when booted via -EFI, and the presence of a CSM makes this work as expected too. - -Having built SeaBIOS with CONFIG_CSM, you should be able to drop the -result (out/Csm16.bin) into your OVMF build tree at -OvmfPkg/Csm/Csm16/Csm16.bin and then build OVMF with 'build -D -CSM_ENABLE'. The SeaBIOS binary will be included as a discrete file -within the 'Flash Volume' which is created, and there are tools which -will extract it and allow it to be replaced; satisfying the -requirements of the LGPL licence. - -A patch to OVMF is required, to prevent it from marking the region from -0xC0000-0xFFFFF as read-only before invoking our Legacy16Boot method. See -http://www.sourceforge.net/mailarchive/forum.php?thread_name=50FD7290.906000... -
On Mon, Dec 15, 2014 at 09:47:44PM -0500, Kevin O'Connor wrote:
The README file and README.CSM file have gotten a bit out of date. Instead of maintaining technical information in the README file, point new users to the SeaBIOS wiki.
FYI, I committed this change.
-Kevin
-
Kevin O'Connor