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.
+resulting file "out/bios.bin" contains the processed bios image.
-The code has been successfully compiled with gcc 4.1.2 and gas
-2.17.50.0.18.
+
+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/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/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
Overview of files:
-The src/ directory contains the bios source code. The post.c code is
-compiled in 32bit mode. The output.c code is compiled twice - once in
-16bit mode and once in 32bit mode. The remaining c files are compiled
-in 16bit mode.
+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 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.
+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 be used to generate valid 16
-bit code. The romlayout.S also defines all the mandatory bios visible
-memory locations.
+assembler - this option enables gcc to generate valid 16 bit code.
-The post code (post.c) is written in 32bits. The 16bit post vector
-(in romlayout.S) transitions the cpu into 32 bit mode before calling
-the initialization code in post.c.
+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, the compiled 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.
+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:
normally do in standard C code.
However, variables stored outside the stack need to be accessed via
-the GET_VAR and SET_VAR macros. 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 constant definitions (those in 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.
+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/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:
+
+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).
projects / seabios.git / blobdiff