From f076a3eeb9a0185b06a2abbba8c798a7761b2bdf Mon Sep 17 00:00:00 2001 From: Kevin O'Connor Date: Mon, 25 Feb 2008 22:25:15 -0500 Subject: Initial checkin. --- README | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 71 insertions(+) create mode 100644 README (limited to 'README') diff --git a/README b/README new file mode 100644 index 0000000..ce80499 --- /dev/null +++ b/README @@ -0,0 +1,71 @@ +This code implements an X86 legacy bios. It is intended to be +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 code has been successfully compiled with gcc 4.1.2 and gas +2.17.50.0.18. + + +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 tools/ 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/blob.16.s). +The gcc "-fwhole-program" option is used to optimize the process so +that gcc can efficiently compile and discard unneeded code. + +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. + +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. + +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. + + +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. 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. -- cgit v1.2.3