TCCBOOT: README

Introduction:
------------

WARNING: don't try to play with TCCBOOT unless you are a kernel
hacker!

TCCBOOT is a bootloader which uses TinyCC to compile C and assembly
sources and boot the resulting executable. It is typically used to
compile the Linux kernel sources at boot time.

TCCBOOT boots the same way as a Linux kernel, so any boot loader which
can run a 'bzImage' Linux kernel image can run TCCBOOT. I only tested
it with ISOLINUX, but LILO or GRUB should work too.

TCCBOOT reads C or assembly sources from a gzipped ROMFS filesystem
stored in an Initial Ram Disk (initrd). It first reads the file
'boot/tccargs' which contains the TinyCC command line (same syntax as
the tcc executable). The TinyCC invocation should output one binary
image 'kernel'. This image is loaded at address 0x00100000. TCCBOOT
then does a jump to the address 0x00100000 in 32 bit flat mode. This
is compatible with the ABI of the 'vmlinux' kernel image.

Compilation:
-----------

TCCBOOT was only tested with Linux 2.4.26. In order to build TCCBOOT,
you must first compile a 2.4.26 kernel because for simplicity TCCBOOT
uses some binary files and headers from the Linux kernel. TCCBOOT also
needs the source code of TinyCC (tested with TinyCC version
0.9.21). You can modify the Makefile to give the needed paths.

Example:
-------

An "Hello World" ROMFS partition is included (initrd.img). You can
rebuild it from the example/ directory. You can test it with the QEMU
PC emulator with the 'qemu-tccboot' script.

Kernel compilation:
------------------

For your information, the patch 'linux-2.4.26-tcc.patch' gives the
necessary modifications to build a Linux kernel with TCCBOOT (NOTE: it
is not suffisant to build the kernel with its own Makefiles - I never
tried). The corresponding kernel configuration is in file
linux-2.4.26-config. Patches are necessary for the following reasons:

- unsupported assembly directives: .rept, .endr, .subsection
- '#define __ASSEMBLY__' needed in assembly sources
- static variables cannot be seen from the inline assembly code
- typing/lvalue problems with '? :'
- no long long bit fields
- 'aligned' attribute not supported for whole structs, only for fields
- obscur preprocessor bug

Some of these problems could easily be fixed, but I am too lazy
now. It is sure that there are still many bugs in the kernel generated
by TinyCC/TCCBOOT, but at least it can boot and launch a shell.

License:
-------

TCCBOOT is distributed under the GNU General Public License.

Fabrice Bellard.