TCCBOOT 0.1 review

TCCBOOT project is a boot loader able to compile and boot a Linux kernel directly from its source code. TCCBOOT is only 138 KB big

License: GPL (GNU General Public License) File size: 184K Developer: Fabrice Bellard

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TCCBOOT project is a boot loader able to compile and boot a Linux kernel directly from its source code.

TCCBOOT is only 138 KB big (uncompressed code) and it can compile and run a typical Linux kernel in less than 15 seconds on a 2.4 GHz Pentium 4.

TCCBOOT is based on the TinyCC compiler, assembler and linker. TinyCC is an experiment to produce a very small and simple C compiler compatible with the GNU C compiler and binary utilities.

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.