BKUNIX — what could have been...

Background

Starting from the mid-1980s in the USSR a home microcomputer BK-0010 was produced that used the instruction set of DEC LSI-11. Its first version, released in 1985, had 32 KB of RAM, out of which up to 16 KB had to be reserved for the video memory. A tape recorder was the external storage device. A great variety of games, educational and text processing software, as well as a few programming systems have been written for it.

Later, a floppy controller with an extra 16 KB of RAM became available, and a flurry of “operating systems” (pseudographic file managers à la Norton Commander), some of them supporting the MS-DOS and the RT-11 file systems, achieved popularity.

When an improved version (BK-0011M, featuring 128 KB of RAM, switchable video buffers, a timer, and a VT-100 compatible terminal emulator) got released, RT-11 (SJ — single job) had been ported to it, and it was the only “real” operating system available on the BK family of computers.

Why BKUNIX?

After learning about the existence of Mini-Unix and LSX — the minimalistic Unix kernels derived from Unix V6 — and reviving the latter from an incomplete floppy image dump, I realized that it could have been possible to run a real Unix on BK-0010. Indeed, LSX in its minimal configuration required only 40 KB of RAM, and a BK with a floppy controller could provide up to 44 KB.

As all the LSX and Mini-Unix sources were written in an ancient dialect of the C language (no unsigned, void, long; weird — by today’s standards — syntax of assignment operators and initializations), the first step was to patch the source to conform to the newer style of C.

An initial attempt to use GCC for cross-development failed: it produced a buggy and size-inefficient code. We have settled on using the Ritchie C compiler from a BSD distribution and the Johnson Portable C compiler (PCC). In the process, quite a few bugs in PCC got fixed.

Due to a peculiarity of the BK-0010 memory mapping we had to bring the kernel up into the 120000-160000 range of addresses. This, and the fact that the better compilers caused a noticeable reduction in code size, allowed us to squeeze back the mount/umount system calls.

Before we proceed with the development of a purely BK-specific kernel, we would like to release the code in a state when it still can work on an LSI-11 (e.g. in the SIMH simulator).

Authors

• Leonid Broukhis
• Serge Vakulenko

License

The source files of the V6 Unix operating system have been released under the BSD license by Caldera International, Inc. See the accompanying file Caldera-License for details.