Getting Started: References, Equipment, and Software
Most software ports begin with the naive assumption that the UNIX kernel is merely a C program with a handful of functions, supporting other utility C programs on demand. While in essence this is true, in practice this is a vast oversimplification. Nevertheless, in the tradition of great projects, we acquired a few tools and other items before getting down to work:
- The Design and Implementation of the 4.3BSD UNIX Operating System by Leffler, McKusick, Karels, and Quarterman (Addison-Wesley, 1989) and Programming the 80386 by Crawford and Gelsinger (Sybex, 1987) were purchased from a bookstore in Berkeley. Since no one on our team possessed any extensive technical background on either the 386 or the IBM PC, the 80386 book was our sole resource for the microprocessor. The 4.3BSD book illuminated some of the obscure areas and requirements of the BSD UNIX operating systems kernel. We highly recommend these books. Both books have become somewhat shopworn during the process -- the 80386 book has had it's covers taped twice, primarily due to being thrown repeatedly across the room in the general direction of the trash can. This book, while the best resource available on the subject, is not as complete as one might hope, primarily because the 80386 is a complex animal and is enigmatic in the correct use of its many features. Segmentation exception handling descriptions should not be taken literally, although the book was of great value when writing the first versions of exception handling code. Some portions of the software were even determined empirically. (Intel was not eager to provide any information.) The single biggest problem encountered in our project was that of inadequate 80386 documentation.
- A completely blank, inexpensive standard 386 AT clone was the selected hardware platform. To minimize expenses and to emphasize commonality, we chose to support only the basic 386 platform.
- Using exploratory programs written in Borland's Turbo C, we were able to explore the typical AT hardware. These exercises permitted us to better understand the information contained in IBM's Technical Reference Guide Personal Computer AT, a classic if not obscure work. We then tested the mechanisms inside the AT to make certain we knew what must be provided in order to generate the necessary software driver support for BSD UNIX.
- Our initial kernel source was the 4.3BSD Tahoe release (available for an obscure machine, the CCI Power 6/32, and as similar to the 386 as a can opener), at that time the most stable and recent release.
All of these references and the equipment were examined prior to generating even the first line of code. An understanding of the architectures of the hardware and software is critical to developing an appropriate 386BSD specification. Thus, we were able to ensure a successful port, even when unanticipated problems arose.
|
|