DESIGNING THE SOFTWARE SPECIFICATION:

This, the first article, is the first published mention of 386BSD. By this time, the project had been operational for 18 months, and William Jolitz was at Berkeley working on the Net/2 release.
In this installment, we discussed the beginning of our project and the initial framework that guided our efforts, in particular, the development of the 386BSD specification.

We'd gathered books and equipment to begin the port in 1989. Most critical was the Crawford and Gelsinger book.
We went through many PCs, most of them portables. Compaq sent a DeskPro 386, and SystemPro 386 & 486.

The 386BSD specification was in two parts - one that detailed getting to a operational system that could build itself and basic console applications, and a more extensive community involvement part, called "A Modest Proposal".

Choosing how far to go in supporting X86 architecture in order to get a basic BSD UNIX system to be able to bootstrap the futre efforts.

We resolve conflicts between UNIX worlds by choosing a middle way - one that isn't a pure standard, but one that doesn't fight standards commonality.

We decided to shoot for a system that emphasized novel 386 support code to create a basic BSD environment on the 386. We assumed that by making it freely available, others will extend the work to remaining areas.

Most popular microprocessors use either segmentation or paging to manage memory address space access. The 386 is rare in that it possesses both. In fact, since segmentation, (see Figure 3(a)), is placed on top of paging (see Figure 3(b)), you are expected to use segmentation in some form any time memory is paged. And, most important, BSD relies on paging.

Reconciling segmentation to UNIX has never been easy, and with 386BSD its an even greater chore. The issues of supporting X86 segments in a Berkeley UNIX world.

386BSD ignored as much as possible the segmentation hardware of the x86, preferring to use a "flat" address space.

386BSD executed programs with a virtual address similar to a VAX running UNIX. In February 1991, this changed so that the format of the process virtual address space memory usage could be arranged differently.

A UNIX legacy, the "u." or per-process data structure, which held the kernel-related data of a process, was present on 386BSD prior to February 1991.

The 386 Paging Memory Management allocates memory in 4KB and 4MB allocation units. This impacts the way programs execute and share file data.

One surprise with 386BSD was that the 80386 doesn't honor write protection on the user page addresses, requiring a work-around. This was fixed in 80486 and all subsequent X86 processors.

386BSD started out in 1989 with a derivative virtual memory system from the VAX by way of a 68030. In February/March 1991, it was cutover to a totally different one cut out of CMU's MACH system, and released with Net/2.

The "u." in more detail, handling kernel stack overflows in 386BSD.

Hardware context switch state description and the part where 386BSD context switching intrudes into the machine independent code semantics.

Catching terminal processor faults.

Sometimes you're forced to use processor features - like hardware context switching. Origionally, the earliest versions of 386BSD didn't use the hardware context switch TSS feature - but you still had to have one anyways.

How to call the system's kernel from a process, using existing industry standards accross the X86 platform.

The AT platform itself requires support in order to have an operational system. And with growth of elovling industry standards, this morphs over time into newer ones as well.

The actual memory referenced on processor, system, and I/O device busses, or physical memory addresses have standard assignments for the kernel to manage for the operating system to function.

ISA devices that attach to the AT standard through controllers field a broad number of devices various categories.

To find controllers and devices, we use tables to instruct device drivers where to probe and attach found peripherals and connect them with low-level drivers and high-level kernel subsystems.

Part of the specification also details the interrupt control mechanism that allows device interrupts to be blocked/unblocked and caught by device drivers interrupt handling code.

How to bootstrap the system from hardware, loading the kernel program, itself a protected mode executable from secondary / nonvolatile / disk storage.

The state of the 386BSD world back in mid-1990 is synopsized here. By the time this appeared, EISA and other "beyond AT" support had been added. CSRG only let other UC institutions have code, although a complete binary and source release was present and tested for 6 months.

Support the processor, and support the ISA bus peripherals are the objectives for the first parts of 386BSD.

Catching potentially restartable 386 processor faults with 386BSD.

Understanding the boundary between research and development with BSD, and where a balance between commercial efforts can be struck.