DESIGNING THE SOFTWARE SPECIFICATION:Porting Unix to the 386: 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.
We'd gathered books and equipment to begin the port in 1989. Most critical was the Crawford and Gelsinger book.
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".The Definition of the 386BSD Specification
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.Conflicts in Priorities
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.386BSD Port Goals: A Practical Approach
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.386 Memory Management Vitals
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.Segmentation and 386BSD
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.Kernel Linear Address Space Overhead
386BSD ignored as much as possible the segmentation hardware of the x86, preferring to use a "flat" address space.Virtual Address Space Layout
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.Per-Process Data Structures
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 1991.386 Virtual Memory Address Translation Mechanism
The 386 Paging Memory Management allocates memory in 4KB and 4MB allocation units. This impacts the way programs execute and share file data.User to Kernel Communication Primitives
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.Berkeley UNIX Virtual Memory System Strategy
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.Structure of Per-Process Data (u.)
The "u." in more detail, handling kernel stack overflows in 386BSD.Process Context Description
Hardware context switch state description and the part where 386BSD context switching intrudes into the machine independent code semantics.Other Processor Faults
Catching terminal processor faults.Microprocessor Idiosyncrasies
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.System Call Interface
How to call the system's kernel from a process, using existing industry standards accross the X86 platform.System Specific (ISA) Issues
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.Physical Memory Map
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 Device Controllers
ISA devices that attach to the AT standard through controllers field a broad number of devices various categories.ISA Device Auto Configuration
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.Interrupt Priority Level Management
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.Bootstrap Operation
How to bootstrap the system from hardware, loading the kernel program, itself a protected mode executable from secondary / nonvolatile / disk storage.Summary: Where is 386BSD Now?
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.Suggested Readings Microprocessor and System Specification Issues
Support the processor, and support the ISA bus peripherals are the objectives for the first parts of 386BSD.Page Fault and Segmentation Fault Mechanism
Catching potentially restartable 386 processor faults with 386BSD.Porting Unix to the 386: Research and the Commercial Sector
Understanding the boundary between research and development with BSD, and where a balance between commercial efforts can be struck.