ОС/8: Операционная система для мини-ЭВМ PDP-8

Primary operating system used in the DEC PDP 8 minicomputer

OS/8 is the primary operating system used on the Digital Equipment Corporation's PDP 8 minicomputer. PDP 8 operating systems which precede OS/8 include:
R L Monitor, also referred to as MS/8. P?S/8, requiring only 4K of memory. PDP 8 4K Disk Monitor System
PS/8 ("Programming System/8"), requiring 8K. This is what became OS/8 in 1971. Other/related DEC operating systems are OS/78, OS/278, and OS/12. The latter is a virtually identical version of OS/8, and runs on Digital's PDP 12 computer. Digital released OS/8 images for non commercial purposes which can be emulated through SIMH.

OS/8 provides a simple operating environment that is commensurate in complexity and scale with the PDP 8 computers on which it ran. I/O is supported via a series of supplied drivers which uses polled (not interrupt driven) techniques. The device drivers have to be cleverly written as they can occupy only one or two memory pages of 128 12 bit words, and have to be able to run in any page in field 0. This often requires considerable cleverness, such as the use of the OPR instruction (7XXX) for small negative constants. The memory resident "footprint" of OS/8 is only 256 words; 128 words at the top of Field 0 and 128 words at the top of Field 1. The rest of the operating system (the USR, "User Service Routines") swaps in and out of memory transparently (with regard to the user's program) as needed. This disk resident (or more properly, system device resident) part of OS/8 is the first 14K words (56 blocks) of the SYS: device, including swap space. The remaining memory contents are preserved between some of the basic user commands. This is exploited by the process by which executable programs are built: for example, an assembly language program is first assembled into a BN (binary) file. Then one or more BN files are loaded into memory using the LOAD (ABSLDR) command. You could then execute the loaded program with the START command, debug it with ODT, or SAVE it to a SV file which can be later retrieved with the GET command or executed immediately with R or RUN commands. Information about the currently loaded program is maintained in the CCB (Core Control Block), which is located on the SYS: device. This information includes which parts of memory are used by the program (which parts should be saved) and the program's starting address. This concept of preserved memory being associated with a user or job also appears in other DEC operating systems, including TOPS 10 for the PDP 10, where such memory is known as a context. OS/8 can boot from a hard disk and present the command prompt in under half a second.

Early versions of OS/8 have a very rudimentary command line interpreter with very few basic commands: , , , , , and With version 3 they add a more sophisticated overlay called CCL (Concise Command Language) that implements many more commands. OS/8's CCL is directly patterned after the CCL found on Digital's PDP 10 systems running TOPS 10. In fact, much of the OS/8 software system is deliberately designed to mimic, as closely as possible, the TOPS 10 operating environment. (The CCL command language is used on PDP 11 computers running RT 11, RSX 11, and RSTS/E, providing a similar user operating environment across all three architectures: PDP 8s, PDP 10s, and PDP 11s.) The basic OS and CCL implements many rather sophisticated commands, many of which still do not exist in modern command languages, not even in MS DOS, Windows, or Unix like operating systems. For example, the command automatically finds the right compiler for a given source file and starts the compile/assemble/link cycle. The and commands permit the use of logical device names in a program instead of physical names (as required in MS DOS). For example, a program can write to device , and with an initial "" then the file is created on physical device RXA2 (the second floppy disk drive). VAX/VMS and the Amiga's operating system AmigaOS (and other OSes built around TRIPOS) make considerable use of this feature. The command is capable of setting many system options by patching locations in the system binary code. One of them, a command under OS 78, is , which re enables the MONITOR commands that are not part of OS 78. The command can reconfigure the OS on the fly, even adding device drivers, often without having to reboot the OS.

There are two systems for handling ASCII text. ASCII files are stored as three 8 bit characters per pair of 12 bit words. The first two characters (marked with bits a0–a7 and b0–b7 below) are stored whole in their words, while the third character (bits c0–c7) is stored with half of its bits in word 1 and the other half in word 2. The insufficiency of a three bit year field, capable of storing only eight years, was recognized when COS 310 was developed.

A single user BASIC and two multi user versions of BASIC are available as options. The single user BASIC uses several overlays to provide the full functionality of the language; when OS/8 is booted from a DECtape, a noticeable delay occurred each time BASIC is required to switch overlays as they need to be read from tape. The multi user versions of BASIC (EDU20 and EDU25) differ only in whether or not they support block replaceable devices (DECtape or disk). Due to cost constraints, many PDP 8s have punched paper tape readers as their only mass storage I/O device. EDU20 loads from paper tape and can do output to a paper tape writer if the machine has one, whereas EDU25 understands the structure of a filesystem, can load from DECtape or disk, and can create files on DECtape or disk. Both can run multiple BASIC programs simultaneously using a primitive task scheduler that round robins among the attached terminals. Memory is always tight because the PDP 8 uses core memory, which was extremely expensive compared to RAM technology. In 8K of 12 bit words EDU20 can support up to 4 terminals at once, although more memory was recommended. EDU25 requires an additional 4K memory bank (for a minimum of 12K) because the code contains a disk device driver and a filesystem handler. While running, EDU20 and EDU25 are self contained programs that don't use any OS/8 system calls. Immediately upon being invoked from the OS/8 command interpreter, they overwrite OS/8's entire resident portion – all 256 words of it. Upon startup, EDU25 saves the contents of memory to DECtape or disk and restores it upon exit. But EDU20 cannot do this as it is targeted at hardware configurations without any block replaceable device.

In addition to a freely available FORTRAN II compiler, there is also a rather complete FORTRAN IV compiler available. This compiler generates code for the optional FPP 8 floating point processor, which is essentially a separate CPU, only sharing memory with the PDP 8 CPU. With the FPP 8 option installed, the FORTRAN runtime code detects it and uses the FPP 8 to run the main program code, and the PDP 8 CPU runs as an I/O processor. Lacking the FPP 8, the runtime code instead calls an FPP 8 interpreter running on the PDP 8 CPU, so the program runs at reduced speed. This FORTRAN IV compiler in version 1 has the interesting bug that DO loops counted incorrectly: DO loops would count 1,2,3,5,6,7, (skipping 4). A quick patch was released to fix this.