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OS/8
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==Overview== OS/8 provides a simple operating environment<ref>An annotated list of commands is available at {{cite web |url=http://www.pdp8.net/os/os8/index.shtml |title=OS/8 MONITOR COMMANDS}}</ref> 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<ref name=CHDkmn.8e>{{cite web |url=http://www.chdickman.com/pdp8 |title=My PDP8/E}}</ref> 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 line interface#Command prompt|command prompt]] in under half a second.
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