Editing JOSS (section)

==Implementation==
[[File:IBM_Selectric_typeball.jpg|thumb|JOSS used a custom type ball, similar to this example, for its [[IBM Selectric typewriter]] terminals to provide mathematical symbols.]]

JOSS-II on the PDP-6 was divided into two parts, matching the internal memory layout of the machine. The machine was equipped with two banks of [[core memory]], with 16k 36-bit words in each bank. The JOSS system itself, which included the [[operating system]], user and file management code, terminal handler and the interpreter, used up much of the first of these banks. The second bank was used for user programs. Although this was a relatively large amount of memory for the era, the system was so heavily used that the 16k word user store was not enough, and it was backed up with a [[magnetic drum]] for [[Memory paging|paging]] support.{{sfn|Bryan|1966|p=3}} The drum was driven by external hardware and did not require attention from the main processor.{{sfn|Bryan|1966|p=4}}

In order to support multiple user programs, the PDP-6 hardware was modified to examine bit-20 of any address reference. If this bit was set, the address was in "user space" and was modified so that address zero pointed to the base address of that user. In this way, user programs could be moved about in memory without causing problems for the system, and the operating system could switch from user to user simply by changing a single internal register.{{sfn|Bryan|1966|p=3}}

Permanent storage was provided by a Storage Products [[hard drive]] with 5.7 million words of storage. Like the drum, the actual [[input/output]] to and from the drum was handled externally, although in this case the movement was triggered by user actions to load and store their programs. An IBM-compatible [[tape drive]] was used to move data to and from the drive as needed, an operation that was also independent of the CPU. Two [[DECtape]] units were also available and worked in the same fashion as the IBM drive.{{sfn|Bryan|1966|p=5}} Demonstration programs were available, such as a [[mortgage calculator]] and [[early mainframe games]].{{sfn|Marks|1971|p=26}} Because large programs caused paging that slowed the computer for all, ''The JOSS Newsletter'' often asked users to use less storage.{{sfn|Marks|1971|pp=32-33}} 

Terminals were handled through a custom "concentrator" that consisted of a mechanical [[Strowger switch]] that could connect any of the 300 to 400 possible terminal plugs to any of 40 outputs. A further eight lines were dedicated to [[Teletype Model 33]] inputs, as opposed to the JOSS-style Selectrics. Those 48 lines were then connected to an electronic [[multiplexer]] connected to the CPU. The multiplexer was [[interrupt]] driven, meaning idle terminals did not use up any cycles.{{sfn|Bryan|1966|p=5}}

The custom JOSS terminals were built by DEC. These consisted of a Selectric mechanism built into a custom chassis containing the communications system, power supply, and other components. A custom font ball was used to provide basic mathematical symbols like ≠ so two-character combinations like <> did not have to be used. The communications system was based on a 6-bit character code in an 8-bit packet with start and stop bits. The lines were driven at 120&nbsp;bit/s to match the maximum 15 character per second speed of the Selectric mechanism. Shift-in and shift-out codes were used to move the ribbon from green to black and back. These codes were also noticed by the communications system and changed lights on the front panel to indicate whether the computer or user had control of the terminal at that moment.{{sfn|Bryan|1966|p=6}}