Blit (computer terminal)

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Teletype DMD 5620

In computing, the Blit was a programmable bitmap graphics terminal designed by Rob Pike and Bart Locanthi Jr. of Bell Labs in 1982.

History[edit]

The Blit technology was commercialized by AT&T and Teletype. In 1984, the DMD (dot-mapped display) 5620 was released,[1] followed by models 630 MTG (multi-tasking graphics) in 1987 and 730 in 1989. The 5620 used a Western Electric 32000 processor (aka Bellmac 32) and had a 15" green phosphor display with 800×1024×1 resolution (66×88 characters in the initial text mode) interlaced at 30 Hz. The 630 and 730 had Motorola 68000 processors and a faster 1024×1024×1 monochrome display (most had orange displays, but some had white or green displays).

The folk etymology for the Blit name is that it stands for Bell Labs Intelligent Terminal, and its creators have also joked that it actually stood for Bacon, Lettuce, and Interactive Tomato. However, Rob Pike's paper on the Blit explains that it was named after the second syllable of bit blit, a common name for the bit-block transfer operation that is fundamental to the terminal's graphics.[2] Its original nickname was the jerq, inspired by Three Rivers' PERQ graphic workstation.

Functionality[edit]

When initially switched on, the Blit looked like an ordinary textual "dumb" terminal, although taller than usual. However, after logging into a Unix host (connected to the terminal through a serial port), the host could load (via special escape sequences) software onto the Motorola 68000 processor of the display. This software could make use of the terminal's full graphics capabilities and attached peripherals such as a computer mouse. Normally, users would load the window systems mpx (or its successor mux), which replaced the terminal's user interface by a mouse-driven windowing interface, with multiple terminal windows all multiplexed over the single available serial-line connection to the host.

Each window initially ran a simple terminal emulator, which could be replaced by a downloaded interactive graphical application, for example a more advanced terminal emulator, an editor, or a clock application. The resulting properties were similar to those of a modern Unix windowing system; however, to avoid having user interaction slowed by the serial connection, the interactive interface and the host application ran on separate systems—an early implementation of distributed computing.

Window systems[edit]

Pike wrote two window systems for the Blit, mpx for 8th Edition Unix and mux for 9th Edition, both sporting a minimalistic design. The design of these influenced the later Plan 9 window systems and rio. When the Blit was commercialized as the DMD 5620, a variant of mpx called "layers" was added to System V.3.[3]

See also[edit]

References[edit]

This article is based in part on the Jargon File, which is in the public domain.

External links[edit]

  • Bart Locanthi, Rob Pike: Blit (MPEG) (YouTube), the classic animated short about the windowing terminal project (it was necessary to explain how mice worked back then; this was 1982, two years before the Mac) (MPEG)
  • Source code (contains proprietary code) 5620