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Turbo 2000 for Atari

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This short article by Dan Polansky briefly covers Turbo 2000 system for Atari 8-bit computers. The system allowed the magnetic tape recorder/datasette (used with compact cassettes) to read and write in 2270 bits per second[1] instead of the original 600. As a consequence, a 90 minute cassette could contain approximately 1.5 MB; assuming a game has no more than 48 KB, this yields at least 30 games per 90 minute cassette. The system was of Czechoslovak origin, made by Jiří Richer in 1987 in Prague. It required a hardware modification of the XC12 datasette and accompanying software, usually available on a cartridge. The software had to turn ANTIC chip off[2]; the only display available during loading were black and white moving strips[2].

A key motivation for this modication seems to be that, in the late 1980ies in Czechoslovakia, Atari disk drives were very expensive, unlike the XC12 datasettes. And XC12 datasettes were needlessly slow with their 600 bits/s, where the effective rate was even lower since the original Atari format split files into data blocks/packets separated by gaps (a packet storing 128 bytes[3]), whereas Turbo 2000 had only one gap separating the file header from the body. If we reckon with the gaps, the effective rate may perhaps be 500 bits/s for multi-block files[4], resulting in a speed-up of over 4 times.

There were multiple systems labeled as Turbo in Czechoslovakia.[1] The technical differences and compatibility are unclear. One variant was called Super Turbo, which allowed flexible bit rate in the range of 2700 to 5000 bit/s.[5] Super Turbo seems to have been compatible with Turbo 2000 in some sense.[6]

A tentative speculation about turning off ANTIC: the loading and saving code would have used busy wait to do timing to track the length of one bit. It would need a fixed number of CPU cycles burned by the given 6502 code. However, ANTIC's direct memory access (DMA) would make the timing too variable; when ANTIC reads RAM to access a display list, player/missile graphics data or video memory ("playfield"), 6502 cannot access RAM and has to wait. Moreover, it is possible that ANTIC is less busy during vertical blank (the beam is off screen); if so, the same 6502 code is faster during vertical blank. See also Wikipedia: ANTIC.

A variant of XC12 labeled XC13 was allegedly sold, ready for Turbo 2000 (and thus requiring no more hardware modification).[7] Absent images of XC13 and the scarcity of sources, it is not clear how reliable that information is.

References

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  1. ↑ 1.0 1.1 Turbo 2000 - úvod by Jiří Pecher, atari-800.cz
  2. ↑ 2.0 2.1 Turbo 2000, atarimax.com
  3. ↑ De Re Atari. Atari. 1982. https://archive.org/details/ataribooks-de-re-atari/page/n187. 
  4. ↑ * TURBO 2000 - Jiří Richter., blog.3b2.sk
  5. ↑ Princip úpravy Turbo 2000 by Jiří Pecher, atari-800.cz
  6. ↑ Dataset Atari XC12 - SUPER TURBO - ver.Čmuchař 2., blog.3b2.sk
  7. ↑ Atari 8-Bit Computers: Frequently Asked Questions, landley.net

Further reading

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