Application
Numerous coprocessors were developed for the Tube. Most commonly seen was a MOS Technology 6502 processor which allowed unmodified BBC Micro programs to run faster and with more memory, as long as they used the API for all I/O. There was also a Zilog Z80 processor to run CP/M and a National Semiconductor 32016 processor running Panos (and unofficially a UNIX variant).
These coprocessors formed the basis of the Acorn Business Computer series, the higher end machines being repackaged BBC Micros with a coprocessor attached via the Tube. The Master Series supported two Tube connections, allowing for a coprocessor fitted inside the case and another connected externally, but only one could be used in any powered session. An internal 6502 processor could be fitted, or an Intel 80186 based system for DOS compatibility (although in practice this was limited).
The Tube was also used during the initial development of the ARM processor. An evaluation board was developed that again used the BBC Micro as a host system for I/O operations.
Acorn had strongly discouraged BBC Micro programmers from directly accessing system memory and hardware, favouring official API calls. This was ostensibly to ensure applications could be seamlessly moved to the Tube 6502 coprocessor, since direct access from there was impossible. When a program called one of the MOS entry points, a replacement subroutine in the coprocessor's ROM passed a corresponding message to the host which carried out the operation and passed back the result. In this way an application could run identically on the host or the coprocessor. Other CPU models used a custom API, which was typically an orthogonal translation of the 6502 API into a native format.
Read more about this topic: Tube (BBC Micro)
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