BeebMaster - Second Processors

Second Processors

The BBC range of computers is unique amongst 8-bit machines in many ways. One of those ways is the option to add a second processor to the machine. The advantage in doing this is that memory available and processing speed are increased.

All second processor units produced for the BBC micro carry their own on-board RAM. When the second processor is running, the language selected eg. BASIC or Wordwise, is copied across the Tube - the interface between the BBC's processor and the second processor - and runs from the second processor. The remainder of the second processor RAM is available to the user. None of the second processor memory is taken up by the screen as the screen displays continues to be mapped into the BBC memory map.

The BBC's processor no longer controls the user programme and is free to control the screen, keyboard, disc interface, analogue to digital interface and 1 MHz bus. Whilst the "host" processor in the BBC is carrying this out, the second processor is free to run the user's programmes. The result is a virtual doubling of the execution speed of a programme.

All the second processors produced for the BBC Model B are external units which connect to the machine via the Tube socket. The units manufactured by Acorn were supplied in the familiar "cheese wedge" boxes. These are compatible with the BBC Master Series but in addition, the Master Series has provision on the main circuit for the addition of internal co-processors.

The most common second processors were the 6502 Second Processor and 65C102 co-processor produced by Acorn. As these processors are essentially the same as the BBC's main processor, any software intended to run on the standard BBC computer can be executed across the Tube.

The 6502 Second Processor and 65C102 Co-Processor provide 64K of on-board RAM. 16K of this RAM is used to hold the language ROM in the second processor's volatile memory. The remaining 48K is available to the user. With HiBASIC running, PAGE is set at 2048 and HIMEM is 47104. Even running standard BASIC, PAGE is still at 2048 and HIMEM is at 32768 in all modes as the screen display is held in the BBC's RAM.

In the Econet system, use of a second processor is essential to allow a BBC computer to function as anything other than the most basic fileserver. For Level 2 and upwards, the additional memory and speed available is essential to run the fileserver software.

When an internal 65C102 co-processor is fitted to a BBC Master 128, the machine becomes known as the Master Turbo.

In addition to the second processors based on the 6502, second processors were available based on other processors. Alternatives included the Z80, 80186, 32016 and ARM all produced by Acorn. Third parties issued second processors based on the Z80, 6008 and 68009, with Torch making a particular speciality of a couple of different types of Z80-based second processors.

The twenty-first century has seen a revival in the interest in second processors amongst Beeb enthusiasts, some of whom have manufactured modern-day co-processors. The first of these was the Sprow ARM7 Co-Processor, an internal co-processor for the Master series with an ARM7 processor and 16MB (or more) of RAM made using surface mount technology.

Since then, there have been several 6502-based co-processor designs issued by other parties, including at least one which can take a Western Digital 14MHz 6502 variant.

Click here to learn about the Acorn Cheese Wedges