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Solidisk Fourmeg CPU and ROM expansion with 32K RAM (top)
The Solidisk Fourmeg CPU and ROM expansion has a socket (blue) for the 4MHz 65C02 (or 2MHz 6502/65C02), 4 ROM sockets (1 x 64K and 3 x 32K) and 1 x TC5565PL-16 8K static RAM.
Solidisk Fourmeg CPU and ROM expansion with 32K RAM (bottom)
On the bottom of the Solidisk Fourmeg CPU and ROM expansion is the optional 32K shadow/sideways RAM. The Fourmeg board plugs into the BBC micro's 6502 socket, normally there would be a blue extension socket on the 2 rows of pins above the 32K board, but the are missing.
Solidisk Fourmeg-32K (top)
The Solidisk Fourmeg-32K Expansion's features are:
Solidisk Fourmeg-32K (bottom)
In the ordinary BBC micro, the CPU runs at 2MHz. By adding faster memory and increasing the speed to 4 MHz, Solidisk enabled most programs to run in half the time. The new 4MHz CPU is equivalent to that used in the Master and Acorn Turbo board. It will run most Master software (e.g. Acorn BASIC4) and at twice the speed of the Master. The Fourmeg CPU can be changed from 4MHz to 2MHz at the flick of a switch.
The Fourmeg board also has 4 ROM sockets, as used in the BBC B+ and Master. Up to 160K of ROM software can be fittted (1 x 64K and 3 x 32K ROMs). The ROMs installed in these sockets can run at 4MHz as can programs running in the onboard RAM. The rest of the BBC Micro is unaffected.
The Fourmeg board can be expanded with 32K of RAM and a 16K Manager ROM which can be used as shadow and sideways RAM (see the first 2 pictures above), as on the BBC B+. In shadow mode 27K of free RAM is available regradless of MODE or DFS. In non-shadow mode the RAM can be used as two sideways RAM banks used to hold the images of two sideways ROMs, or as a RAMDISK, or printer buffer. The shadow and sideways RAM can also run at 4MHz.
The Solidisk Twomeg CPU and ROM Expansion is the same as the Fourmeg but has either a 2MHz 6502 or 65C02 CPU.
The Solidisk Fourmeg CPU and ROM Expansion is installed by removing the BBC Micro's 6502 CPU and plugging blue socket on the bottom of the Fourmeg board into the empty IC1 socket in the BBC Micro. The CPU is then either installed in the Fourmeg board or replaced with the 4MHz 65C02 CPU. There are various wires that connect to the BBC Micro motherboard including a switch for the 2MHz/4MHz CPU clock.
Here is the Solidisk Twomeg and Fourmeg RAM/ROM Expansions manual.