BBC Microcomputer Service Manual Oct 1985 Section 1 BBC Micro ...

3.3 CRT controller + video processor + Teletext hardware
Random Access Memory on the Microcomputer is provided by either 8 or 16
dynamic memory devices (ICs 53-68). These devices store 16K bits each
and therefore in the Model B, the data inputs and outputs of one pair
of devices are paralleled for each of the 8 data bits, DO to D7. To
address 16K bits requires 14 address lines, and this is achieved on the
4816 by having 7 inputs and latching in the addresses in two halves by
using a row address strobe (RAS) and a column address strobe (CAS). Two
octal buffers (81LS95) have to be used to multiplex the appropriate
processor address lines onto the RAM address lines. (ICs 12 and 13).
However, the 6845 CRT controller (IC2) also needs to access the RAM,
and what is more, it accesses it differently depending on whether it is
working in the Teletext mode or in one of the other graphics modes.
Therefore two more pairs of octal buffers are used, ICs 10 and 11 for
the Teletext mode, and ICs 8 and 9 for the other modes, the main
difference being that in these modes the three least significant
address bits are produced by the character row address lines from the
CRTC in order to give the bit-mapping of the characters in the RAM
memory rather than having the ROM character generator as in the
Teletext mode.
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