Acorn BBC Master 128, Service Manual

Page: 21 / 87

Figure 2
This system allows for the screen bit map to be removed from the main
CPU memory map, of which it occupies a significant proportion. It
will, however, only work if the screen is being accessed by opcodes
from a known region - i.e. the MOS VDU drivers. A mechanism is also
provided to permit 'illegal' screen access:
* Bit X in ACCCON, when set, causes all accesses to region (a) to be
re-directed to LYNNE. This occurs irrespective of the opcode address,
hence considerable care will have to be exercised in its use. When
cleared, the memory map returns to its usual format.
In the same way that the BASIC variable HIMEM will always have the
value &8000 when LYNNE is used, it is desirable for the variable PAGE
to have the value &E00, irrespective of the current filing system.
16 Issue 1