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3.4 Memory Devices.
The 8-bit 6502A microprocessor can directly address 64 kilobytes of memory. In the BBC Microcomputer,
32 kilobytes of this memory is contained within Read Only Memories (ROM's). ROM is non-volatile; that
is, it retains data when the power to it is removed. The Machine Operating System (MOS) of the
microcomputer is contained within 15 kilobytes of this read-only memory on a 16-kilobyte device,
(normally IC 51). The machines address decoding circuitry "maps-out" the remaining 1 kilobyte, since this
area is devoted to JIM, FRED and SHEILA. The BASIC interpreter is contained within another mask-
programmed, 16 kilobyte ROM (normally IC52).
In early versions of the BBC Microcomputer the 0.1 operating system was contained within four 2732
type, 4 kilobyte ultra violet light Erasable programmable Read Only Memory devices (EPROMs). These
were located in IC positions 52,88,l00, l0l, the BASIC interpreter ROM being in IC position 51. In this case
the devices were accessed at 1MHZ. The Machine Operating System (MOS) was in EPROM due to
delays in producing the mask programmed ROM version. The reason for using four 4K devices was
probably due to the high cost of 8K and 16K EPROMs at the time.
Early versions of the series 1 (l.0) operating system, supplied with machines fitted with a disc interface,
were contained within two 8-kilobyte EPROMs on a small "carrier board". When replacing this with a
Series 1 Operating System ROM it should be noted that the carrier board has rather large diameter pins,
which sometimes damage the IC socket into which the board was fitted. Chapter 6 gives details on a
remedy for this.
There are positions for five read-only memories on the main PCB, the operating system and BASIC
normally occupying two of these, as previously mentioned. The remaining three positions are for fitting
utility or additional language ROMs and are often referred to as the "paged" or "sideways" ROM positions.
In order to select between these, IC 76, which is a 74L5163, must be fitted and two circuit board links
removed (see chapter 4). This modification is only required on issues 1,2 and 3 of the main PCB. In order
to be able to make use of the paged ROM facility it is also necessary to have a Series 1 Machine
Operating System fitted.
If the sideways ROM to be selected is an 8 or 16 kilobyte device, then it can be switched into the ROM
memory space by writing to a 74L5163 4-bit synchronous binary counter (IC76), which is referred to as
the ROM select latch. In cases where four, 4 kilobyte ROMs are fitted in the paged ROM sockets, a
74L5139 two-to-four line decoder (part of IC 20) selects which of the four devices is being selected by
lines A12 and A13 of the address bus.