Microcomputer Circuits and Processes

address bus
r-----
AND gates
RD .--...,---)...-----t
256 x 8 bits
memory
WR._-1-...,---).... __ --1
enable •..•• ---..l. ~
Figure 2.12
256-byte memory from figure 2.11 shown as a single board with buffer, and control lines,
and logic.
Note also how the enable connection, which controls the tristate
buffer hanging on the data bus as usual, is also ANDed with the read
(RD) and write (WR) connections. If the enable is high, then RD and
WR may perform their functions, but if the enable is low, RD and WR
have no effect and the memory cells remain unchanged. This is useful
when larger memory systems are designed, as you will find out.
MORE MEMORY - DEVICE SELECTION
We have put together 256 bytes of memory quite quickly, but that is not
an awful lot, especially when you consider that a Sinclair ZX home
computer comes with 16 or 48 kilobytes. Before we see how to wire up
larger amounts of memory, there is the vocabulary to be sorted out.
First, an 8-bit number like 01110100 is called a byte. Our 256-byte
memory size is often called a page of memory, and four pages make up
1 kilobyte (or lK, pronounced 'kay' as in 'OK'). Although 4 x 256
= 1024, this is what computer people mean when they talk about lK.
So 64K of memory means 64 x 1024= 65 536 bytes. It may sound like
feet and inches, but at least everyone, including you, knows what it
means.
To make up lK of memory all we need is four 256-byte boards. But
we are only able to address one of these with our 8-bit address bus. To
address the other boards, the address bus has to be expanded. If we
doubled the size of the address bus to 16-bits wide, then we would have
more potential. You may think we would use each of the extra eight
wires on the bus to select a 256-byte memory board. This is shown in
figure 2.13, and would give us a maximum of 8 x 256 = 2048 (2K) bytes
of memory. Each of the extra address lines is connected to the enable
input on a memory board. This would work, but is not normally done,
for two reasons. Firstly, the memory addresses will not be continuous as
you go from one page to another. For example, the second board would
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