Microcomputer Circuits and Processes

CHAPTER 3
RUNNING A SIMPLE
PROGRAM
In Chapter 2 we saW how a CPU, memory, and input-output devices
communicated with each other using bussed data and address signals,
RD, WR, and ALE signals, all of which did the right thing at the right
time to make the system work. Now these data movements and their
controlling signals do not happen by magic; it is the job of the computer
program, written by you. In this chapter we shall see how a simple
program, stored in memory, is able to make a simple computer run. The
computer is a hypothetical4-bit machine called SAM (Simple Although
Meaningful). SAM employs most of the ideas discussed in Chapter 2,
although it is a bit simpler.
SAM'S
HARDWARE
SAM has a 4-bit multiplexed (shared) address-data bus, and the usual
RD, WR, and ALE signals. It is connected to memory only, there is no
input-output device in use at the moment, so we can forget about the
MilO signal. The memory is also simple, just one board, so we do not
have to worry about decoding. SAM is shown in figure 3.1.
RD
WR
ALE
CPU
,
memory
K
"
"
4-bit shared bus )
v
Figure 3.1
SAM.
What is inside the CPU? This is shown in figure 3.2. You can see a
number of registers, which are simple 4-bit memories used to hold
4-bit binary numbers. These communicate with the 4-bit bus via the
usual tristate buffers. The registers are: reg A, reg B, the accumulator, the
program counter, and the instruction register. They all have very specific
jobs to do; for example, the accumulator holds all results of maths-type
operations. The purposes of the other registers will be revealed as you
read on. There are two other important units on the CPU chip; the
AL U or arithmetic logic unit, which does what it says: arithmetic and
logic jobs, like adding, subtracting, ANDing, and ORing. Then there
are the clock and timing, and the instruction register circuits. These
circuits provide the RD, WR, ALE signals and also signals to enable the
register buffers.
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