Microcomputer Circuits and Processes

INTERPRETING
THE INSTRUCTIONS
The instruction 1111 (addition) is fetched from memory and is loaded
into the instruction register. Instructions always go into this register.
From here, they pass into the instruction decode logic. The number
1111 is interpreted as an addition, so an enable signal is sent, for
example, to the ALU to tell it to do an addition. The actual guts, the
logic circuits inside the instruction decoder and inside the ALU, are too
complex to describe here, but if you are keen, get hold of data sheets for
the CD40181 or CD4057 circuits, made by RCA, and have a look at
these.
A SELECTION FROM SAM'S INSTRUCTION SET
Here is how to write a small program for SAM to add two numbers.
This program will be made up of a few simple instructions. We must use
the 1111 addition instruction, but also we will need other instructions
to move data in and out of memory. Begin by looking at a MOVE
instruction, represented by 0010.
move immediate data
to reg A
i
what the
instruction does
MVIA 0010
1
mnemonic
i
op-code
The 4-bit number 0010, which the instruction decoder interprets as
a 'move', is called the operation-code or op-code. The abbreviation for
the move, MVI A, is called the mnemonic. But what does this particular
MOVE instruction do?
When the CPU fetches 0010 from memory, the instruction decoder
tells it that it must look at the immediately following memory location.
There, it will find a 4-bit number, say 1100, which it must move into
CPU register A. This is illustrated in figure 3.6.
PC ~ 0 0 1 0 MVI A instruction
1 1
0 0 data
reg A
:;-1 1 1 0 01
Figure 3.6
MVI A instruction. The instruction 0010 causes the next number, 1100, to be moved into
register A.
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