Microcomputer Circuits and Processes

adding is represented by 1111. Figure 3.4 shows this instruction stored
somewhere in SAM's memory. A whole program, made up of more
instructions plus some data, is stored in memory as a long list of 4-bit
numbers.
other instructions
or data
-
4-bit memory
--0 1 1 0
1 1 1
-
--
1-
instruction
-- 0 1 0 1
0 1 1 1
ADD
Figure 3.4
A section of memory, showing the instruction for addition in one memory location.
How does the CPU actually execute a program? First, it must fetch
the instruction from the memory, by READing memory. But how does
it know which memory location to read next? This is the job of the
program counter register in the CPU, which holds a 4-bit number - the
address of the instruction to be dealt with next. When SAM is switched
on, the program counter (PC) is reset to 0000. As the program runs, so
the PC is incremented, usually fairly steadily, as shown in figure 3.5.
addresses
(in binary)
PC~
1------1
PC~
1-------1
~
0000
0001
0010
001 1
PC
~
points to zero
at power-up
... later ...
... later .. ,
Figure 3.5
Showing how the program counter points to memory locations. Low memory addresses
are near the top.
Some instructions like JUMP or CALL SUBROUTINE may send
the PC bouncing around anywhere within memory. Wherever the
instruction comes from, the CPU now has to interpret it, and execute it.
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