Microcomputer Circuits and Processes

Execute
cycle number
The program in detail looks like this:
The number 0111 is put into reg A, where it will be used as an address in
the MOV M -+ Acc instruction in EXECUTE cycle 2, and the
MOV Acc -+ M instruction in EXECUTE cycle 5.
2 The contents of memory whose address 0111 is in reg A is moved into
the accumulator. So 0110 (6 10 ) ends up in the accumulator.
3 and 4 The number immediately following the ADI instruction, the number
0101 (5 10 ), is added to the accumulator, and the results stored in the
accumulator.
5 The accumulator contents 1011 (11 10 ) are moved to memory whose
address (0111)is in reg A.
6 Stop. The computer comes to a halt.
The memory location referred to by reg A which initially held the
number 6 10 , ends up receiving the result (11 10 ).
RUNNING THE PROGRAM
Now you are familiar with SAM's instructions, and have seen the
assembled program, we come to the real task of this chapter: looking in
detail at data flows and signal changes as the program is run.
Remember from Chapter 2 that the CPU contains a clock, and three
clock periods are needed to carry out either a READ or a WRITE
operation. These two operations are all that is needed to run the
program. Remember that the instructions live in memory, so each
instruction has to be FETCHed before it can be EXECUTEd. This
gives us two cycle types, FETCH and EXECUTE, which run in
sequence as shown in figure 3.11.
1 clock
period
~
CPU clock
FETCH I EXECUTE FETCH I EXECUTE FETCH
1st instruction
)!<
2nd instruction '3rd instruction
Figure 3.11
Showing how each instruction is made up of FETCH and EXECUTE cycles, which in
turn are made up of three CPU clock periods.
What actually goes on during the EXECUTE cycle depends on
what instruction is being executed. But the FETCH cycles are always
the same for any instruction; so begin by looking at a fetch cycle.
33