Microcomputer Circuits and Processes

The FETCH cycles
FETCH cycles, which get instructions from memory, are each made up
of three clock periods. Preceding EXECUTE cycles, FETCH cycles
always involve the same movements of data and changes in control
signals, so we need to look at only one FETCH cycle. Figures 3.12 to
3.14 show what happens during each of the three clock periods making
up one FETCH cycle. In each of figures 3.12 to 3.25 shading shows
where data is being transferred, and where changes in registers are
happening.
FETCH cycle, clock period 1
When the computer is switched on, the program counter points to
memory location 0000. This is the place where the first instruction of
the program lives, in this case instruction 0010, which is a MOVE
IMMEDIATE instruction. So during clock period 1,the contents of the
program counter are put on to the bus by a signal from the clock and
timing circuits. These circuits also send an address latch enable (ALE)
signal to the address latch, which latches the address 0000 from the bus.
The address latch then points to address 0000.
CPU
MEM
, 0 0 1 0
o 1 1 1
o 1 1 0
1 1 1 1
o 1 0 1
o 1 1 1
o 0 0 0
o 1 1 0
Figure 3.12
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