Microcomputer Circuits and Processes

Second EXECUTE cycle MOV M--+Acc, clock period 1
The instruction 'Move the contents of memory into the accumulator'
has been fetched into the instruction register. This has code 0110. Now
this instruction must be executed. Remember that the address from
where the data must be read is held by register A. So this address must
first be loaded into the address latch. To do this, the instruction decoder
enables register Ns buffer, thus putting the address DIllon to the bus.
The instruction decoder also sends out an ALE signal which loads this
new address into the address latch. Memory location 0111 is thus
Figure 3.18 addressed.
CPU
MEM
001 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
Second EXECUTE cycle MOV M--+Acc, clock periods 2 and 3
Here, in two clock periods, the contents of memory location 0111,
which is the number 0110 (6 10 ), is moved· into the accumulator. The
instruction decoder sends a read (RD) signal to memory putting the
number 0110 on to the bus, and then it enables the accumulator's buffer,
allowing the number 0110 to pass into the accumulator. This is the end
of the second EXECUTE cycle, which has loaded 0110 into the
Figure 3.19 accumulator, as directed by register A.
CPU
MEM
o
o
38