Microcomputer Circuits and Processes

Fifth EXECUTE cycle MOV Acc-+M, clock period 1
The result of the addition, now in the accumulator, must be written
back into memory at address 0111. The 'Move accumulator into
memory' instruction achieves this. Remember that the address of the
memory location where the number is to be written is contained in
register A. So, during clock period 1 of this instruction, the instruction
decoder enables register Ns buffer which puts the number 0111 on to
the bus. The instruction decoder also sends an ALE signal to memory
which latches 0111 into the address latch. Memory location 0111 is now
being addressed. This is the location that will receive the result of the
Figure 3.24 addition.
CPU
MEM
o
o
1 1
o 1
Fifth EXECUTE cycle MOV Acc-+M, clock periods 2 and 3
During these last two clock periods of this EXECUTE cycle, the data in
the accumulator is moved into memory location 0111. The instruction
decoder enables the accumulator's buffer, so putting the accumulator's
contents, 1011,on to the bus. Also the instruction decoder sends a write
(WR) pulse to memory which writes the data 1011 into memory
location 0111, this address being stored in the address latch. Note that
the number 1011 is written over the previous occupant of this address,
Figure 3.25 the number 0110, without first having to clear out the memory location.
CPU
MEM
41