Microcomputer Circuits and Processes

The accumulator is a very useful register, since it has access to the
ALU, and any data put into the accumulator can then be subjected to
arithmetic and logic operations. So instructions to move data in and
out of SAM's accumulator are vital. First, how to get data into the
accumulator from memory.
move memory data into Mav M-+Acc 0110
accumulator, via reg A
In the same way as the ADI instruction, the accumulator is expected to
receive data. But, unlike the immediate instruction, the data is not in the
following memory location. So where is it? The address of the data to be
moved into the accumulator is contained in register A. In figure 3.9,
register A has previously been loaded with the number 0111. When the
CPU executes an Mav M -+Acc instruction, it first looks at register A
to find out which memory location contains the data. The CPU then
gets the data, here 0001, from that location, and puts it into the
accumulator. This way of moving data is quite useful, since data may be
got from any memory location using just the one Mav M -+ Acc
instruction, by changing the address stored in register A.
PC 0 1 1 0 1 instruction MOV M-Acc found
0101
011 0
0111 000 1
l'
memory
addresses
accumulator
3 data moved into
accumulator
Figure 3.9
The instruction MOV M-+Acc. Note how register A must be interrogated, to get the
address from where the data is to be obtained.
Perhaps a second example of this way of doing things will help you
see what is going on. This is the reverse instruction to the above, moving
data out of the accumulator into memory.
move accumulator data MaV Acc-+M 0111
into memory, via reg A
Here the number stored in the accumulator, which could be the
result of an arithmetic operation, is to be put into memory somewhere.
The destination of the data is an address stored in register A. Look at
figure 3.10. Here the number 0011 in the accumulator has to be moved
into memory. The address where it is to go, 0101, has been previously
31