Microcomputer Circuits and Processes

Third EXECUTE cycle ADI, clock period 3
If you look at the structure of SAM's CPU, you will see that the arithmetic
logic unit (ALU), which is about to perform the addition, works
on two numbers, one in the accumulator and the other in register B.
That number must get into register B, and that is what happens during
this clock period. The instruction decoder sends a read (RD) signal to
memory, reading number 0101 on to the bus. It also enables register B's
Figure 3.22 buffer, letting the number 0101 off the bus and into register B.
CPU
MEM
a
a
The program is now ready for the addition, but all three clock periods
of this EXECUTE cycle are used up. The instruction decoder knows
this (it knew it as soon as the instruction was fetched), and so it enters a
new EXECUTE cycle.
Fourth EXECUTE cycle ADI, clock periods 1, 2, and 3
In this second EXECUTE cycle for the instruction ADI, the instruction
decoder, together with clocK and timing circuits, move the numbers
from register B and the accumulator, ask the ALU to add them and put
their sum into the accumulator. The accumulator now contains 0110
+0101 which is 1011. Note that in SAM there is no provision for
overflows such as you would get if you instructed SAM to add 1111
Figure 3.23 +0001. Real microprocessors do have such a provision.
CPU
MEM
40