In this final stage, we will complete our microprocessor.
Look at the circuit on the next page. This figure is the complete microprocessor circuit.
We have already designed all the blocks used in this circuit except for the “instruction
decode.” We will discuss that circuit in a minute. To begin with, let‟s look at the parts
of the circuit that were not in the circuit on lab D.
First‟ you will note that we have added more ROM memory. This additional memory is
needed so that there is enough memory to store a program, which is just a series of
instructions that the microprocessor is to perform. Instructions are given numeric
codes, so that they can be stored in memory. For our microprocessor, the first code in
a program is at address 0.
Many instructions, such as loading the number 3 into the accumulator, require data. In
this case, the data is the number 3. When instructions require data, the data follows
after the instruction code. Say, for example, that there is an instruction code at
address Q (where Q could be any address, 0 through F). If the instruction requires
data, the data will be at address Q+1. If the instruction requires more data, the
additional data will be at address Q+2. After the data (if any) for that instruction will
come the next instruction code. This next instruction may again require data, or it may
not. More instructions will follow until the program is complete. The possible memory
layouts of an instruction are illustrated below.