Enhance the 8-Bit CPU
We have learned an 8-bit CPU in the last lab. The ALU of the 8-bit CPU has
operations such as AND, NOT, and XOR. Can you
enhance the ALU by adding more operations?
2. The 8-bit CPU:
Obtain the following 8-bit CPU working circuit file:
The circuit just obtained is a 8 bit version of the 1 bit CPU we built
in the previous lab with a few additions:
- there is now a 256 x 8 bit memory attached to the bus through
a Memory Address Register(MAR)
- there is now a Program Counter (PC) with a built in increment
by one function.
- the ALU has been expanded to perform the invert operation on
- all the switches have been grouped into one area to simulate a
3. More Explanations:
- The circuit is simply an extension of what you have already done.
Notice the similarities e.g. temp, accumulator, IR, B and C registers,
and the external input.
- Notice the differences. For example, the memory has a memory address
there is a decoder associated with the ALU;
and there is a PC and an adder to produce a PC+1 value.
- The difference between absolute and immediate addressing:
- In absolute addressing, the operand is located in
the address specified in the instruction.
For example: NOT 5,B means NOT (or invert) the contents
of location 5, and save it in register B.
- In immediate addressing, the operand is specified in
For example: AND #5,B means AND the value 5,
with register B and save the result in B.
Many devices may be connected to a common bus by using tri-state buffers.
4-bit tri-state buffers are used in the 8-bit CPU circuit to control the
8-bit data flow.
Copyright: Department of Computer Science, University of Regina.