Enhance the 8-Bit CPU

1. Purpose

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:

          ~ftp/pub/class/301/ftp/8bitcpu.cct

• 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 the Accumulator(ACC)
• all the switches have been grouped into one area to simulate a control unit.

3. More Explanations:

1. 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.
2. Notice the differences. For example, the memory has a memory address register;
   there is a decoder associated with the ALU;
   and there is a PC and an adder to produce a PC+1 value.
3. 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 the instruction.
     For example: AND #5,B means AND the value 5, with register B and save the result in B.

4. CONCLUSION

 
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.

5. Lab Assignments