Pico CPU: Preparation

The main goals of this lab are:
  1. to build up an instruction set form a set of design requirements
  2. to implement your design in VHDL
  3. to test the functionality of the CPU
At the end of this lab you should be able to:
  1. building a full functional system step by step from a relatively abstractdefinition
  2. understand the idea of building a fully synchronous system
  3. setting up a test setting for your system
In This section, you will get the basic design requirement and will build up the Instruction Set Architecture of your Pico-CPU, and will draw the block diagram of your system.

Build up your CPU instruction set

We Would like you to build up a set of RISC instruction set based on our application requirements. In your Pico-CPU you dont have any kind of Pipeline, stack, instruction level parallelism, exception, or any kind of fancy stuff. The design requirements are:
  1. The instructions should consist of an Operation Code (Op-Code) and one operand

  2. we would like to design an 8 bit CPU, so the length of the operands are going to be 8 bits. The operand will be either a data or an address.

  3. Op-Code is 8 bits but 3 bits are reserved, see figure 1. So you have 32 available instructions to implement. but we don't need necessarily 32 operations. Implementing more that 32 instructions is not allowed.

    Instruction Format
    figure 1
  4. We would like to cover the following functionalities in our CPU:
    1. Addition/Subtraction
    2. Increment/Decrement
    3. Arithmetic and Logical Shift
    4. Bitwise AND, OR and XOR
    5. Negation
    6. Load/Store
    7. Unconditional Branch (jump)
    8. Branch if zero / Branch if Overflow
    9. Clear Registers/Flags
    10. NOP/HALT
  5. Your Pico-CPU's DataPath Unit, should have an accumulator register which we call AC. The results of our operations will be written back to AC after ALU calculation. You MAY have one more register in your DPU which is transparent to programmer but its not obligatory to have this register.
  6. There is no restrictions on how you implement the functionalities that are required. As an example, you can do subtraction by combining negation and addition. and there are lots of different approaches that you can take, And its totally up to you.
  7. Important: In our design we dont implement relative or indexed addressing. Only direct addressing is used.
Important: You MUST fill table 2 with your designed instruction set. We will collect these tables at the beginning of exercise session. Students who do not bring the instruction set table to the lab, will not be allowed to start the exercise.

Draw Block diagram of your design

In this section we need you to draw the basic block diagram of your design based on the design requirements of the previous section. we need the following diagrams to be presented when you come to the lab: (not only the wires but the direction of flow of data should be drawn)
  1. Draw the connections for Top level block diagram in figure 2.
    CPU Diagram Space
    figure 2

  2. Draw the connections for Control unit block diagram in figure 3.
    CU Diagram Space
    figure 3

  3. Draw the connections for DataPath unit block diagram in figure 4 (Note: you can either use or bypass the Register in Datapath).
    DPU Diagram Space
    figure 4

Questions

  1. How much do you think you can compress the instruction set (in other words, what is the lower bound on the number of instructions)?
  2. How will fewer and more simplistic instructions will effect programming at user end? give an illustrative example that removing one instruction will effect the code.
  3. What will be the effect of reducing the number of instruction on hard- ware side?
  4. What will you personally prefer to have, larger instruction set or smaller one? and why?
  5. How will you implement a multiplication operation with your proposed instruction set?
  6. How can you implement delays with your proposed instruction set?