Exercise #9: Pico-CPU (Design and test your Pico-DPU)

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General Instruction
In this lab we design a simple 8-bit CPU. to go through the lab easily, we have divided your tasks into three different parts, introductory materials, preparation before entering the lab and the tasks that you have to perform during the lab.

Important: You MUST go through the preliminary readings before you start your lab.
Important: You MUST finish lab preparation before entering the lab for Pico-CPU.
Important: You MUST bring instruction table which you make during preparation, to lab on every session. (for labs 9,10 and 11)

Important Note: Please make sure you read Jiri Gaisler's "A structured VHDL design method". This document would be used as our coding reference throughout the labs. Your designs would are not acceptable if you do not follow 2 process architectrue as a design rule.

Design your Pico-DPU

According to our Instruction Set, our Pico-CPU should be able to perform the following operations:

DPU Flags and Comparator

We would like to have the following flags in our ALU: These flags will be used for jump operations in Control Unit. Some processors keep these flags in a separate register called "Status Register", which is accessible to the programmer, but for our processor, we will keep them only accessible through jump operations.

To implement Zero flag we need to be sure that whether all the bits are zero or not, we can just directly use logical AND of negative of all the accumulator register bits together (equivalent to NOR) and produce our zero signal as shown in figure 1.
Zero Flag
figure 1

Over flow happens when we are handling signed values and the result of operation is bigger than our bitwidth and this results in change in sign bit of the result. An example of this is adding 72 + 81 which should be 153 but since we have a signed representation, it will result in -103. An easy way to determine OV flag is to figure out if both inputs are positive or both are negative, then the result has the same sign. So if we are adding/subtracting two 8-bit signed operands namely A and B, and we get result R, then we can build our OV signal as:


Zero Flag
figure 1

Testing your DPU

To test our DPU, we need to provide control signals for different ALU operations along with data form control unit side and memory side. Block diagram of the test setup is show in figure 2.


DPU Test
figure 2

Questions

before you proceed, you should be able to answer the following questions.