Problem 1: CPU.circ
Build a 4-bit single cycle CPU that can implement the given instructions.
Instruction Format
Our CPU will be using fixed length instructions. Our CPU will also have two types of instruction formats: R-type and I-type. In R-type instructions both operands come from registers. In I-type instructions, the first operand comes from a register and the second will be contained within the instruction.
R-Type
| Name | Bits | Description |
| OpCode | 15 – 12 | Determines what operation should be performed |
| C | 11 – 8 | The destination register. The C in RegC = RegA OP RegB |
| A | 7 – 4 | The first source register. The A in RegC = RegA OP RegB |
| B | 3 – 0 | The second source register. The B in RegC = RegA OP RegB |
I-Type
| Name | Bits | Description |
| OpCode | 15 – 12 | Determines what operation should be performed |
| C | 11 – 8 | The destination register. The C in RegC = RegA OP Imm |
| A | 7 – 4 | The first source register. The A in RegC = RegA OP Imm |
| Immediate | 3 – 0 | The second source register. The Imm in RegC = RegA OP Imm |
Instructions
| Operation | Encoding (The value in the OpCodeField) | Description |
| STOP | 0000 | The CPU ceases execution |
| NOP | 0001 | Do nothing |
| LOAD | 0010 | RegC = Immediate |
| MOVE | 0011 | RegC = RegA |
| ANDR | 0100 | RegC = RegA AND RegB |
| ANDI | 0101 | RegC = RegA AND Immediate |
| ORR | 0110 | RegC = RegA OR RegB |
| ORI | 0111 | RegC = RegA OR Immediate |
| XORR | 1000 | RegC = RegA XOR RegB |
| XORI | 1001 | RegC = RegA XOR Immediate |
| NOT | 1010 | RegC = NOT RegA |
| NEGATE | 1011 | RegC = -RegA |
| ADDR | 1100 | RegC = RegA + RegB |
| ADDI | 1101 | RegC = RegA +Immediate |
| SUBR | 1110 | RegC = RegA – RegB |
| SUBI | 1111 | RegC = RegA – Immediate |
Inputs
| Pin | Size (in bits) | Explanation |
| Instruction | 16 | The instruction located at Instruction_Address |
| ClkIn | 1 | The Clock. Connect this to the clock ports of your registers/flip-flops. Do nothing else with this. |
Outputs
| Pin | Size (in bits) | Explanation |
| Instruction_Address_Out | 5 | The address of the instruction you want to execute |
| Reg0-15 | 4 | The values in the register file. This has already been connected for you |
CPU Components
Your CPU should have
- A Program Counter (PC)
- This stores and keeps track of what instruction you are on
- Instruction Decoder
- This is a bunch of combinational logic that sets the control signals inside of your CPU
- Register File
- A bunch of registers as well as ways to specify which ones you want. This has already been created for you.
- The only outputs of the register file that you are allowed to use are A_Out and B_Out. The rest are for testing purposes and should not be used. Using them will result in a 50% penalty in your grade.
