In this Project , we were tasked with designing a 4-bit processor capable of executing a set of instructions. To achieve this, we utilized and extended previously developed components, including:
- 4-bit Add/Subtract unit
- 3-bit Adder
- 3-bit Program Counter (PC)
- k-way b-bit Multiplexers
- Register Bank
- Program ROM
- Instruction Decoder
- 7-Segment Display
- Slow Clock
This group project involved distributing workload among team members, designing components, and refining them to ensure functionality. The processor was successfully implemented and tested on the Basys3 board.
This is a simple microprocessor capable of executing a simple set of instructions. In order to build the microprocessor, we developed and extended the following components.
- 4-bit Add/Subtract unit - This unit have the ability to add and subtracting numbers represented using 2’s complement and this was built by modifying 4-bit RCA.
- 3-bit adder – This unit is used to increment the program counter. This was also built by modifying 4-bit RCA.
- 3-bit Program Counter (PC) - Program Counter is used to keep track with the next instruction to be executed. This was built by using three D-flipflops and it can be reset to 0 when required.
- Multiplexers – Here we have used a set of k-way b-bit multiplexers to enable the components in the microprocessor.
- Register bank – There are eight registers in the register bank and each one can store 4-bit value at a time. 3-to-8 Decoder in the register bank select which register to be enabled using the “register enabled signal”.
- Program rom – This is the component which stores the Assembly Program. As the microprocessor only understands machine language, we hard coded the instructions as binary values in the program rom.
- Instructions Decoder - The main function of the instruction decoder is activating necessary components based on the instructions we wish to execute.
The processor supports the following instructions:
| Instruction | Description | Format |
|---|---|---|
| MOVI R, d | Moves immediate value d to register R. |
1 0 R R R 0 0 0 d d d d |
| ADD Ra, Rb | Adds values in registers Ra and Rb and stores the result in Ra. |
0 0 Ra Ra Ra Rb Rb Rb 0 0 0 0 |
| NEG R | Computes the 2's complement of register R. |
0 1 R R R 0 0 0 0 0 0 0 |
| JZR R, d | Jumps to address d if the value in register R is zero; otherwise increments the program counter. |
1 1 R R R 0 0 0 0 d d d |
The processor was extended with the following operations:
| Feature | Description |
|---|---|
| JMP | Jumps to a specific program ROM line unconditionally. |
| Comparator | Compares two 4-bit binary values and outputs 1 if they are equal. |
| Multiplier | Multiplies two 4-bit binary values and outputs the result. |
| Complement | Computes the 1's complement of a 4-bit binary value. |
Extended Instruction set
| Instruction | Description | Format |
|---|---|---|
| MOVI R, d | Moves immediate value d to register R. |
1 0 R R R 0 0 0 d d d d |
| ADD Ra, Rb | Adds values in registers Ra and Rb and stores the result in Ra. |
0 0 Ra Ra Ra Rb Rb Rb 0 0 0 0 |
| NEG R | Computes the 2's complement of register R. |
0 1 R R R 0 0 0 0 0 0 0 |
| JZR R, d | Jumps to address d if the value in register R is zero; otherwise increments the program counter. |
1 1 R R R 0 0 0 0 d d d |
| MUL Ra, Rb | Multiplies values in registers Ra and Rb and stores the result in Ra. |
1 1 0 Ra Ra Ra Rb Rb Rb 0 0 0 0 |
| JMP R, d | Jumps to dth instruction unconditionally. |
1 0 0 0 0 0 0 0 0 0 d d d |
| NOT R | Computes the complement of register R. |
1 1 1 R R R 0 0 0 0 0 0 0 |
| COM Ra, Rb | Compares values in registers Ra and Rb and outputs a result to LED. |
1 0 1 Ra Ra Ra Rb Rb Rb 0 0 0 0 |
- Center Button: Resets the processor. Hold for 2-3 seconds to reset all registers and the program counter.
- LED0-LED3: Displays the value in Register 7.
- Seven-Segment Display: Outputs the value in Register 7.
- LED15: Overflow flag.
- LED14: Zero flag.
- LED07: Comparator flag.
All simulation diagrams and VHDL codes are available in the following PDF: