My first Microprocessor design

Specifications

1.  RAM:                                    32*8 bits of RAM

2.  Data Bus:                               8 bit

3.  User Visible registers:             A, B, C, D

4. Control and status registers:     MAR, MBR, IR, PC, TEMP

5. ALU:                                      8 bit, performs Addition, complementing and logical NAND operation

6. Flag:                                       Zero, carry

The block diagram of the microprocessor is shown below:

The microprocessor is based on RISC approach. Hence it supports minimum number of instructions.

Instruction set

Instruction is 8 bit. The first 3 bits denote the type of instruction. The last 2 bits give the register address.

The instruction set is as follows

1. SFA memory:  Store from register A to given memory address

2. STA memory: Store to register A from given memory address

3. JMP memory:  Performs unconditional jump to the given memory address

4. ADD A, R:        Performs addition between the given register and register A and stores in register A

5. CMP:                                Complements the contents of register A

6. NAND A, R:    Performs logical NAND between register A and given register contents

7. MVI A, Data:  performs immediate addressing

8. MOV A, R:      Moves the data from register A to given register

The instruction decoder and the timing generator is shown below:

The combinational logic used to generate the required signal for the different latches present in the registers, ALU was drawn based on the micro-instruction analysis. Which I’ll add shortly.

The combinational logic thus designed is as shown below:

 This is just a simple design of a microprocessor. I'm trying to understand the more complex microprocessors present in today's market. And hope fully will design an efficient design someday!