Worst 8bit CPU

This is a von Neumann 8bit CPU and has a tiny instruction set.

Specifications

• 7 not powerfull instructions: NOT, OR, AND, ADD, JMP, SEL, CC
• 3 internal 8bit registers
• 256 Bytes of RAM
• 256 Bytes of ROM for programs
• 8 bit parallel output

Instruction set

Instructions

Instruction	Operand	Description	Cycles
NOT	A	Not A	1
OR	A	Or A	1
AND	A	And A	1
ADD	A	Full 8 bit adder	1
JMP	A	Absolute jump to A address in ROM	1
SEL	A	Expose address A of RAM	1
CC	A	Clear Carry flag	1

All instructions are 1 byte long and the operand is 1 byte. How compute "A OR B"?

By using the accumulator modifier that allow to use the accumulator as operand. Let's define "new" instructions:

Instruction	Operand	Cycles
NOT_a	A	1
OR_a	A	1
AND_a	A	1
ADD_a	A	1

So A OR B is:

AND_a 0x00; // clear the accumulator
OR_a A;
OR_a B;

Therefore in practice A OR B take 3cycles.

OPCODE

Instruction	Opcode
NOT	0x00
OR	0x01
AND	0x02
ADD	0x03
JMP	0x20
SEL	0x08
CC	0x10
NOT_a	0x04
OR_a	0x05
AND_a	0x06
ADD_a	0x07

For JMP the operand is the address to jump to. Value must be even. (if odd it can execute data as instruction, that can be dangerous...)

For SEL the operand is the address of the RAM to expose.

Architecture

Von Neumann architecture with 3 internal registers and 256 Bytes RAM and 256 Bytes ROM.

CPU

Program Counter

Program counter is a 8 bit loadable counter with clear input. The overflow output has no purpose is this version. If the program is designed correctly, the program counter will either never overflow, or design to seamlessly from 0xFF address 0x00.

Counter use JKFlipFlop.

Fetcher

The fetcher read ROM and alternatively write in instruction or data register. Even address are for instructions and odd for data.

Instruction Decoder

The instruction decoder decode the instruction and set the control signals. Is divided in two part, one for ALU and one for flow control.

This is an heterogenous instruction decoder. This helps to alleviate the overwhelming aspects of the design. Only ALU use demux decoder, other control flow use One-hot decoder.

Compute unit

This unit is the heart of the CPU. It is composed of the ALU, internal register and the RAM.

ALU

The ALU is a simple 8 bit ALU with 4 operations: NOT, OR, AND, ADD. ALU instruction decoder is directly connected to control logic, It is a 4 to 1 demux.

Logic Unit

Logic unit is responsible for NOT, OR and AND.

Full 8bit Adder

This is a 8 bit adder with carry in and carry out. No more, no less.

Register

The first register 0x00 is the output register

RAM

Ram with 8 bit address register on the left.