Ben Eater CPU Simulation

This is a simulation of Ben Eater's Breadboard CPU in Logisim.

In order for this simulation to run properly, you will need Logisim Evolution (Holy Cross Edition). Once you have installed Logisim, you can open the "NearExact.circ" file from within the simulation. The EaterCPU.cpu file is for use with customasm.

Notes

• This simulation will only work in Logisim Evolution Holy Cross Edition.
• The CPU is made to be as exact as possible given the limitations of the software used to make the simulation.
  • Also, some signal flags were added into the circuit before Ben Eater added them to his breadboard processor. So the control word differs a bit.
• Simulation performance will vary from user to user. I was able to get a maximum clock frequency of just over 500 Hz.

Sample Programs

• CountUp will count from 0 to 255 and then wrap back around to zero.
• CountDown will count from 255 to 0 and then wrap back around to 255.
• AddThree will count by 3 from 0 to 255 and then wrap back around to 0.
• Double will add the previous result to itself, then wrap back around to 1.
• Fibonacci will count the fibonacci number from 1 to 255 and then restart.

Output Decoder

The OnesPlace, TensPlace and HundredsPlace files contain the respective data for each decimal place on the seven-segment display, just in case the simulation starts off without any data in the respective ROMS.

Microcode

The Microcode ROM is a 17-bit ROM with bit 0 being for the Jump if Carry flag. This is what makes the CPU Turing-complete. Bits are Highest to lowest going from left to right.

Permissions

• You have permission to fork/clone this repository and do with the clone/fork as you please as long as you give me credit for the original simulation.
• Only create pull requests if your fork intends to keep the simulation up to date.

Instruction Set

The instruction set of this simulated processor is exactly the same as Ben Eater's processor. The upper half of the opcode (bits 4-7) is the instruction code. The lower half of the opcode is a 4-bit address or 4-bit number. The opcode listing is in hexadecimal notation.

Instruction	Opcode	Description
NOP	0x	Do nothing. Operand x is ignored.
LDA x	1x	Loads the A register from address x.
ADD x	2x	Loads the value at address x into the B register, adds B to A, and stores the result in the A register. Sets or clears the carry and zero flags.
SUB x	3x	Loads the value at address x into the B register, subtracts B from A, and stores the result in the A register. Sets or clears the carry and zero flags.
STA x	4x	Stores the contents of the A register into memory at address x.
LDI x	5x	Loads x into the lower half of the A register.
JMP x	6x	Unconditionally jumps to address x and continues execution from there.
JC x	7x	Jumps to address x if the carry flag is set and continues execution from there.
JZ x	8x	Jumps to address x if the zero flag is set and continues execution from there.
OUT	Ex	Copies the contents of the A register into the output register. Operand x is ignored.
HLT	Fx	Halts execution of the processor until a reset signal is received. Operand x is ignored.