EightBitCPUSim

Simulation of Ben Eater's 8 Bit CPU Build Project

Background

A friend of mine was working on Ben Eater's 8-Bit CPU Project, that implements an enhanced SAP-1 architecture, and they descided to create a web based simulater for the project

This is my take on the project. Included is a simple python assembler project that will output the textual binary for inclusion in the simmulator's cpu memory.

My friend has now created a web based assembler that I have included in this repository.

Assembler

The assembler is loosely based around the style and syntax of the Z-80 assembly language. It currently supports the following:

Comments

Lines that start with ; are considered comments and ignored.

Registers

The A register (Accumulator) is the only register available in the ld command. The output register is considered unique and populated via the out opcode which moves the contents of the Accumulator into the display register.

Behavior of ld - Load

There are three situations for the use of the load command:

ld a,3 Loads the value 3 into the accumulator. Uses the LDI opcode.

ld a,(X) Loads the value at memory location (X) into the accumulator. Uses the LDA opcode

ld (X),a Loads the value in the accumulator into memory location (X). Uses the STA opcode.

Labels and Branching

There is basic support for labels and jumps which allows for conditional branching. Labels start with : . Memory locations will be mapped by the assembler, allowing for code such as the following:

Start:
  ld a,1
  out
  jmp Foo
Bar:
  ld a,3
  out
  hlt
Foo:
  sub (Decrement)
  jz Bar
; This line is never executed
  hlt
Decrement:
  .byte 1

This program will output 1 then 2 then 3 since the zero flag is set. The following jumps are supported:

jmp Unconditional Jump

jc Jump if Carry-Flag is set

jz Jump if Zero-Flag is set

Variables

Variables may be declared after labels. The assembler will automatically assign a pointer and manage references when building the machine code. Variables must start with .byte after a label.

Counter:
  .byte 3
Y:
  .byte 6

Sample Program

The following computes 3 * 29 and displays the output:

; Multiplies X by Y
Top:
  ld a,(X)
  sub (Decrement)
  jc Continue
  ld a,(Product)
  out
  hlt
Continue:
  ld (X),a
  ld a,(Product)
  add (Y)
  ld (Product),a
  jmp Top

Decrement:
  .byte 1
Product:
  .byte 0
X:
  .byte 3
Y:
  .byte 29

Screenshot