Big

Computer

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Intro

A very simple CPU emulator written in c++, inspired by LMC for me to learn how a cpu works in more detail.

Current features:

• Fetch-decode-execute cycle
• 32-bit architecture (256 CPU instructions, not all implemented)
• A 16x16 pixels "monitor" with a 12-bit colour depth

Building

Make sure you have:

• g++
• cmake
• sfml (for pixel plotting)

$ git clone https://github.com/j4cobgarby/big-boy-computer.git
$ mkdir big-boy-computer/build && cd big-boy-computer/build
$ cmake ../src -G "<Insert your favourite Makefiles type here>"
$ make

Overview of the registers

int64_t ac; // Accumulator - stores the result of the last calculation
word_t pc; // Program counter - store address of the next instruction
word_t ir; // Instruction register - stores top 8 bits of instruction
sword_t ar; // Address register - stores bottom 24 bits of instruction

Writing a program in machine code

Introduction

Currently, to write a program for the big boy computer (bbc), you need to edit an array in main.cpp called program, which holds the instructions. Because of this, every change in the machine code program will mean you need to recompile bbc.

Starting

Have a look at INSTRUCTIONS (file) for a comprehensive list of all the CPU instructions along with a synopsis of an instruction.

A program must start with (i.e. the first word of a program in ram must be) a number representing how many pieces of data you want to initialise. The CPU then skips ahead this amount of instructions, assuming that they're pieces of data.

Example

Say you wanted to be able to access a memory address with the value of 10 (an ASCII newline), your program may look like this (base 2, note the padding zeroes are of course optional):

0000000000000000000000000000001
0000000000000000000000000001010
... (rest of program) ...
0000000000000000000000000000000

Or equally (base 16)

00000001
0000000a
... (rest of program) ...
00000000

And, annotated:

00000001    # Tell the processor that the next 1 word is data, not an instruction
0000000a    # This address in RAM is now 10
... (rest of program) ...
00000000    # And all programs must end with 0, which is the halt instruction

Writing your program to file

This is very simple:

• Get a hex editor
• Literally just type out your program's instructions as hex