MyCustomISA

MyCustomISA is a custom 16-bit CPU architecture designed to run on both an FPGA and a virtual machine. It is an educational project aimed at exploring the fundamentals of CPU design from the ground up — word size, memory mapping, registers, flags, interrupts, and instruction sets — with every decision documented and justified.

The CPU targets a 64KB address space, drives a 128×128 black-and-white display, and is simple enough to implement end-to-end while remaining powerful enough to run real assembly programs.

---

Table of Contents

1. Designing the Memory Map

   • Word size and address space
   • Memory regions (code, stack, framebuffer, MMIO)
   • Boot state and register initialisation
   • Flags and the interrupt system
   • Complete 64KB memory map

2. Designing the Instruction Set

   • Instruction encoding in 16-bit words
   • Operand types and addressing modes
   • Arithmetic, logic, and control flow instructions
   • PUSH / POP and the call convention
   • MVINT and the trap instruction

3. Writing the Assembler [TODO]

   • Tokenising and parsing assembly source
   • Resolving labels to concrete addresses
   • Emitting binary output

4. Building the Virtual Machine [TODO]

   • Fetch–decode–execute loop
   • Emulating MMIO (keyboard, display, timer, debug)
   • Interrupt dispatch and context save/restore
   • Debugger interface and state inspection

5. FPGA Implementation [TODO]

   • Top-level architecture and clock domain
   • Datapath and control unit in HDL
   • Mapping MMIO to physical peripherals
   • Synthesis and timing closure

---

MyCustomISA — a custom 16-bit ISA for FPGA and virtual machine implementation.