STEPLA-1 8-bit Hardwired CPU

A complete 8-bit Harvard architecture CPU designed and simulated in Logisim-Evolution, built entirely from individual logic gates down to the gate level.

Version: 2.4.2
Status: Under Development

What Is STEPLA-1?

STEPLA-1 is a fully functional 8-bit CPU where every component; registers, decoders, ALU, control unit is built from individual 74-series logic gates. No black box components. Every signal path is visible, traceable, and documented.

Unlike EEPROM-based designs, STEPLA-1 uses a fully hardwired control unit a gate-level AND/OR matrix where every control signal is a physical gate whose inputs you can probe with a multimeter. This makes the machine transparent in a way that microcode-based designs cannot be.

Key Features

Architecture

• 8-bit Harvard architecture
• 4-bit opcode space, 16 instructions
• 4 general purpose registers (RA-RD)
• Separate instruction and data RAM (256 bytes each)
• Bootstrap Control Unit (BCU) for cold-boot ROM → RAM transfer

Control Unit

• Fully hardwired PLA-inspired gate matrix
• Hierarchical decode: opcode decoder + step decoder + AND matrix
• No EEPROM, no microcode pure combinational logic
• Every control signal traceable to individual gates

Performance

• Variable cycle instructions: 3 to 5 clock cycles
• Early-exit conditional branching (25% latency reduction)
• Synchronous load-to-one reset (33% efficiency gain over v2.3)
• Calculated IPC: 0.200-0.333, weighted average 0.263
• Target clock: 4 MHz on physical breadboard
• Effective throughput: ~1 MIPS at target frequency

Instruction Set

Why STEPLA-1 Is Different

Most educational CPU projects use one of two approaches:

• EEPROM microcode (Ben Eater's SAP-1): Simple to implement, opaque in operation
• HDL simulation (VHDL/Verilog): Powerful but abstracted from physical reality

STEPLA-1 takes a third path: discrete gate simulation that maps directly to physical components. Every Logisim gate corresponds to a real 74-series IC you can buy at an electronics market. The simulation IS the schematic.

Comparison with SAP-1:

Repository Structure

STEPLA-1/
├── simulation/
│   ├── STEPLA-1.circ          # Main Logisim-Evolution file
│   ├── control_unit.circ      # Control unit subcircuit
│   ├── register_file.circ     # Register selector subcircuit
│   └── BCU.circ               # Bootstrap control unit
├── programs/
│   ├── fibonacci.asm          # Fibonacci sequence demo
│   └── counter.asm            # Basic counter program
├── docs/
│   └── STEPLA-1_Spec_v2.4.2.pdf  # Full specification
└── README.md

Getting Started

Requirements

• Logisim-Evolution v3.8.0 or later

Running the Simulation

1. Clone this repository
2. Open simulations/8bitcomp.circ in Logisim-Evolution
3. Load programs/fibonacci.asm into the ROM
4. Toggle switch to 1 and press reset button.
5. Run the clock at desired frequency.

Loading a Program

Programs are loaded via the Bootstrap Control Unit automatically. On simulation start the BCU copies the instruction ROM contents to instruction RAM before releasing control to the CPU. See Section 8 of the specification for BCU operation details.

Documentation

The full 43-page specification covers:

• Control unit theory and PLA architecture
• Complete ISA with encoding rules
• Every instruction's T-state microoperation sequence
• Clock phase synchronization and dual-edge design
• BCU boot protocol and T0 null state
• Signal conditioning for physical breadboard build
• Timing analysis with real component datasheets
• Known limitations and v3.0 roadmap

📄 [Read the Full Specification](/STEPLA-1 Control Unit Design Specifictation Manual v2.4.2.pdf)

Physical Build

STEPLA-1 is designed for physical construction using 74HCT series logic:

Key components:

• Logic: 74HCT08, 74HCT32, 74HCT14, 74HCT86
• Registers: 74HCT377, 74HCT175, 74HCT74
• Decode: 74HCT154, 74HCT138, 74HCT139
• Counter: 74HCT163
• ALU: 74HCT283 (×2 cascaded)
• Memory: AS6C62256 SRAM, AT28C64B EEPROM
• Bus: 74HCT244, 74HCT245

Target clock speed: 4 MHz (verified via timing analysis, critical path: 101ns, half-cycle budget: 125ns at 4 MHz)

Roadmap

v2.5.0 (planned)

• Overflow flag (VF) and Negative flag (NF)
• Signed arithmetic support
• JV and JN conditional jump instructions
• Proteus ISIS simulation with real component models

v3.0.0 (planned)

• 16-bit instruction word
• 16-register general purpose file
• Hardware stack (PUSH/POP/CALL/RET)
• Dual asynchronous control units
• Pre-fetch buffer approaching 1.0 CPI

Acknowledgments

This project builds on the work of:

• Ben Eater — whose 8-bit breadboard computer series demonstrated that CPU design can be made completely transparent
• Leon Nicolas — whose cascaded RAM implementation in Logisim provided the structural insight that led to STEPLA-1's hardwired control matrix
• Albert Paul Malvino — Digital Computer Electronics
• Patterson and Hennessy — Computer Organization and Design
• Charles Petzold — Code: The Hidden Language of Computer Hardware and Software

Contributing

Contributions welcome. Areas of particular interest:

• Physical breadboard build documentation
• Additional assembly programs
• Assembler implementation
• Proteus simulation port
• v3.0 architecture discussion

Please open an issue before submitting large changes.

License

This project is open source. MIT License