# Sparkle HDL Development History

This document tracks the development phases and implementation milestones of Sparkle HDL.

## Phase 7: Example CPU & Formal Verification (Complete)

**Goal**: Demonstrate real-world hardware design with formal verification

**Completed Components**:
- **Sparkle-16 CPU**: 16-bit RISC processor with 8 instructions
- **ISA Definition**: Complete instruction encoding/decoding (LDI, ADD, SUB, AND, LD, ST, BEQ, JMP)
- **ALU**: Arithmetic Logic Unit with 9 formal correctness proofs
- **Register File**: 8 registers with R0 hardwired to zero
- **Memory Interface**: Instruction/data memory with SimMemory and SRAM modules
- **CPU Core**: Complete fetch-decode-execute state machine with simulation
- **Verification Framework**: ISA correctness, ALU proofs, instruction classification
- **Example Programs**: Arithmetic operations and control flow demonstrations

**Verification Status**:
- ✅ ALU correctness proven (9 theorems)
- ✅ ISA opcode correctness (encode/decode bijection)
- ✅ ISA instruction classification (branches, register writes)
- ⏳ Full instruction encode/decode bijectivity (in progress)
- ⏳ State machine invariants
- ⏳ R0=0 invariant
- ⏳ Memory safety proofs

**Files Added**:
- `Examples/Sparkle16/ISA.lean` - Instruction set architecture
- `Examples/Sparkle16/ALU.lean` - Arithmetic Logic Unit
- `Examples/Sparkle16/RegisterFile.lean` - 8-register file
- `Examples/Sparkle16/Memory.lean` - Memory interface
- `Examples/Sparkle16/Core.lean` - CPU core with state machine
- `Examples/Sparkle16/ISAProofTests.lean` - ISA correctness tests
- `Sparkle/Verification/Basic.lean` - Fundamental BitVec lemmas
- `Sparkle/Verification/ALUProps.lean` - ALU correctness proofs
- `Sparkle/Verification/ISAProps.lean` - ISA encoding/decoding correctness

**Key Achievements**:
- First complete CPU implementation in Sparkle
- Real-world demonstration of formal verification in hardware
- Proof that hardware and verification can coexist in same codebase
- Example of complex state machine synthesis

## Phase 6: Primitive Module Support (Complete)

**Goal**: Support vendor-specific blackbox modules (ASIC/FPGA primitives)

**Implementation**:
- **Blackbox Support**: Declare technology-specific modules without defining them
- **Vendor Integration**: Support for ASIC/FPGA vendor libraries (TSMC, Intel, Xilinx, etc.)
- **Common Primitives**: Helper functions for SRAM, ROM, clock gating cells
- **Module Instantiation**: Seamless instantiation of primitive modules

**Features**:
- Technology-independent code with vendor-specific backends
- Memory blocks (SRAM, ROM, register files)
- Clock gating cells for power optimization
- IO pads and technology-specific cells
- Proper module instantiation in generated Verilog

**Files Added**:
- `Sparkle/Primitives.lean` - Primitive module support
- `Examples/PrimitiveTest.lean` - SRAM and clock gating examples

**Use Cases**:
```lean
-- Define SRAM primitive from vendor library
def sram256x16 : Module :=
  primitiveModule "SRAM_256x16" [
    ("addr", .input (.bitVector 8)),
    ("dout", .output (.bitVector 16)),
    ("clk",  .input .bit)
  ]
```

## Phase 5: Feedback Loops (Complete)

**Goal**: Enable stateful circuits with feedback paths

**Implementation**:
- **Signal.loop Primitive**: Fixed-point combinator for feedback loops
- **Counter Support**: Enable circuits where output feeds back to input
- **State Machines**: Support for stateful hardware designs
- **Loop Closure**: Automatic wire allocation and connection for feedback paths
- **Variable Mapping**: Scoped tracking of loop variables during compilation

**Features**:
- `Signal.loop` primitive for explicit feedback
- Automatic detection and resolution of cycles
- Proper initialization and reset semantics
- Support for complex state machines
- Clean Verilog generation with feedback wires

**Files Added**:
- `Examples/LoopSynthesis.lean` - Feedback loop examples

**Technical Details**:
- Uses fixed-point combinator semantics
- Generates temporary wires for feedback paths
- Maintains proper execution order in Verilog
- Supports nested loops and complex control flow

## Phase 4: Verilog Backend (Complete)

**Goal**: Generate synthesizable SystemVerilog from IR

**Implementation**:
- **Code Generation**: Clean, synthesizable SystemVerilog output
- **Type Mapping**: Lean types → Verilog types (logic, bit, packed arrays)
- **Operator Mapping**: IR operators → Verilog syntax
- **Register Generation**: Proper always_ff blocks with reset
- **Module Generation**: Complete modules with proper ports

**Features**:
- Clean, readable output matching hand-written Verilog style
- Proper indentation and formatting
- Support for all standard operators (arithmetic, logical, bitwise, comparison)
- Correct bit width inference
- Technology-independent code generation

**Files Added**:
- `Sparkle/Backend/Verilog.lean` - SystemVerilog code generator
- `Examples/VerilogTest.lean` - Verilog generation examples
- `Examples/FullCycle.lean` - Advanced examples (MAC, FIR filter, traffic light, FIFO)

**Advanced Examples**:
- Multiply-accumulate (MAC)
- 4-tap FIR filter
- Traffic light controller
- Shift register
- FIFO buffer

## Phase 3: Compiler (Complete)

**Goal**: Automatically compile Lean code to hardware IR

**Implementation**:
- **Primitive Registry**: Maps Lean functions (BitVec.add, BitVec.and, etc.) to hardware operators
- **Translation Kernel**: Translates Lean expressions to hardware IR using metaprogramming
- **Synthesis Commands**: `#synthesize` and `#synthesizeVerilog` for automatic compilation
- **Signal Synthesis**: Automatic compilation of `Signal.register` and `Signal.mux` primitives
- **Automatic Clock/Reset**: Detects registers and adds clock/reset inputs automatically

**Features**:
- Automatic translation of BitVec operations to hardware
- Support for let-bindings and local variables
- Proper handling of if-then-else (generates mux)
- Name hygiene with `_gen_` prefix for generated wires
- Error reporting for unsupported constructs

**Supported Constructs**:
- Combinational logic (arithmetic, logical, bitwise operations)
- Registers with `Signal.register`
- Multiplexers with `Signal.mux` or if-then-else
- Feedback loops with `Signal.loop`
- Constants and literals
- Let-bindings

**Limitations**:
- Complex higher-order functions may need manual IR construction
- Pattern matching requires desugaring
- Some dependent type features not supported

**Files Added**:
- `Sparkle/Compiler/Elab.lean` - Metaprogramming compiler
- `Examples/SynthesisTest.lean` - Automatic synthesis examples (BitVec)
- `Examples/SignalSynthesis.lean` - Signal-to-IR synthesis examples

## Phase 2: Netlist IR (Complete)

**Goal**: Create a compositional intermediate representation for hardware

**Implementation**:
- **Hardware Types**: Bit, BitVector, Array
- **AST**: Expressions (const, ref, op), Statements (assign, register, inst)
- **Circuit Builder**: Compositional monad (`CircuitM`) for building netlists
- **Name Hygiene**: Automatic unique wire naming with collision avoidance

**Features**:
- Typed IR with proper bit width tracking
- Support for all common hardware operators
- Register inference and state tracking
- Module composition and hierarchy
- Wire allocation and connection management

**Files Added**:
- `Sparkle/IR/Type.lean` - Hardware type system
- `Sparkle/IR/AST.lean` - Netlist AST
- `Sparkle/IR/Builder.lean` - Circuit builder monad
- `Examples/ManualIR.lean` - Manual IR construction examples

**IR Operations**:
- Arithmetic: add, sub, mul, div, mod
- Logical: and, or, xor, not
- Comparison: eq, ne, lt, le, gt, ge
- Bitwise: shl, shr, and, or, xor
- Other: mux, concat, slice, zeroExtend, signExtend

## Phase 1: Simulation (Complete)

**Goal**: Cycle-accurate functional simulation of hardware

**Implementation**:
- **Domain Configuration**: Type-safe clock domains with configurable period, edge, and reset
- **Signal Semantics**: Stream-based signals `Signal d α ≈ Nat → α`
- **Hardware Primitives**: `register`, `registerWithEnable`, `mux`, bundling
- **BitPack**: Type class for hardware-representable types

**Features**:
- Pure functional simulation semantics
- Functor/Applicative/Monad instances for Signal
- Type-safe clock domain tracking
- Simulation testing before synthesis
- Direct correspondence with hardware behavior

**Files Added**:
- `Sparkle/Core/Domain.lean` - Clock domain configuration
- `Sparkle/Core/Signal.lean` - Signal types and simulation
- `Sparkle/Data/BitPack.lean` - Hardware type serialization
- `Examples/Counter.lean` - Simulation examples

**Simulation Primitives**:
- `Signal.register` - Creates a register (delays by 1 cycle)
- `Signal.registerWithEnable` - Conditional register update
- `Signal.mux` - Multiplexer (if-then-else for signals)
- `Signal.bundle` - Combine multiple signals
- `Signal.unbundle` - Extract signals from bundle

## Initial Implementation

**First Commit**: Basic project structure

**Features**:
- Lean 4 project setup with Lake
- Initial module structure
- Basic type definitions
- Example placeholders

**Files Created**:
- `lakefile.lean` - Lake build configuration
- `Main.lean` - Entry point
- Basic directory structure

## Development Principles

Throughout all phases, Sparkle has maintained these principles:

1. **Type Safety First**: Leverage Lean's dependent types to catch errors at compile time
2. **Simulation Before Synthesis**: Always test designs functionally before generating hardware
3. **Clean Code Generation**: Generated Verilog should be readable and match hand-written style
4. **Proof Integration**: Verification should be seamless, not an afterthought
5. **Educational Focus**: Code should be clear and demonstrative of concepts
6. **Composability**: Build complex designs from simple, reusable components

## Statistics

- **Total Phases**: 7
- **Total Modules**: 15+ core modules
- **Example Designs**: 10+
- **Formal Proofs**: 15+ theorems
- **Lines of Code**: ~5000+ (excluding generated)
- **Supported Operations**: 20+ hardware operators
- **Development Time**: Incremental development with clear milestones

## Future Directions

Planned enhancements beyond Phase 7:

- **Vector Types**: Parameterized hardware `Vec n α` for regular structures
- **Module Hierarchy**: Multi-level designs with proper instantiation
- **Waveform Export**: VCD dump for GTKWave visualization
- **More Proofs**: State machine invariants, protocol correctness
- **Optimization Passes**: Dead code elimination, constant folding, CSE
- **FIRRTL Backend**: Alternative IR for formal verification tools
- **Testbench Generation**: Automatic test harness creation
- **Formal Equivalence**: Prove Verilog matches Lean semantics