LISA

LISA (inspired by LLVM ISA) is a small synthesizable processor whose architectural instruction stream is a simplified LLVM-like bytecode format.

Directory Layout

• run.sh: command-line runner for local checks/build/simulation and CI
• synth/synth.ys: Yosys synthesis script for the top module
• synth/reports/: synthesis netlist and logs
• rtl/lisa_defs.vh: shared opcode/uop constants
• rtl/lisa_imem.v: byte-addressable instruction memory with fetch window
• rtl/lisa_fetch_unit.v: variable-length instruction fetch helper
• rtl/lisa_decoder.v: decodes opcodes, operands, immediate fields, branch targets, PHI tags
• rtl/lisa_ssa_regfile.v: fixed-size SSA ID -> register storage
• rtl/lisa_int_alu.v: integer add/sub/mul
• rtl/lisa_lsu.v: load/store unit glue
• rtl/lisa_data_mem.v: byte-addressable little-endian data memory
• rtl/lisa_control_flow_unit.v: branch/jump/return next-PC logic
• rtl/lisa_bytecode_core.v: top-level FSM (fetch -> decode -> execute -> writeback)
• tb/tb_lisa_bytecode_core.v: self-checking example testbench

Simplified Bytecode Encoding

All instructions start with:

• byte 0: opcode
• byte 1: instruction length (bytes)

Supported instructions:

• iconst: [01][07][dest][imm0][imm1][imm2][imm3]
• add: [02][05][dest][srcA][srcB]
• sub: [03][05][dest][srcA][srcB]
• mul: [04][05][dest][srcA][srcB]
• load: [05][04][dest][addrSSA]
• store: [06][04][dataSSA][addrSSA]
• br: [07][09][condSSA][tLo][tHi][fLo][fHi][tagT][tagF]
• jmp: [08][05][targetLo][targetHi][tag]
• ret: [09][03][srcSSA]
• phi: [0A][07][dest][srcA][srcB][tagA][tagB]
• halt: [FF][02]

LLVM Concept Mapping

• LLVM SSA value IDs map directly to entries in the SSA register file (rtl/lisa_ssa_regfile.v).
• phi uses a predecessor-edge tag (pred_tag) updated by branch/jump instructions.
• Control flow is modeled with explicit bytecode targets and tags.
• Memory operations use a byte-addressable data memory and 32-bit little-endian loads/stores.

Running from the Command Line

chmod +x run.sh
./run.sh check
./run.sh sim
./run.sh synth

Expected test result:

• return value = 100
• data memory at address 0x0010 = 12

Useful commands:

• ./run.sh lint: Verilator lint
• ./run.sh check: Yosys structural RTL check
• ./run.sh build: build simulation binary
• ./run.sh sim: build + run testbench
• ./run.sh synth: run Yosys synthesis (synth/synth.ys)
• ./run.sh ci: CI entrypoint (lint + check + sim + synth)
• ./run.sh clean: remove simulation artifact

Synthesis artifacts:

• Netlist: synth/reports/lisa_bytecode_core_synth.v
• Log: synth/reports/synthesis.log

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