[sql-hkr]

Summary

Closes #44.

This PR implements a frame-pointer based stack-frame model (using the AVR Y register pair R29:R28) for local variables and function parameters, enabling robust recursive function calls and preserving caller state.

Key Changes

• CPU & Assembler (cpu.py, assembler.py):
  • Implemented LDD Rd, Y+q / Z+q and STD Y+q / Z+q, Rr instructions for indirect addressing with displacement.
  • Supported reading/writing Stack Pointer via standard AVR I/O ports 0x3D (SPL) and 0x3E (SPH) in IN and OUT.
  • Added operand parsing for Y+q, Z+q in the assembler.
• Compiler (compiler.py):
  • Added _collect_locals to discover all function parameters and local variable declarations across blocks and statements.
  • Implemented function prologue: saves caller Y, reads SP into Y (IN R28, 0x3D; IN R29, 0x3E), allocates local stack space via SBIW R28, <locals_size>, and updates SP.
  • Implemented function epilogue: restores stack space via ADIW R28, <locals_size>, updates SP, restores caller Y, and returns (RET).
  • Switched local scalar variable reading/writing and prefix/postfix increment/decrement to LDD / STD.
  • Added peephole optimization rule eliminating redundant LDD immediately following STD.
• Tests:
  • Added unit tests for recursive Fibonacci (fib(7)), recursive Factorial with local variables, nested multi-frame calls, and global variable interactions inside functions.
  • Maintained 100% statement coverage across cpu.py, compiler.py, and assembler.py (342 tests passing).

Verification

• uv run pytest --cov=tiny8.compiler --cov=tiny8.cpu --cov=tiny8.assembler (100% coverage, 342 passed)
• uv run ruff check src/ tests/ (all passed)
• uv run ruff format --check src/ tests/ (clean)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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