MIPS Interpreter Implementation

Overview

This skill provides guidance for implementing a MIPS interpreter/emulator capable of running compiled MIPS ELF binaries. The key insight is that building an interpreter is fundamentally an engineering task requiring iterative implementation, not a pure research problem. Start coding early and iterate.

Critical Anti-Pattern: Analysis Paralysis

The most common failure mode is excessive research without implementation. Avoid:

• Reading every source file before writing any code
• Trying to understand every detail of the target program
• Researching exhaustively before creating a basic structure

Instead: Create a skeleton implementation early, then fill in components as understanding grows.

Implementation Strategy

Phase 1: Scaffold First (Start Here)

Create the basic interpreter structure immediately, before deep research:

// vm.js - Create this skeleton FIRST
class MIPSInterpreter {
  constructor() {
    this.registers = new Uint32Array(32);
    this.memory = null;  // Will be ArrayBuffer
    this.pc = 0;
    this.hi = 0;
    this.lo = 0;
  }

  loadELF(buffer) { /* TODO */ }
  step() { /* TODO */ }
  run() { /* TODO */ }
  handleSyscall() { /* TODO */ }
}

Phase 2: ELF Loading

Implement ELF parsing to load the binary:

1. Parse ELF header to get entry point and program headers
2. Load PT_LOAD segments into memory at specified virtual addresses
3. Initialize PC to entry point (typically 0x400000 range for MIPS)
4. Handle BSS section (zero-initialized, may be large)

Key ELF details for MIPS:

• Machine type: 0x08 (MIPS)
• Check endianness from ELF header (e_ident[EI_DATA])
• Entry point from e_entry field
• Program headers define memory layout

Phase 3: Core Instruction Execution

Implement instruction decode and execute loop:

1. Fetch: Read 4 bytes at PC
2. Decode: Extract opcode (bits 31-26) and determine format (R/I/J)
3. Execute: Perform the operation
4. Advance: Update PC (normally PC += 4, handle branches/jumps)

Start with these essential instruction categories:

• ALU: ADD, ADDU, SUB, AND, OR, XOR, NOR, SLT, SLTU
• Immediate: ADDI, ADDIU, ANDI, ORI, XORI, SLTI, SLTIU, LUI
• Shifts: SLL, SRL, SRA, SLLV, SRLV, SRAV
• Memory: LW, LB, LBU, LH, LHU, SW, SB, SH
• Branches: BEQ, BNE, BGTZ, BLEZ, BLTZ, BGEZ
• Jumps: J, JAL, JR, JALR
• Special: SYSCALL, MFHI, MFLO, MULT, MULTU, DIV, DIVU

Phase 4: Syscall Emulation

Implement syscalls the target program uses. Common syscalls:

• File I/O: open, close, read, write
• Memory: brk, mmap
• Process: exit
• Time: gettimeofday

Map syscall numbers to handlers. Syscall number is in $v0 ($2), arguments in $a0-$a3 ($4-$7).

Phase 5: Test and Iterate

Run the binary, observe failures, fix issues:

• Unimplemented instructions will crash - implement as discovered
• Missing syscalls will fail - add as needed
• Memory issues will manifest - debug with logging

Verification Strategy

Incremental Testing Milestones

1. ELF loads: Entry point and segments loaded correctly
2. First instruction executes: PC advances, registers change
3. First function call works: JAL/JR, stack operations
4. First syscall handled: write() outputs something
5. Program runs to completion: exit() called normally

Debugging Techniques

• Log PC and instruction at each step (disable for performance later)
• Print register state on syscalls
• Compare against reference MIPS emulator (SPIM, MARS) for simple programs
• Create minimal test programs to verify specific instructions

Common Pitfalls

1. Ignoring Delay Slots

MIPS has branch delay slots - the instruction after a branch executes before the branch takes effect. Many interpreters skip this for simplicity (works for most compiled code).

2. Sign Extension Errors

• LB, LH sign-extend; LBU, LHU zero-extend
• Immediate values in I-type instructions are sign-extended (except for logical ops)
• Failing to sign-extend causes subtle bugs

3. Endianness Mismatch

• MIPS can be big or little endian (check ELF header)
• JavaScript TypedArrays use host endianness
• May need DataView for explicit endianness control

4. Memory Size Underestimation

• BSS sections can be very large (>1GB for programs like Doom)
• Use sparse memory representation or on-demand allocation
• Don't allocate full address space upfront

5. Unsigned vs Signed Arithmetic

• JavaScript numbers are signed; use

  >>> 0

  for unsigned comparison
• ADDU, SUBU don't trap on overflow; ADD, SUB do (rarely matters)
• SLT vs SLTU: signed vs unsigned comparison

6. Multiplication Result Registers

• MULT/MULTU put 64-bit result in HI:LO
• MFHI/MFLO retrieve the values
• Don't forget to implement these

Time Management

For a complex interpreter task:

• Hour 1: Scaffold + ELF loading
• Hour 2: Core ALU and memory instructions
• Hour 3: Branches, jumps, and basic syscalls
• Hour 4+: Iterate based on runtime failures

An incomplete but running interpreter beats thorough research with no code.

Resources

Refer to

references/mips_instruction_reference.md

for instruction encoding details and opcode tables.