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+---
+title: "Mcode IR"
+description: "JSON-based intermediate representation"
+---
+
+## Overview
+
+Mcode is a JSON-based intermediate representation that can be interpreted directly. It represents the same operations as the Mach register VM but uses string-based instruction dispatch rather than binary opcodes. Mcode is intended as an intermediate step toward native code compilation.
+
+## Pipeline
+
+```
+Source → Tokenize → Parse (AST) → Mcode (JSON) → Interpret
+                                                → Compile to Mach (planned)
+                                                → Compile to native (planned)
+```
+
+Mcode is produced by the `JS_Mcode` compiler pass, which emits a cJSON tree. The mcode interpreter walks this tree directly, dispatching on instruction name strings.
+
+## JSMCode Structure
+
+```c
+struct JSMCode {
+  uint16_t nr_args;        // argument count
+  uint16_t nr_slots;       // register count
+  cJSON **instrs;          // pre-flattened instruction array
+  uint32_t instr_count;    // number of instructions
+
+  struct {
+    const char *name;      // label name
+    uint32_t index;        // instruction index
+  } *labels;
+  uint32_t label_count;
+
+  struct JSMCode **functions; // nested functions
+  uint32_t func_count;
+
+  cJSON *json_root;        // keeps JSON alive
+  const char *name;        // function name
+  const char *filename;    // source file
+  uint16_t disruption_pc;  // exception handler offset
+};
+```
+
+## Instruction Format
+
+Each instruction is a JSON array. The first element is the instruction name (string), followed by operands:
+
+```json
+["LOADK", 0, 42]
+["ADD", 2, 0, 1]
+["JMPFALSE", 3, "else_label"]
+["CALL", 0, 2, 1]
+```
+
+The instruction set mirrors the Mach VM opcodes — same operations, same register semantics, but with string dispatch instead of numeric opcodes.
+
+## Labels
+
+Control flow uses named labels instead of numeric offsets:
+
+```json
+["LABEL", "loop_start"]
+["ADD", 1, 1, 2]
+["JMPFALSE", 3, "loop_end"]
+["JMP", "loop_start"]
+["LABEL", "loop_end"]
+```
+
+Labels are collected into a name-to-index map during loading, enabling O(1) jump resolution.
+
+## Differences from Mach
+
+| Property | Mcode | Mach |
+|----------|-------|------|
+| Instructions | cJSON arrays | 32-bit binary |
+| Dispatch | String comparison | Switch on opcode byte |
+| Constants | Inline in JSON | Separate constant pool |
+| Jump targets | Named labels | Numeric offsets |
+| Memory | Heap (cJSON nodes) | Off-heap (malloc) |
+
+## Purpose
+
+Mcode serves as an inspectable, debuggable intermediate format:
+
+- **Human-readable** — the JSON representation can be printed and examined
+- **Language-independent** — any tool that produces the correct JSON can target the ƿit runtime
+- **Compilation target** — the Mach compiler can consume mcode as input, and future native code generators can work from the same representation
+
+The cost of string-based dispatch makes mcode slower than the binary Mach VM, so it is primarily useful during development and as a compilation intermediate rather than for production execution.