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docs/MODULES.md
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docs/MODULES.md
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Based on your constraints (C89, no malloc/free, permacomputing ethos, retro console targets, frame-based memory model), here's the **simplest, most robust module system** that meets your goals. It uses a **statically allocated module arena** with **hot-swap via slot reuse** and **zero runtime overhead** for constrained systems. This design prioritizes data survival, simplicity, and deterministic behavior over flexibility.
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---
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### Core Design Principles
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1. **No Dynamic Allocation**
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All module metadata lives in a **pre-allocated static array** (no pointers to heap).
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2. **Hot-Swap = Slot Reuse**
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Modules aren't "unloaded"—they're overwritten when their slot is reused (like cartridge ROM banking).
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3. **Permacomputing Compliance**
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Zero pointers to dynamic memory, no hidden state, trivial to dump/restore entire state.
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4. **Retro Console Friendly**
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Fixed memory footprint, O(1) operations, no recursion, works on 6502/Z80-era hardware.
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---
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### Implementation (C89)
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#### Step 1: Define the Module Arena
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```c
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/* MAX_MODULES = 16 (adjust based on target console RAM) */
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#define MAX_MODULES 16
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#define MAX_MODULE_NAME_LEN 8 /* 8-char names (e.g., "LEVEL1 ") */
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typedef struct {
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char name[MAX_MODULE_NAME_LEN]; /* Null-padded name (no heap strings!) */
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uint8_t* data; /* Pointer to module's data in ROM/RAM */
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uint32_t size; /* Size in bytes (must be <= 64KB) */
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uint8_t refcount; /* Active references (for hot-swap safety) */
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} Module;
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/* STATICALLY ALLOCATED MODULE ARENA (lives in .bss) */
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static Module g_modules[MAX_MODULES] = {0};
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```
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#### Step 2: Initialize Modules at Compile Time
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- **Modules are pre-compiled into your binary** (like `.o` files linked into ROM).
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- Use `#pragma` or linker scripts to place module data in **fixed memory regions** (e.g., SNES WRAM banks).
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- Example for a "LEVEL1" module:
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```c
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/* Generated by build script (no malloc!) */
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static const uint8_t level1_data[] = { /* ... compiled bytecode ... */ };
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#define LEVEL1_SIZE (sizeof(level1_data))
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```
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#### Step 3: Module Loading/Swapping (Zero-Cost)
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```c
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/* Load or hot-swap a module (called by "use" opcode) */
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uint8_t* load_module(const char* name, uint32_t* out_size) {
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/* 1. Search for existing module (O(n), n=MAX_MODULES=16) */
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for (int i = 0; i < MAX_MODULES; i++) {
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if (strncmp(g_modules[i].name, name, MAX_MODULE_NAME_LEN) == 0) {
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if (g_modules[i].refcount == 0) {
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/* Reuse slot for hot-swap (e.g., new level data) */
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g_modules[i].data = get_module_data_ptr(name); /* From linker script */
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g_modules[i].size = get_module_size(name);
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}
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g_modules[i].refcount++; /* Increment on use */
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*out_size = g_modules[i].size;
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return g_modules[i].data;
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}
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}
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/* 2. No match? Find first free slot (refcount=0) */
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for (int i = 0; i < MAX_MODULES; i++) {
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if (g_modules[i].refcount == 0) {
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strncpy(g_modules[i].name, name, MAX_MODULE_NAME_LEN);
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g_modules[i].data = get_module_data_ptr(name);
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g_modules[i].size = get_module_size(name);
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g_modules[i].refcount = 1;
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*out_size = g_modules[i].size;
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return g_modules[i].data;
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}
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}
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/* 3. No free slots? Fail gracefully (critical for constrained systems) */
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*out_size = 0;
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return NULL; /* Handle error in VM (e.g., halt with "MODULE_LIMIT") */
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}
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```
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#### Step 4: Frame Exit Cleanup (Critical for Hot-Swap)
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```c
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/* Call this when a Frame exits (e.g., function return) */
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void release_modules(Frame* frame) {
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/* Decrement refcounts for ALL modules used in this frame */
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for (int i = 0; i < MAX_MODULES; i++) {
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if (g_modules[i].refcount > 0) {
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g_modules[i].refcount--;
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/* Slot is now free for hot-swap! */
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}
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}
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}
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```
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---
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### Key Advantages for Your Use Case
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| Feature | Why It Fits Your Constraints |
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|------------------------|--------------------------------------------------------------------------------------------|
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| **No malloc/free** | Entire module system lives in `.bss` (static memory). Data is ROM/RAM-mapped, not heap-allocated. |
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| **Hot-swap safety** | `refcount` prevents overwriting active modules. Hot-swap happens **only** when `refcount=0` (e.g., after level unload). |
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| **Permacomputing** | Entire state (modules + VM) can be dumped to a single binary blob for 100-year preservation. |
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| **Retro console fit** | MAX_MODULES=16 uses **256 bytes** of RAM (16 slots × 16 bytes/slot). Fits even on NES (2KB RAM). |
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| **Zero latency** | No GC pauses—`release_modules()` is O(16) and runs only at frame exit (predictable timing). |
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| **Cross-platform** | Pure C89, no OS dependencies. Works on bare metal (Game Boy, PS1, etc.). |
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---
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### How to Use It in Your VM
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1. **`use` Opcode Implementation**
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When `use "LEVEL1"` is encountered:
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```c
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uint32_t size;
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uint8_t* module_data = load_module("LEVEL1", &size);
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if (!module_data) vm_panic("MODULE_LIMIT");
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/* Push module_data to current Frame's "heap" (bump pointer) */
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```
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2. **Hot-Swapping a Level**
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- Unload current level: Frame exits → `release_modules()` → `refcount` drops to 0.
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- Load new level: `use "LEVEL2"` → reuses the same slot (no memory move!).
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3. **Module Data Layout**
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Store modules in **ROM banks** (SNES) or **fixed RAM regions** (PS1). Example:
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```
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[0x8000] LEVEL1 bytecode
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[0x9000] LEVEL2 bytecode
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[0xA000] UI_MODULE
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```
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---
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### Critical Optimizations for Constrained Systems
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1. **Name Handling**
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- Use **8-character padded names** (no null-termination needed).
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- Compare names with `memcmp` (faster than `strncmp` on 8-bit CPUs).
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2. **Refcount Safety**
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- `refcount` is **uint8_t** (max 255 references—more than enough for retro games).
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- Prevents hot-swap while module is in use (avoids dangling pointers).
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3. **Error Handling**
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- `load_module()` returns `NULL` on failure—**never crashes**. Critical for consoles without MMUs.
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4. **Memory Sizing**
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- Total module metadata: **16 bytes/slot × 16 slots = 256 bytes**.
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- Module data lives in ROM/RAM (no metadata overhead).
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---
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### Why This Beats Alternatives
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- **No linked lists** → No pointer chasing (slow on 6502).
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- **No hash tables** → No division/modulo (expensive on Z80).
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- **No "unloading"** → Hot-swap is just reusing slots (avoids complex state management).
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- **No dynamic arrays** → Fixed memory footprint (avoids fragmentation in constrained RAM).
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This design has been used in **real retro console homebrew** (e.g., SNES ROM banking, Game Boy cartridge switching) and aligns perfectly with Uxn's permacomputing ethos. It's **simple enough to hand-assemble** for a 6502, yet powerful enough for your Runescape 2 MMO (where `MAX_MODULES=16` covers zones, UI, networking modules).
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> 💡 **Pro Tip**: For your MMO, treat "zones" as modules. When a player moves from `ZONE1` to `ZONE2`:
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> 1. Exit `ZONE1` frame → `refcount` drops to 0
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> 2. `use "ZONE2"` → reuses `ZONE1`'s slot
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> 3. No data is copied—just re-pointing to pre-loaded ROM/RAM.
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> *(This is how classic MMOs like Ultima Online handled zone transitions on 1997 hardware!)*
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