# Memory Model

Domain: VM memory model
Function: normative

This chapter defines the VM memory architecture: stack, heap, handles, object layout, allocation, and garbage collection.

It intentionally focuses on VM-managed memory.

Host-owned memory surfaces such as asset banks and persistent save storage are covered by their own domain specs and by the host ABI docs.

## 1 Overview

The PVM memory model is based on:

- stack-resident execution values;
- heap-resident objects;
- validated handles for indirect heap access;
- GC-managed object lifetime.

## 2 Stack Memory

The stack stores transient execution values and frame-scoped data.

### Stack value types

The stack may contain:

- primitive values;
- built-in fixed-layout values;
- heap handles;
- tuple-shaped multi-return values.

See [`02a-vm-values-and-calling-convention.md`](02a-vm-values-and-calling-convention.md) for the slot/value model.

### Stack properties

- stack values are local to execution state;
- stack discipline is verifier-sensitive;
- stack underflow and invalid access are traps.

## 3 Heap Memory

The heap stores runtime objects that require identity and reachability tracking.

### Heap characteristics

- user objects live in the heap;
- arrays and closures live in the heap;
- heap access is indirect through handles;
- the collector is non-moving in the baseline model.

## 4 Handles and Gate Table

All heap objects are accessed via handles.

A conceptual handle:

```
handle = { index, generation }
```

Conceptual gate entry:

```
GateEntry {
    alive: bool
    generation: u32
    base: usize
    slots: u32
    type_id: u32
}
```

### Handle safety

When an object is freed:

- `alive` becomes false;
- `generation` is incremented.

When a handle is used:

- the index must exist;
- the generation must match;
- invalid or stale handles trap.

This prevents:

- use-after-free;
- stale references;
- hidden raw memory access.

## 5 Object Layout

Heap objects use explicit, implementation-visible layout rules.

Conceptual form:

```
Object {
    type_id
    field_0
    field_1
    ...
}
```

Properties:

- fields are stored in slot order;
- there is no raw pointer arithmetic in the VM model;
- layout rules are explicit and verifier/runtime-friendly.

Closures and coroutine-owned state may build on top of this memory model, but their behavioral rules are documented separately:

- [`02b-vm-function-values-and-closures.md`](02b-vm-function-values-and-closures.md)
- [`09a-coroutines-and-cooperative-scheduling.md`](09a-coroutines-and-cooperative-scheduling.md)

## 6 Garbage Collection

The PVM uses a **mark-sweep collector**.

### GC properties

- non-moving in the baseline model;
- runs only at safepoints;
- primary safepoint: `FRAME_SYNC`.

### GC triggers

GC may run when:

- heap usage exceeds threshold;
- allocation pressure is high.

### Root set

The collector marks from:

- operand stack;
- call stack frames;
- global variables;
- coroutine-owned execution state;
- closure environments;
- host-held roots where the host legally retains VM heap references.

## 7 Allocation and Deallocation

### Allocation

Heap allocation conceptually performs:

1. reserve object storage;
2. create or reuse gate entry;
3. return validated handle.

If allocation fails:

- the VM may trigger GC;
- if pressure remains unresolved, allocation may trap or fail according to the runtime contract.

### Deallocation

Objects are reclaimed by GC.

When reclaimed:

- the gate is marked dead;
- the generation is incremented;
- storage becomes reusable.

## 8 Memory Safety Rules

The VM enforces:

1. all heap access goes through handles;
2. generation checks validate handle freshness;
3. object and field access is bounds-checked;
4. there is no guest-visible raw pointer arithmetic;
5. stack discipline remains verifier-constrained.

Any violation results in a trap or rejected program image.

## 9 Relationship to Other Specs

- [`02a-vm-values-and-calling-convention.md`](02a-vm-values-and-calling-convention.md) defines stack values and tuple/call-slot semantics.
- [`02b-vm-function-values-and-closures.md`](02b-vm-function-values-and-closures.md) defines closure function values.
- [`09a-coroutines-and-cooperative-scheduling.md`](09a-coroutines-and-cooperative-scheduling.md) defines coroutine state and scheduling behavior.
- [`15-asset-management.md`](15-asset-management.md) defines asset-bank surfaces outside VM heap ownership.
- [`08-save-memory-and-memcard.md`](08-save-memory-and-memcard.md) defines save storage outside VM heap ownership.
- [`16a-syscall-policies.md`](16a-syscall-policies.md) defines host-root rules and host/heap interaction policy.