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+# Polkadot Virtual Machine (PVM)
+
+This package implements the **Polkadot Virtual Machine (PVM)** as specified in the **Gray Paper Appendix A and B v0.7.2 (GP)**. 
+It provides a deterministic execution environment for services in the JAM network.
+The comments refer to the respective equations in the GP.
+
+## Overview
+
+The PVM is a stack-based virtual machine with:
+- **32-bit address space** (2³² bytes)
+- **13 general-purpose registers** (φ)
+- **Page-based memory** (4KB pages)
+- **Gas metering** for execution cost accounting
+- **Host calls** (ecalli) for interacting with the runtime environment
+
+## Directory Structure
+
+```
+internal/pvm/
+├── common.go          # Core types, memory, registers, and context structures
+├── exit_reason.go     # Exit/error types (halt, panic, out-of-gas, page fault)
+├── initialisation.go  # Memory and register initialization (eq. A.37, A.42, A.43)
+├── instance.go        # VM instance creation and core execution primitives
+├── instruction_codes.go # Opcode constants (A.5.1–A.5.13)
+├── instructions.go    # Gas costs, register definitions, opcode validation
+├── invocations.go     # Whole-program and basic invocation (ΨM, ΨH, Ψ)
+├── mutations.go       # Instruction implementations (Trap, Load, Store, etc.)
+├── program.go         # Blob parsing, deblobbing, instruction decoding
+├── step.go            # Single-step execution (Ψ1)
+└── host_call/         # Host call implementations
+    ├── common.go           # Shared constants, result codes, utilities
+    ├── general_functions.go # Gas, Fetch, Lookup, Read, Write, Info
+    ├── accumulate_functions.go # Bless, Assign, Designate, Transfer, etc.
+    └── refine_functions.go     # HistoricalLookup, Export, Machine, Invoke, etc.
+```
+
+## Core Components
+
+### Memory (`common.go`)
+
+Memory is organized into segments with access control:
+
+| Segment | Purpose | Access |
+|---------|---------|--------|
+| **ro** | Read-only data (program constants) | Read-Only |
+| **rw** | Read-write data (heap, globals) | Read-Write |
+| **stack** | Call stack | Read-Write |
+| **args** | Program arguments | Read-Only |
+
+- **Page size** (`Z_P`): 4KB (2¹²)
+- **Zone size** (`Z_Z`): 64KB (2¹⁶)
+- Addresses below 2¹⁶ are forbidden (reserved zone)
+- `sbrk` extends the heap within the RW segment
+
+### Registers (`instructions.go`)
+
+| Reg | Name | Purpose |
+|-----|------|---------|
+| R0  | ra   | Return address (2³² − 2¹⁶ for return-to-host) |
+| R1  | sp   | Stack pointer |
+| R2  | t0   | Temporary |
+| R3  | t1   | Temporary |
+| R4  | t2   | Temporary |
+| R5  | s0   | Saved |
+| R6  | s1   | Saved |
+| R7  | a0   | Argument/return value |
+| R8  | a1   | Argument/return length |
+| R9  | a2   | Argument |
+| R10 | a3   | Argument |
+| R11 | a4   | Argument |
+| R12 | a5   | Argument |
+
+### Exit Reasons (`exit_reason.go`)
+
+| Error | Symbol | Description |
+|-------|--------|-------------|
+| `ErrHalt` | ∎ | Normal program termination |
+| `ErrPanic` | ☇ | Irregular termination (trap, invalid operation) |
+| `ErrOutOfGas` | ∞ | Gas exhausted |
+| `ErrPageFault` | F | Invalid memory access |
+| `ErrHostCall` | h | Host call in progress |
+
+## Execution Model
+
+### Invocations
+
+1. **InvokeWholeProgram** (ΨM) — Full program invocation:
+   - Parses blob
+   - Initializes memory and registers
+   - Instantiates VM
+   - Runs until halt, panic, out-of-gas, or page fault
+
+2. **InvokeHostCall** (ΨH) — Runs until a host call or termination
+
+3. **InvokeBasic** (Ψ) — Executes a single basic block or until host call
+
+4. **step** (Ψ1) — Executes one instruction
+
+### Program Format
+
+Programs are delivered as **blobs** containing:
+- **Program memory sizes** (RO, RW, stack, initial heap pages)
+- **RO data** — Read-only constants
+- **RW data** — Initialized read-write data
+- **Code and jump table** — JAM-encoded bytecode
+
+The **deblob** operation extracts:
+- **Code** — Instruction stream
+- **Bitmask** — Instruction boundaries
+- **Jump table** — For indirect jumps (`djump`)
+
+### Instruction Set
+
+The PVM implements a RISC-like instruction set (see `instruction_codes.go`):
+
+- **Control flow**: Trap, Fallthrough, Jump, JumpInd, LoadImmJump, LoadImmJumpInd, Branch variants
+- **Memory**: Load/Store (U8, U16, U32, U64, signed/unsigned variants)
+- **Arithmetic**: Add, Sub, Mul, Div, Rem, shifts, rotations
+- **Bitwise**: And, Or, Xor, AndInv, OrInv, Xnor
+- **Other**: MoveReg, Sbrk, CountSetBits, Leading/TrailingZeroBits, SignExtend, Cmov
+- **Host**: Ecalli — triggers a host call with an immediate index
+
+## Host Calls
+
+Host calls allow programs to interact with the runtime. The `host_call` subpackage implements:
+
+### General Functions
+- **gas** — Return remaining gas
+- **fetch** — Fetch chain constants, entropy, authorizer hash, work metadata
+- **lookup** — State lookup
+- **read** — Read from service state
+- **write** — Write to service state
+- **info** — Service/block metadata
+
+### Accumulate Functions
+- **bless** — Set manager, assigners, designate service
+- **assign** — Assign cores to services
+- **designate** — Designate service for creation
+- **checkpoint** — Create checkpoint
+- **new** — Create new service
+- **upgrade** — Upgrade service
+- **transfer** — Transfer balance
+- **eject** — Eject service
+- **query** — Query service state
+- **solicit** — Solicit service
+- **forget** — Forget solicitation
+- **yield** — Yield execution
+- **provide** — Provide preimage
+
+### Refine Functions
+- **historical_lookup** — Look up historical state
+- **export** — Export segments
+- **machine** — Get machine state
+- **peek** — Peek at memory
+- **poke** — Write to memory
+- **pages** — Memory page info
+- **invoke** — Invoke nested PVM
+- **expunge** — Remove data
+
+### Result Codes
+
+Host calls return status via `Code`:
+- `OK` — Success
+- `NONE` — Item does not exist
+- `WHAT` — Name unknown
+- `OOB` — Out of bounds
+- `WHO` — Index unknown
+- `FULL` — Storage full
+- `CORE` — Core index unknown
+- `CASH` — Insufficient funds
+- `LOW` — Gas limit too low
+- `HUH` — Invalid solicitation/forget state
+
+## Context Types
+
+- **AccumulateContext** — Used during accumulation phase (block building)
+- **RefineContextPair** — Used during refinement (execution with integrated PVM map)
+- **IntegratedPVM** — Captures code, RAM, and instruction counter for nested invocations
+
+## Constants (from specification)
+
+| Constant | Value | Description |
+|----------|-------|-------------|
+| `AddressSpaceSize` | 2³² | Total address space |
+| `PageSize` | 4KB | Memory page size |
+| `MemoryZoneSize` | 64KB | Zone size |
+| `InputDataSize` | 16MB | Max input/args size |
+| `DynamicAddressAlignment` | 2 | Indirect jump alignment |
+| `AddressReturnToHost` | 2³² − 2¹⁶ | Special return address |
+