prometeu-studio/docs/specs/pbs_old/Prometeu Runtime Traps.md

Runtime Traps v0 — Prometeu VM Specification

Status: Proposed (requires explicit owner approval)

Scope: Prometeu VM / PBS v0 execution model

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1. Motivation

Prometeu aims to be a deterministic, industrial-grade virtual machine. To achieve this, execution errors that are:

• caused by user programs,
• predictable by the execution model,
• and recoverable at the tooling / host level,

must be explicitly represented and ABI-stable.

This specification introduces Runtime Traps as a formal concept, consolidating behavior that already existed implicitly in the VM.

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2. Definition

A Runtime Trap is a controlled interruption of program execution caused by a semantic violation detected at runtime.

A trap:

• terminates the current execution frame (or program, depending on host policy)
• does not corrupt VM state
• returns structured diagnostic information (TrapInfo)
• is deterministic for a given bytecode + state

A trap is not:

• a debugger breakpoint
• undefined behavior
• a VM panic
• a verifier/load-time error

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3. Trap vs Other Failure Modes

Category	When	Recoverable	ABI-stable	Example
Verifier error	Load-time	❌	❌	Stack underflow, bad CFG join
Runtime trap	Execution	✅	✅	OOB access, invalid local
VM panic	VM invariant violation	❌	❌	Handler returns wrong slot count
Breakpoint	Debug only	✅	❌	Developer inspection

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4. Trap Information (TrapInfo)

All runtime traps must produce a TrapInfo structure with the following fields:

TrapInfo {
  code: u32,        // ABI-stable trap code
  opcode: u16,      // opcode that triggered the trap
  pc: u32,          // program counter (relative to module)
  message: String,  // human-readable explanation (non-ABI)
}

ABI Guarantees

• code, opcode, and pc are ABI-relevant and stable
• message is diagnostic only and may change

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5. Standard Trap Codes (v0)

5.1 Memory & Bounds

Code	Name	Meaning
TRAP_OOB	Out of bounds	Access beyond allowed bounds
TRAP_INVALID_LOCAL	Invalid local	Local slot index out of bounds

5.2 Execution & Types

Code	Name	Meaning
TRAP_ILLEGAL_INSTRUCTION	Illegal instruction	Unknown/invalid opcode encountered
TRAP_TYPE	Type violation	Type mismatch for operation or syscall argument types
TRAP_DIV_ZERO	Divide by zero	Division/modulo by zero
TRAP_INVALID_FUNC	Invalid function	Function index not present in function table
TRAP_BAD_RET_SLOTS	Bad return slots	Stack height mismatch at return

5.3 System

Code	Name	Meaning
TRAP_INVALID_SYSCALL	Invalid syscall	Unknown syscall ID
TRAP_STACK_UNDERFLOW	Stack underflow	Missing arguments for syscall or opcode

This list is closed for PBS v0 unless explicitly extended.

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6. Trap Semantics

6.1 Execution

When a trap occurs:

1. The current instruction does not complete
2. No partial side effects are committed
3. Execution stops and returns TrapInfo to the host

6.2 Stack & Frames

• Operand stack is left in a valid but unspecified state
• Call frames above the trapping frame are not resumed

6.3 Host Policy

The host decides:

• whether the trap terminates the whole program
• whether execution may be restarted
• how the trap is surfaced to the user (error, log, UI, etc.)

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7. Verifier Interaction

The verifier must prevent traps that are statically provable, including:

• stack underflow
• invalid control-flow joins
• invalid syscall IDs
• incorrect return slot counts

If a verifier rejects a module, no runtime traps should occur for those causes.

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8. What Is Not a Trap

The following are VM bugs or tooling errors, not traps:

• handler returns wrong number of slots
• opcode implementation violates OpcodeSpec
• verifier and runtime disagree on stack effects

These must result in VM panic, not a trap.

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9. Versioning Policy

• Trap codes are ABI-stable within a major version (v0)
• New trap codes may only be added in a new major ABI version (v1)
• Removing or reinterpreting trap codes is forbidden

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10. Summary

Runtime traps are:

• an explicit part of the Prometeu execution model
• deterministic and ABI-stable
• reserved for user-program semantic errors

They are not debugging tools and not VM panics.

This spec formalizes existing behavior and freezes it for PBS v0.

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