The Miden virtual machine

Events

Events interrupt VM execution for one cycle and hand control to the host. The host can read VM state and modify the advice provider. From the VM's perspective, emit has identical semantics to noop - the operand stack and registers remain unchanged.

Event identifiers are field elements derived from well-known strings using EventId::from_name() (first 64 bits of blake3("<name>") as little-endian u64, mod p). The first 256 event ID values are reserved for system events. The VM doesn't enforce structure for stack-provided IDs, but immediate forms restrict inputs to this string-based mapping.

Event names should be as unique as possible to avoid collisions with other libraries. Use a hierarchical naming convention like project_name::library_name::event_name. Generic names may cause conflicts in multi-library environments.

Event Instructions

• emit - Interrupts execution, hands control to host (1 cycle)
• emit.<event_id> - Expands to push.<event_id> emit drop (3 cycles). Immediate IDs must come from event("...") constants or inline event("...").

# Using a constant
const.MY_EVENT=event("miden::transfer::initiated")
emit.MY_EVENT

# Inline form
emit.event("miden::transfer::initiated")

# Equivalent manual stack form (any Felt – not validated):
push.<felt> emit drop

Event Types

System Events - Built-in events handled by the VM for memory operations, cryptography, math operations, and data structures.

Custom Events - Application-defined events for external services, logging, or custom protocols.

Tracing

Miden assembly also supports code tracing, which works similar to the event emitting.

A trace can be emitted via the trace.<trace_id> assembly instruction where <trace_id> can be any 32-bit value specified either directly or via a named constant. For example:

trace.EVENT_ID_1
trace.2

To make use of the trace instruction, programs should be ran with tracing flag (-t or --trace), otherwise these instructions will be ignored.