Decoder transport and processing

DecoderSystem transfers measurement bits from controller memory-mapped I/O (MMIO) registers to a decoding algorithm and returns correction bits. All cores share the transport links and decoder processing capacity. Each core and decoder ID pair has its own input window and accumulated corrections.

Connections

• Input: aligned 32-bit data accesses from MemoryModel, routed through callbacks installed by Simulator.

• Output: status and correction registers read by the CPU; decoder trace events with core, decoder and request IDs.

• Owner: Simulator::decoders_.

• Scheduling: Simulator::barrier() calls step() after clocked memory accesses at that tick. next_boundary() supplies timed wakeups for transfers and jobs. A result returned at a memory edge becomes readable on a later edge.

See decoder configuration and registers for the JSON fields, MMIO commands and timing equations.

Requests, jobs and results

DecoderMmio validates addresses, register permissions and command operands. DecoderSystem accepts its commands, applies queue backpressure and advances transfers, jobs and result state.

A submit write copies the core’s staging registers into a bounded request queue. A full queue keeps the memory write pending. Each request reserves transmission time and incurs the configured propagation delay. Arriving packets append their bits to the selected session’s input window.

A complete window calls the configured decoder callback and creates a timed job. The callback runs synchronously without advancing simulation time. The job starts at least one nanosecond after admission and respects the decoder’s initiation interval. Its configured latency determines completion.

The response link sends completed jobs in completion order, breaking ties by request ID. Returned bits accumulate by XOR until consumed or reset. The result capacity limits sessions with an active job or unread corrections. A full result capacity leaves the completing syndrome packet at the request queue head.

Objects and state

Member	Role
mmio_	DecoderMmio owns per-core registers and translates accesses into typed commands.
sessions_	Per-core, per-decoder partial input, active-job status and accumulated corrections.
requests_, reset_request_	Bounded syndrome queue and one separate reset slot.
jobs_	Decoded results with processing start, completion and response-arrival ticks.
available_	Earliest next job start for each decoder ID.
tx_available_, rx_available_	End of the reserved request and response transmission intervals.

Reset and errors

A decoder reset command travels over the request link using its separate slot. On arrival it clears earlier work for the selected session, including its partial window, queued syndrome requests, active job and unread corrections. Later requests and other sessions remain. Reset takes precedence over completion and result delivery at the same tick; existing transmission and job-start reservations remain in effect.

Simulator reset clears all decoder sessions, queues, jobs and reservations. idle() requires no pending request, reset, job or incomplete input window. Unread completed corrections do not prevent simulation completion.

Construction rejects invalid capacities, timing values and decoder definitions. Invalid MMIO accesses or decoder IDs fault. Input-window overflow, premature result reads and consuming unfinished corrections also fault. A callback that returns the wrong number of correction bits raises BackendFailure.

Implementation and tests

Source: decoder.hpp, decoder.cpp, decoder/mmio.cpp, simulator.cpp and decoding.py.

CTest: decoder.transport.

The tests cover transfer and processing times, queue backpressure, correction accumulation, core routing, 64-bit payloads, reset ordering and invalid accesses. Optional decoder.pymatching checks matrix configuration and correction masks. Cross-project integration.qec runs compiled surface-code decoding and repetition-code feedback programs.