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ADR 0068: Extension first-use is a target-profile hook

Context​

Some targets allocate VECTOR or CUBE context lazily and need a precise trap before the first instruction that would consume that context. PTO does not own one portable enable-register layout, trap-number allocation, ACR route, or software task-state policy. Encoding a particular target's values in the portable model would turn an implementation profile into common PTO semantics.

Decision​

PTO defines two extension kinds and two target-profile hooks. The portable default reports both kinds disabled. Its raise hook returns without changing trap, bundle, queue, memory, or fault state.

An enabling target profile must define the covered instruction set, enable state and ownership, source and manager ACRs, trap envelope and argument mapping, and the ordering point. The trap must occur after decode and target legality but before extension allocation or architectural effects. The saved execution point must permit exact retry, and the target must guarantee forward progress for its context-save path.

Consequences​

PTO implementations that do not select such a profile retain existing behavior. Target profiles can bind a concrete first-use ABI without changing PTO encodings or importing target register fields into the portable model. A profile claim is incomplete without executable zero-effect, retry, and independent-kind evidence.

Last updated on by Kevin Zhou