ADR 0011: Make EBARG the visible PTO v0 trap snapshot

Context

The system-register contract exposes 18 banked EBARG registers, while the earlier reference model saved all return state only in an internal record. That made the visible register values observationally unrelated to ACRE: software could inspect or edit an EBARG word, but recovery ignored the edit.

The architecture also permits profile-defined extended execution state beyond the first-layer register snapshot. PTO v0 needs bounded executable storage for bundle arguments and other state that has no allocated EBARG word.

Decision

In PTO v0, EBARG is the editable first-layer trap snapshot:

• EBARG0[3:0] records the source ACR, bit 4 is snapshot valid, bits 5 and 6 record bundle-active and bundle-body-active, bits 10:7 record bundle kind, bits 13:11 record transfer kind, and bit 14 records the bundle condition. Bits 63:15 are reserved zero.
• EBARG_BPC_CUR, EBARG_BPC_TGT, EBARG_TPC, and EBARG_LRA record the current BPC, next target, resume TPC, and local return address.
• EBARG_TQ0 through EBARG_TQ3 and EBARG_UQ0 through EBARG_UQ3 record all temporary queue entries in newest-to-oldest order.
• PTO v0 has no first-layer loop-boundary or loop-counter registers, so trap save writes zero to EBARG_LB and EBARG_LC; software may still read and write those context words, but recovery does not consume them.
• EBARG_EXTCTX_PTR, EBARG_EXTCTX_META, and EBARG_TPLFLAGS are persistent read/write context words. Ordinary trap save does not overwrite them.
• ECSTATE records the source CORE_STATE; bits 3:0 select the recovery ACR and bit 4 records whether the source was in a bundle body.

ACRE consumes the visible ECSTATE and EBARG PC, queue, return, and bundle- control fields. ACR0 software may therefore edit those fields before recovery. Recovery clears EBARG0.VALID. A missing snapshot, reserved control encoding, body-active without bundle-active, inconsistent source ACR, or odd BPC/TPC is an execution-state-check fault rather than a silent no-op. An unsupported ACRE request-type encoding remains an illegal-instruction fault.

The internal saved context also preserves bundle dimensions, scalar and tile bindings, control/data attributes, fallthrough state, the bundle return target, the bundle-argument kind, and all eight 32-bit predicate registers. The bundle return target is distinct from the local return address held in EBARG_LRA. These fields have no complete EBARG encoding in PTO v0, so successful recovery restores their saved values. EBARG-covered fields remain authoritative and may be deliberately edited by manager software before recovery.

The bounded _TrapContexts record is PTO v0 profile-defined extended EBSTATE, not an alternative visible register file. It retains bundle argument, bundle- argument kind, bundle return target, and commit state that have no allocated first-layer EBARG words. Those fields remain an explicit extended-context profile dependency.

Consequences

Trap recovery observes software edits to the visible return PC and queue state, and every EBARG address has a defined reset and access behavior. Complete portable serialization of the remaining extended bundle state is still tracked separately from this first-layer snapshot closure.

The system-register behavior catalog therefore partitions the range rather than assigning one over-broad behavior to all 18 words: 13 recovery-active snapshot registers, two save-zero/recovery-inert loop-context words, and three save-preserved/recovery-inert extended-context words. Tests exercise all five tail registers across reset, software write, trap save, and recovery.