ADR 0025: Scalar bitfield and byte-reversal bounds

Historical-evidence note: test paths named below record the evidence used when this ADR was accepted; they are not active architecture or release owners. Current ownership is the four-surface ASL tree, with per-ID AVS coverage projected into spec/evidence/release-traceability-readiness.json.

• Scope: scalar BXS, BXU, BIC, BIS, CLZ, CTZ, BCNT, REV, and HL.BFI

Decision

For the scalar bitfield forms, the six-bit imml field encodes the field width minus one, so every encoded value selects a width from 1 through 64. The six-bit imms or immr field independently selects the least-significant source bit from 0 through 63. A field that crosses bit 63 wraps to bit 0.

REV extracts the selected wrapping field and reverses its bytes into the low bits of the result. Bits above the selected width are zero. A selected width that is not a multiple of eight completes normally and returns zero; it is not an illegal instruction and raises no fault. imml and immr remain independent encoded operands and must not be collapsed or substituted for one another.

HL.BFI uses its independently encoded immr as the first destination bit and imms as the last destination bit. The inclusive destination interval wraps through bit 63 when the last bit precedes the first bit. Source bit zero is inserted at the first destination bit, with ascending source bits following the wrapping destination interval.

All source values are read before the first destination write. Consequently, an absolute or temporary-queue destination that aliases a source observes the pre-instruction source value.

Rationale

This disposition makes every value of the two six-bit bounds total without silently treating adjacent fields as one operand. It preserves the existing PTO catalog and executable reference behavior while making the non-byte REV result an explicit architecture rule rather than an incidental implementation choice.

Verification

tests/asl/scalar/model/alu/semantics/scalar-bound-bitfield-contract-001.asl varies imml while holding immr fixed, varies immr while holding imml fixed, exercises minimum, byte-aligned, wrapping, non-byte, and full-width selections, and uses an aliased source/destination. The catalog checker requires this ADR and the decoded boundary witness to remain traceable from PTO-REQ-SCALAR-ALU-001.