# V1 compatibility

V1 remains an explicit regression target with its [specification](v1-specification.md)
and historical `v1.0.0` artifacts. V2 retains opcodes 0–C and their timing for
V1-valid programs, but changes the host protocol, queue sizes and status layout.
The version query distinguishes V1 `0x010140` from V2 `0x020240`.

Use `protocol_emulator.v2.host.connect(transport)` to select the matching host
backend. V1 clients are not promised binary host compatibility with V2. The
installed CLI remains V1; use [the V2 Python toolchain](assembler-toolchain.md)
for the new firmware. V2 ASIC build defaults do not change CLI or Python API protocol selection.
The package/runtime version is `2.0.0rc1`; protocol version queries are unchanged.

Use `make v1-test`, `make v1-lint`, `make v1-synth` and `make v1-formal` for
explicit V1 checks. Pin simulation uses `make -C test -f Makefile.v1pins`.
V1 metadata and its prior physical configuration are retained in `physical/v1`.

The full functional runner always includes both V1 and V2. Existing V1 core
and pin cases remain required, alongside both tops' lint/synthesis and the V1
induction and reset checks. V1's four-byte TX buffering and request/NOP throughput
limits must not be confused with V2's packed writes and 16-entry queues.