PHYMSC Memo

PHYMSC is TOPS-20's Mass Storage Control Protocol (MSCP) disk driver, serving as a key piece of upper-level software support for the CI-20. Its primary role is to interface PHYSIO with SCAMPI to support MSCP disks.

Operating as a controller driver within the TOPS-20 PHYSIO system, PHYMSC provides a passive service, responding to requests from PHYSIO for disk pages, handling device polling, initiating I/O requests, and processing interrupts. It appears to PHYSIO as a standard device controller. PHYMSC only connects to disks in the 576-byte format and manages dual-ported HSC drives by assigning them to even or odd HSCs based on unit number, without automatic port switching.

As a TOPS-20 SYSAP (System Application Program), PHYMSC communicates with disks through SCAMPI services. It relies on SCAMPI to maintain connections to HSC disks, provide information about the remote disk structure, and notify PHYMSC of connection status (e.g., connection breaks, new nodes, node online/offline).

PHYMSC functions as an MSCP Host, implementing core MSCP functions like Get Unit Status, Online, Abort, Read, Write, and Get Command Status, while not using others (e.g., Flush, Erase). A dedicated "poller" component continuously checks the responsiveness of remote MSCP servers, handling reconnections and, if necessary, attempting to reload the remote MSCP server.

Key differences from standard PHYSIO behavior include PHYMSC not aborting transfers on OVRDTA and offloading latency optimization, channel stacking, and position requests to the HSC50 controller. It primarily uses PHYSIO's Controller Data Blocks (KDB) and Unit Data Blocks (UDB) for data structures and maintains a queue of outstanding I/O requests. Future PHYSIO changes are anticipated to support a second serial number word and multiple concurrent requests on the CI. The driver also handles Bat Blocks for media errors reported by the HSC50.