Specifications

Data Structures
1.16 SCSI Connection Descriptor Table (SCDT)
Table 1–18 (Cont.) Contents of SCSI Connection Descriptor Table
Field Name Contents
SCDT$W$_TAG_MAP Quadword bitmap of allocated tags. A bit set in this map indicates a
tag value in use. Tag values in the map can be from 0 to 63 allowing 64
outstanding I/Os in a device.
SCDT$W_DEV_IO_COUNT Number of I/Os currently outstanding on the in-device queue.
SCDT$W_PORT_IO_COUNT Number of I/Os currently outstanding on the incoming port queue.
SCDT$W_WAIT_TAG Synchronizes the port queue for a non-queued I/O request. A tag value is
still allocated though the command is not sent as a tagged command to the
device. The port will not initiate queued I/O if the SCDT$V_QUEUE_WAIT
bit is set until the I/O in SCDT$W_WAIT_TAG has completed.
SCDT$W_MAX_TAG Largest tag value used.
SCDT$W_MAX_QUEUE Class driver imposed limit.
SCDT$L_SEQUENCE Next sequence to be used.
SCDT$L_NEXT_SEQUENCE Next sequence ID to be sent device.
SCDT$L_SCDRP_MAP Pointer to a list of SCDRPs indexed by the tag value. Reduces searching
by SCDT$L_DEV_QFL.
1.17 SCSI Port Descriptor Table (SPDT)
The SCSI port descriptor table (SPDT) contains information specific to a SCSI
port, such as the port driver connection database. The SPDT also includes a
set of vectors, corresponding to the SPI macros invoked by SCSI class drivers,
that point to service routines within the port driver. The SCSI port driver’s unit
initialization routine creates an SPDT for each SCSI port defined for a specific
MicroVAX or VAXstation system and initializes each SPI vector.
The port driver reads and writes fields in the SPDT. The class driver reads the
SPDT indirectly when it invokes an SPI macro.
The SPDT is illustrated in Figure 1–20 and described in Table 1–19.
1–73