Each following Read Track command attempts to advance to the next specified
track, orients to the home address, and transfers each count, key, and data area
until end-of-track.
A pseudo count field is transferred to the channel after the last record on a track is
transferred. If no records were transferred, only the pseudo count field is sent to
the channel. The pseudo count field can be used to locate the end of the track
image character string in the host system’s main storage.
Prestage Trackset – ‘10’ Extended Operation
Code
:
The Prestage Trackset
Operation Code gives notification to the control unit of intent to access a set of
tracks in a future operation. The tracks to be accessed are specified by the
Extended Parameter and the number of tracks to be transferred is specified by the
Count Parameter (byte 3). No data transfer commands may occur in the Locate
Record domain. Any command that follows Prestage Trackset is outside the
domain.
The maximum length of the Extended Parameter is specified in byte 43 of the
Device Characteristics Information (See Appendix A, "Device Characteristics" on
page A-1.).
The Extended Parameter contains a bit map that represents a set of sequentially
addressed tracks. Each bit in the parameter represents one track. A ‘1’ bit indicates
the data associated with the corresponding track is to be prestaged. A ‘0’ bit
indicates the track is to be skipped. Tracks in the bit map represented by the
‘1’ bits are not limited to the tracks contained within the defined extent. (While a
request to prestage a track that is outside of the extent may be honored, a later
attempt to access the prestaged data will be required to be within extent limits.)
The first bit must be a ‘1’ and represents the track whose address is specified in
the Seek Address parameter (bytes 4–7 of the LRE parameters). Subsequent bits
represent consecutively addressed tracks in ascending order. If the first bit is not a
‘1’, the Locate Record Extended command is terminated with status that includes
unit check (Command Reject, Format 0, Message 4, Invalid Parameter).
The number of ‘1’ bits in the bit map must be equal to the value in the count
parameter (byte 3); otherwise Locate Record Extended is terminated with status
that includes unit check (Command Reject, Format 0, Message 4, Invalid
Parameter).
It is possible to construct a valid Prestage Trackset operation that points to tracks
that are outside of the logical volume. For example, Seek to a track that is 5 tracks
away from the end of the volume, include 10 bits in the bit map, and a count
parameter of 10. Internal Disk will prestage the tracks that are within the logical
volume and ignore the inability to prestage the remaining tracks.
The Locate Record Extended orientation bits (byte 0, bits 0 and 1) have valid
settings of either count or home address orientation. However, Internal Disk uses
only home address orientation while executing Prestage Trackset. The first track is
prestaged beginning at the count area of Record Zero and continuing with each
count, key, and data area until end-of-track.
For the following tracks, Internal Disk attempts to advance to the next specified
track, orients to the home address, and transfers each count, key, and data area
until end-of-track.
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