Specifications

ATAPI For Streaming Tape QIC-157 Rev B
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Table 2-1 Byte Count Register Usage
Operation Usage (PIO) Usage (DMA)
Send Command Packet Is used as a parameter to the Packet
Command and is not used to control the
Packet transfer.
Command Packet is always sent via
Programmed I/O and not DMA.
Parameters to the Packet
Command (Task File Contents)
As a parameter to any Packet Command
that will transfer ata, the Byte Count is
used by the Host to communicate the
maximum / preferred amount of data to
be transferred on each DRQ.
The Device can ignore the byte count, as
the actual transfers are controlled via the
ATAPI Device and not the Host.
Parameter Data from the
Device to the Host (e.g. data
from a READ, or INQUIRY
command)
At each DRQ the count contains the
number of bytes that the Host shall
transfer from the Device.
The ATAPI Device can transfer data
whenever it wishes, and as such the Byte
Count shall not be used.
Parameter Data from the Host
to the Device (e.g. data for a
Write, or Mode Select
command)
At each DRQ the count contains the
number of bytes that the Host shall
transfer to the Device.
The ATAPI Device can transfer data
whenever it wishes, and as such the Byte
Count shall not be used.
If the Device requests more data transferred than required by the command protocol, the Host shall
pad when sending data to the Device, and dump extra data into a bit bucket when reading data from
the Device. Only the amount of data specified or implied in an ATAPI packet shall be transferred;
transfer beyond this may cause data corruption.
On odd byte transfers, the only permissible time for an actual Odd Byte Count value will be on the
Last DRQ. Intermediate DRQs shall contain even byte counts.
2.5. Sector Count (ATAPI Interrupt Reason) Register Usage for Packet
Commands
The Interrupt Reason Register contains an expanded definition of the ATA DRQ Status. When the
DRQ is presented in the Status Register for an ATAPI Packet Command, then the contents of this
register indicate if Packet Command or User Data shall be transferred and the direction of transfer.
2.6. Overlapped Command Operation
ATAPI media access commands have immediate status with a delayed process. These commands
return Completion Status immediately with the actual execution of the command continuing. For
these commands, the DSC bit is cleared until actual completion has finished. If a new command is
received before DSC is set, the drive will respond in one of the following ways:
• The new command is queued and BSY is left set; or
• The new command is executed with no impact on the previous command.
1. ATA commands operate differently from packet commands. When a new ATA command is
written to the Command Register while BSY bit is set, the executing command stops execution
and reports a “Check Condition,” “Aborted,” and ABORTED COMMAND Sense Key.
2. New ATAPI media access commands received while the DSC bit is cleared shall cause the BSY
signal to be held until the previous command completes. New ATAPI non-media access
commands will complete normally without impacting any previous command. If a check
condition exists due to a previous command, that condition will be reported as a deferred error