6.6Write Cache
The write cache function of the drive makes a high speed processing in the case that data to be written by a write command is logically sequent the data of previous command and random write operation is performed.
When the drive receives a write command, the drive starts transferring data of sectors requested by the host system and writing on the disk medium. After transferring data of sectors requested by the host system, the drive generates the interrupt of command complete. Also, the drive sets the normal end status in the Status register. The drive continues writing data on the disk medium. When all data requested by the host are written on the disk medium, actual write operation is completed.
The drive receives the next command continuously. If the received command is a "sequential write" (data to be written by a command is logically sequent to data of previous command), the drive starts data transfer and receives data of sectors requested by the host system. At this time, if the write operation of the previous command is still been executed, the drive continuously executes the write operation of the next command from the sector next to the last sector of the previous write operation. Thus, the latency time for detecting a target sector of the next command is eliminated. This shortens the access time. The drive generates an interrupt of command complete after completion of data transfer requested by the host system as same as at previous command. When the write operation of the previous command had been completed, the latency time occurs to search the target sector.
If the received command is not a "sequential write", the drive receives data of sectors requested by the host system as same as "sequential write". The drive generates the interrupt of command complete after completion of data transfer requested by the host system. Received data is processed after completion of the write operation to the disk medium of the previous command.
Even if a hard reset or soft reset is received or the write cache function is disabled by the SET FEATURES command during unwritten data is kept, the instruction is not executed until remaining unwritten data is written onto the disk medium.
The drive uses a write data as a read cache data. When a read command is issued to the same address after the write command, the read operation to the disk medium is not performed.
When an error occurs during the write operation, the drive makes retry as much as possible. If the error cannot be recovered by retry, the drive stops the write operation to the erred sector, and continues the write operation from the next sector if the write data is remained. (If the drive stacks a write command, for that the drive posts the command completion, next to the command that write operation is stopped by error occurrence.) After an error occurs at above write operation, the drive posts the error status to the host system at next command. (The drive does not execute this command, sets the error status that occurred at the write operation, and generates the interrupt for abnormal end. However, when the drive receives a write command after the completion of error processing, the drive posts the error after writing the write data of the write command.)
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