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It should be noted that for ATAPI devices the PACKET command in PIO data transfer mode has nearly the same functionality with respect to PIO DRQ data blocks sizes as does a PACKET command in DMA data transfer mode. In PIO data transfer mode the host specifies the maximum size of any DRQ data block (using the Byte Count Limit or BCL value). A device shall never attempt to transfer a DRQ block larger than the BCL. The device may transfer DRQ data blocks of any size (following the rules for PACKET command data transfers) up to but not larger than the BCL set by the host. The host may select a BCL value that is not related to the device's media block size. So PACKET PIO data transfers are a lot like PACKET DMA data transfers. *** Hale Landis *** www.ata-atapi.com ***
