SCSI and ATAPI

ATAPI refers to devices that use the Packet Interface of the ATA6 (or higher) standard command set. It is basically a way to issue SCSI commands to a CDROM, CD-RW, DVD, or tape drive, attached to the ATA bus.

ATAPI uses a very small number of ATA commands. The most important are the PACKET command (0xA0), and IDENTIFY PACKET DEVICE (0xA1).

PACKET command (0xA0)

Each ATAPI command packet is made of a 'command byte' (from the SCSI command set -- see below for a partial list), followed by 11 'data' bytes. For instance, reading the table of contents is achieved by sending the following byte string to the device, as a "command".

 unsigned char atapi_readtoc[]=  { 0x43 /* ATAPI_READTOC */, 0, 1, 0, 0, 0, 0, 0, 12, 0x40, 0, 0};

IDENTIFY PACKET DEVICE command (0xA1)

This command is a "normal" ATA PIO mode command, used during initialization. It is an exact mirror of the ATA IDENTIFY command, except that it only returns information about ATAPI devices. Use it in exactly the same way as you use IDENTIFY, including the meanings of all the bits in all the 256 words of data returned.

x86 Directions

Important note: on the Primary bus, the standard set of ATA IO ports is 0x1F0 through 0x1F7. In much or all of the ATAPI documentation, you will see this set of IO ports called the "Task File". The term seems very confusing.

If the transfer is complete, BSY and DRQ will clear. Otherwise, wait for the next IRQ, and read or write the same number of words again.

Notes: there is a possible future change planned to increase the length of ATAPI command strings to 8 words. Check the two bottom bits of ATAPI Identify word 0 to verify 6 or 8 word command size.

Once again, if you use polling to check BSY, DRQ, and ERR after sending the PACKET command, then you should probably ignore the ERR bit for the first four loops. (ATAPI calls this the "CHECK" bit, instead of ERR, but it means the same thing.)

Reference from http://wiki.osdev.org/ATAPI.  From here, we can know some ATAPI command and SCSI CDB information.