The Enhanced IDE/Fast-ATA FAQ
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The Enhanced IDE/Fast-ATA FAQ

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ATAPI: CD-ROMs and tapes

11 ATAPI: CD-ROMs and tapes

Contents of this section

11.1 How does ATAPI differ from, and coexist with, ATA(-2)?

For the sake of compatibility with non-ATAPI aware software that might mistake an ATAPI device for a harddrive, the device pretends it isn't there until it's waken up by a special sequence of commands. Once activated, it uses a command protocol that radically differs from that used by harddisks.

The reason is that the ATA command and register set is not adequate to support some CD-ROM command structures. Therefore, only a minimum of traditional ATA commands are supported by ATAPI devices. For most of their functions, these devices rely on the ATAPI Transport Protocol using packets of at least 12 bytes sent, as data, through the Data Register. These packet commands have been derived from the SCSI command set; this makes it reasonably easy to rewrite existing SCSI CD-ROM and tape drivers for ATAPI hardware.

Beware that non-ATAPI aware 'intelligent' controllers, mainly caching controllers, will be mightily confused by packet commands. Traditionally, the data register is only used to transport 512-byte sectors; a 12-byte command packet is a completely different kind of animal and should be treated in a different way by the (intelligent) controller.

11.2 What's so special about the secondary port?

Nothing, in principle. A secondary IDE port has been reserved in the PC I/O map for ages (base address 0170h, IRQ 15), and adapters that could be configured as secondary have been available for quite some time, even though BIOS support was lacking. So while it is a part of EIDE, there is actually nothing new about this feature, except that the possibility of connecting tape drives and CD-ROMs to the ATA adapter has transformed four device support from a luxury into a necessity.

Actually, there is another reason to provide a secondary port for ATAPI devices. There are a number of advanced hardware features for harddisk interfaces, such as prefetch buffers and write behind, that may get in the way of ATAPI compatibility. This means that if the software drivers of an intelligent ATA-2 port are not ATAPI-aware, or simply don't work as they should, you may run into sticky problems. Especially if you have an ATAPI CD-ROM that doesn't support PIO mode 3 transfers, a secondary port that provides only basic ATA features is a good way to avoid a lot of headaches.

Finally, an ATAPI device on the primary port will cause Windows FastDisk drivers that aren't ATAPI aware to fail.

Nothing prevents you from defining more ports like the primary and the secondary one; in addition to these two, the tertiary and quaternary one are semi-standard. See section 10.4 for the port and IRQ assignments of all these ports.

Next Chapter, Previous Chapter

Table of contents of this chapter, General table of contents

Top of the document, Beginning of this Chapter

Created by: Astalalista - Last modification: 2004/Jan/26 at 02:57
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