A Little IDE Math

The ATA-2 Fast ATA specification boasts a 16.6MBps rate for data transfer from a drive'sbuffer to system memory. However, you are unlikely to achieve this rate under real-world IDEimplementations because of the limitations of existing drives and the command overhead that the ATAinterface requires. Even the fastest IDE/ATA hard disks aren't capable of much more than a 10MBpssequential transfer rate (from drive to drive buffer). Furthermore, the buffer transfers on most IDEsystems are 4KB in size. Thus, a drive takes about 400 microseconds (4KB ÷ 10MBps) to read 4KBof data into its buffer at 10MBps. Fast ATA's 16.6MBps transfer rate can easily keep pace, so thereshouldn't be a problem, right? Wrong.

Although the ATA interface is doing its job in this situation, a hidden factor limits the10MBps (the drive's maximum transfer rate in the example) performance ceiling: commandturnaround time. Command turnaround time is the overhead associated with the operations of theATA/ATA-2 command set. (The host system's CPU speed also slightly affects command turnaround time.)Most PCs have command turnaround times in the neighborhood of 275 microseconds. For controllers tokeep pace with the faster ATA/ATA-2 hard disks, command turnaround time needs to be about 150microseconds. Thus, an ATA-2 controller cannot grab data as fast as it arrives in the drive'sbuffer, resulting in a phenomenon known as slipped revolutions. Because the drive's bufferis full and cannot accept more data, the drive platters must spin idly until the ATA interface cancatch up and free space in the buffer. This delay causes other delays that limit the performance ofATA-2 drive subsystems.