CHAPTER 13

740 Chapter 13 Optical Storage
Having buffer or cache memory for the CD/DVD drive offers a number of advantages. Buffers can
ensure that the PC receives data at a constant rate; when an application requests data from the drive,
the data can be found in files scattered across different segments of the disc. Because the drive has a
relatively slow access time, the pauses between data reads can cause a drive to send data to the PC
sporadically. You might not notice this in typical text applications, but on a drive with a slower access
rate coupled with no data buffering, it is very noticeable—and even irritating—during the display of
video or some audio segments. In addition, a drive’s buffer, when under the control of sophisticated
software, can read and have ready the disc’s table of contents, speeding up the first request for data. A
minimum size of 128KB for a built-in buffer or cache is recommended and is standard on many 24x
and faster drives. For greater performance, look for drives with 256KB or larger buffers.
CPU Utilization
A once-neglected but very real issue in calculating computer performance is the impact that any piece
of hardware or software has on the central processing unit (CPU). This “CPU utilization” factor refers
to how much attention the CPU (such as Pentium III/4, Athlon, and so on) must provide to the hardware
or software to help it work. A low CPU utilization percentage score is desirable because the less
time a CPU spends on any given hardware or software process, the more time it has for other tasks,
and thus the greater the performance for your system. On CD-ROM drives, three factors influence
CPU utilization: drive speed, drive buffer size, and interface type.
Drive buffer size can influence CPU utilization. For CD-ROM drives with similar performance ratings,
the drive with a larger buffer is likely to require less CPU time (lower CPU utilization percentage) than
the one with a smaller buffer.
Because drive speed and buffer size are more of a given, the most important factor influencing CPU
utilization is the interface type. Traditionally, SCSI-interface CD-ROM drives have had far lower CPU
utilization rates than ATAPI drives of similar ratings. One review of 12x drives done several years ago
rated CPU utilization for ATAPI CD-ROM drives at 65%–80%, whereas SCSI CD-ROM drives checked
in at less than 11%. By using DMA or Ultra-DMA modes with an ATA interface drive, near-SCSI levels
of low CPU utilization can be realized. Using DMA or Ultra-DMA modes can cut CPU utilization
down to the 10% or lower range, leaving the CPU free to run applications and other functions.
Direct Memory Access and Ultra-DMA
Busmastering ATA controllers use Direct Memory Access (DMA) or Ultra-DMA transfers to improve
performance and reduce CPU utilization. Virtually all modern ATA drives support Ultra-DMA. With
busmastering, CPU utilization for ATA/ATAPI and SCSI CD-ROM drives is about equal at around 11%.
Thus, it’s to your benefit to enable DMA access for your CD-ROM drives (and your ATA hard drives,
too) if your system permits it.
Most recent ATAATAPI CD-ROM drives (12x and above) support DMA or Ultra-DMA transfers, as does
Windows 95B and above and most recent Pentium-class motherboards. To determine whether your
Win9x or Me system has this feature enabled, check the System Properties’ Device Manager tab and
click the + mark next to Hard Disk Controllers. A drive interface capable of handling DMA transfers
lists “Bus Master” in the name. Next, check the hard drive and CD-ROM information for your system.
You can use the properties sheet for your system’s CD-ROM drives under Windows 9x/Me and
Windows 2000/XP to find this information; you might need to open the system to determine your
hard drive brand and model. Hard disk drives and CD-ROM drives that support MultiWord DMA
Mode 2 (16.6MB/sec), UltraDMA Mode 2 (33MB/sec), UltraDMA Mode 4 (66MB/sec), or faster can use
DMA transfers. Check your product literature or the manufacturer’s Web sites for information.
To enable DMA transfers if your motherboard and drives support it, open the System Properties sheet
in Windows 9x or later, click the Device Manager tab, and open the Properties sheet for your hard
drive. Click the Settings tab, and place a check mark in the box labeled DMA.