Dynamic information storage or retrieval – Condition indicating – monitoring – or testing – Including radiation storage or retrieval
Reexamination Certificate
1997-11-17
2001-04-10
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
Condition indicating, monitoring, or testing
Including radiation storage or retrieval
C369S053200
Reexamination Certificate
active
06215747
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the organization of data stored on CD-ROMs and, in particular, a method for organizing data on CD-ROMs that increases data retrieval efficiency in constant angular velocity CD-ROM drives.
BACKGROUND OF THE INVENTION
CD-ROM technology was first developed in 1976 and became, by the early 1980s, a widely accepted media for the distribution of recorded music. The potential application of CD-ROM technology for high capacity, low cost data storage and data distribution for computer systems was quickly recognized. CD-ROMs have since become the standard media for distribution of computer software.
FIG. 1
displays basic physical characteristics of a CD-ROM. A CD-ROM
101
is a transparent polycarbonate disk with a radius of 6 cm
102
. At the center of the disk is a spindle hole
103
with a radius of 7.5 millimeters. A manufacturer stores data on a CD-ROM by impressing a pattern of flat areas, called lands, and depressions, called pits, onto the surface of the CD-ROM. The surface is then covered with a reflective metallized film, followed by a protective coat of lacquer. Data is read from the CD-ROM by shining a laser beam onto a rotating CD-ROM. Light from the laser beam is reflected by the lands and dispersed by the pits. The light reflected from the lands is detected by a photodiode detector. The lands and pits are arranged along a single spiral track
104
starting near the center of the disk and spiraling out to the edge of the disk. The single spiral track is approximately 4,500 meters in length and 600 nanometers wide. The adjacent turns of the spiral are about 1.6 microns apart.
FIGS. 2A and 2B
display the low-level data formatting of a CD-ROM. The single spiral track of the CD-ROM is divided into data sectors. A standard size CD-ROM that stores 60-minutes of recorded music contains 270,000 data sectors. If the outer 5 millimeters of the disk is used for data storage, the resulting CD-ROM that stores 74 minutes of recorded music contains a total of 333,000 data sectors. The data sectors are arranged sequentially along the spiral track
201
from a starting point near the center of the disk outward towards the edge of the disk. Each sector
202
contains 12 bytes of synchronization information
203
, followed by four bytes of header information
204
, followed by 2,048 bytes of data
205
, with the final 288 bytes used for error correcting codes
206
.
FIG. 3
displays the read operation of a CD-ROM drive. The laser beam and photodiode detector are mounted on an assembly
301
that moves along a radial vector
302
of the CD-ROM. The CD-ROM is spun in a clockwise direction
303
by a motor. As the CD-ROM is spun, the photodetector/laser assembly moves radially outwards in order to sequentially access each successive sector arranged along the spiral track. In order to read a set of contiguous sectors starting at some arbitrary point on the surface of the CD-ROM, the photodiode/laser assembly is first moved outward to the approximate radius of the CD-ROM corresponding to the first sector of the set of contiguous sectors. This first operation is called seeking. Then each of the contiguous sectors is read in order as the photodiode/laser assembly follows along the track of the spinning disk outward from that point. This latter process is called data transfer. The total time required to read a number of contiguous sectors from a CD-ROM is called the access time, expressed by the following equation:
access time=seek time+data transfer time (1)
FIG. 4
displays the higher-level formatting of the data on a CD-ROM. A CD-ROM contains three data sections. The first is a label section
401
that contains a volume title for the CD-ROM. The second section contains directory information that indicates to the software controlling the CD-ROM drive how the remaining data is laid out on the CD-ROM. The format of the directory information may vary depending on the operating system of the computer accessing the CD-ROM. Directories are usually logically ordered as tree-structured hierarchies. Following the directory information are the data files
403
. The data files are analogous to data files traditionally stored on magnetic disks within computer systems. A data file contains a number of bytes of data, and may have further internal formatting, depending on the operating system of the computer for which the data file has been created. Each data file is stored on the CD-ROM in one or more contiguous data sectors. The label, directory and file data sections follow one another and are laid out from the starting point of the track near the center of the CD-ROM outward towards the edge of the CD-ROM. Any unused capacity of the CD-ROM occurs between the last used sector of the CD-ROM and the outer edge of the CD-ROM, forming a band of unused sectors at the outer edge of the CD-ROM.
FIG. 5
displays a polar-coordinate-based mathematical representation of a spinning CD-ROM. A location P
501
on the surface of the CD-ROM is designated by the length r of the radial vector
502
from the center of the CD-ROM to the location P and by the angle &thgr;
503
by which radial vector is displaced from an arbitrary reference vector
504
. The length of the arc S
505
corresponds to the length of the spiral track from the reference vector
504
to the point P
501
. The length of this arc is approximately described by the following equation:
S=&thgr;r (2)
Differentiating both sides of the above equation with respect to time produces the following equation:
dS/dt=d&thgr;/dt r (3)
The rate of change of the length of the arc S with respect to time, dS/dt, represents the linear velocity of the point P along a track of the spinning CD-ROM. The rate of change of the angle &thgr; with respect to time, d&thgr;/dt, represents the angular velocity of the spinning CD-ROM. Thus, the following formula represents the linear velocity of a point moving on a spinning CD-ROM, V, in terms of the angular velocity of the CD-ROM, &ohgr;, and the length of a radial vector describing the location of the point, r:
V=&ohgr;r (4)
The linear velocity of a point moving on a spinning CD-ROM is thus equal to the angular velocity of the CD-ROM disk times the length of the radial vector from the center of the CD-ROM disk to the point.
Lower-speed CD-ROM drives commonly have variable speed motors that spin the CD-ROM disk at different angular velocities in order to keep the linear velocity of points moving under the photodetector/laser assembly constant. The data transfer rate is obviously directly related to the linear velocity at which lands and pits move under the photodetector/laser assembly. In constant linear velocity (“CLV”) CD-ROM drives, the data transfer rate is therefore constant over the entire spiral track of the CD-ROM.
FIG. 6
displays a graph of angular velocity versus radius for a CD-ROM read by a CLV CD-ROM drive. The vertical axis
601
of the graph represents the angular velocity. The horizontal axis
602
represents the length of the radial vector from the center of the CD-ROM to a particular data sector. The curve displayed in the graph shows how a CLV drive varies the angular velocity at which it spins a CD-ROM depending on the radius at which the photodetector/laser assembly is reading sectors from the spiral track. The values are given for a standard 150 KB/sec data transfer rate. The angular velocity varies from about 15 revolutions per second at a radius of 1 cm,
603
, to about five revolutions per second at a radius of 6 cm,
604
. Thus, a CLV drive spins the disk faster in order to access the innermost sectors of a CD-ROM and spins the CD-ROM slower in order to access the sectors at the outer edge of the CD-ROM. Referring to equation (1) shown above, it can be seen that, because the data transfer time is constant over the entire surface of the CD-ROM for a CLV drive, and because the seek time increases with increasing values of location of a data file, the access time for a file on the CD-RO
Dorsey & Whitney LLP
Micron Electronics Inc.
Tran Thang V.
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