Chapter 2. Ibm Diskette; Disk Layout - IBM 3742 Reference Manual

Chapter 2. IBM Diskette
The IBM diskette (Figure 5) is a flexible Mylar* disk con-
tained within a protective cartridge about
8.0
inches square.
Data is recorded and read serially on the disk in the same
way data is processed on magnetic tape. Records can be
accessed directly as with system-attached magnetic disk
drives. The diskette falls between the punched card and
one-half inch magnetic tape in terms of data storage capa-
city and functional characteristics. It combines the small
batch data storage properties of punched cards with many
of the features of one-half inch magnetic tape and has the
added advantage of direct access. In addition, the diskette
fits easily into the procedures of most punched card instal-
lations, replacing cards with minimum changes required.
When compared to the punched card, the disk has the fol-
lowing advantages.
• Reusability to reduce media cost.
• Ease of erasure and error correction.
• Very favorable ratio of size and weight to data stored.
One small disk can usually hold the entire day's output
from the typical key entry station.
• Improved data readout speeds. This speed is advanta-
geous when either the 3741 Model 2 or 3747 data con-
verter is attached to a data processing system via com-
munications.
• The ability to easily back up and reread data for retrans-
mission in case of communication errors.
• Portability and mailability of useful quantities of data.
• Machine-assisted direct access to offline batch data files.
DISK LAYOUT
The disk unit reads and writes on only one side of the disk.
The physical layout of the disk is shown in Figure 6. Each
disk is divided into an index track and 73 data tracks or
circles similar to the grooves on a record. The.track is div-
ided into 26 sectors. Thus, any data record can be located
by an address consisting of a track number and a sector
number. Twenty-six records of up to 128 characters each
can be written on each track. The capacity of the disk is
1898 data records.
*Trademark of E I duPont de Nemours & Co., Inc.
6
Figure 5. 18M- Diskette
1 Data Record
Address: Track 03
Sector 07
Figure 6. Physical Disk Layout
Sector 07