Computer Corner [Jan. 1990--Electronic Servicing & Technology]

Working with a hard disk system

By Glenn R. Patsch

You may have wondered how the hard disk got its name. Floppy disks are flexible and you can actually bend them, al though you should not. Hard disks contain rigid, flat surfaces referred to as platters. These flat surfaces are coated with a magnetic material to store information. (IBM refers to a hard disk as a fixed disk.) Floppy disks are removable; hard disks are fixed in place.

Hard disks are actually quite fragile, and the computer should not be bumped, jarred or moved while it is operating. Otherwise, you could dam age the heads or the surface of the hard disk. Before the computer is ever moved, the heads should be parked.

This operation may be performed automatically by the hard disk, depending on the computer manufacturer. If not, you should park the heads by using a software utility designed to do that.

Hard disks are not difficult to under stand. Before DOS (or any other operating system) can be used by a hard disk, it must be properly set up. A hard disk must be low-level formatted to establish the sector size, interleave and location of the tracks. The sector size is usually 512 bytes, which is the smallest amount of space on the disk. If you need to store 513 bytes of data, you must use two sectors.

The interleave defines the order of the tracks on the disk. A one-to-one inter leave places all of the tracks in numerical order. Changing the interleave can speed up or slow down the flow of in formation from the disk to the processor. All of this depends on how fast the disk rotates, where the sectors are located, and how fast the processor can accept information. By changing the interleave, you are changing the order of the sectors to try to have the next one in the right place to compensate for a slow processor, a slow disk or both.

Low-level formatting is usually done by the manufacturer of the disk.

Patsch is a consultant specializing in the selection, evaluation and installation of IBM personal computer and compatible hardware and software.

Sectors

Once a hard disk has been low-level formatted, it is ready to be divided into one to four sectors referred to as partitions. The DOS FDISK command is used to create these partitions. Each partition becomes a different logical drive.

The first partition, the C: drive, contains DOS and is used to boot up the PC. The bootable partition is referred to as the active partition and can be any one of the four partitions, although it is usual ly the first one. The second partition would be the D: drive. The third partition would be the E: drive, and the fourth would be the F: drive.

Partitions also can each have a different operating system, which would al low you to have DOS and UNIX on the same disk. Depending on the physical capacity of the disk, each partition can be a maximum of 32Mbytes for DOS 3.3 and older versions of DOS. PC-DOS 4.0 from IBM and MS-DOS 3.31 from Compaq and others allow partitions larger than 32Mbytes, up to a maximum of four trillion bytes. After the disk is divided into partitions, each partition must be formatted with the FORMAT command.

The DOS FORMAT command pre pares the disk to accept files by creating a directory and file allocation table (FAT). The directory keeps track of the file names and stores the file size, the time and the date the file was created.

The directory also stores the file attributes, which are read only, hidden, sys tem and archive.

The FAT keeps track of which clusters are used and which are free. The directory points to the first cluster for the file.

Sectors are grouped into clusters. Some disks have one sector per cluster; others use several sectors per cluster. Large capacity disks have several sectors per cluster to keep the size of the FAT manageable. There are actually two copies of the FAT because it is so important.

Now that the disk is formatted, it should be bootable. The DOS FORMAT command with the /s option or the SYS command will transfer the hidden files IBMBIO.COM and IBMDOS. COM (IO.SYS and MSDOS.SYS for MS-DOS) and the file COMMAND.

COM to the hard disk.

Creating directories

Directories should be created on the disk for each application program. The DOS MKDIR or MD (make directory) command will create a new directory.

To create a directory for a Lotus 123 spreadsheet program, for example, use MD 123 to create the 123 directory. A path can be set to use the various application programs with the PATH command. PATH c:;c:DOS;c:123; will search the root, DOS and 123 directories for programs to run. Place the PATH command in the AUTOEX EC.BAT files, which will automatically execute when the PC is turned. Your AUTOEXEC.BAT file might look like this:

ECHO OFF ECHO IBM PS/2 MODEL 70 WITH VGA MONITOR PATH C:;C:DOS;C:123; PROMPT $P$G The CONFIG.SYS file contains con figuration information for the computer. It might look like this: BUFFERS=15 FILES =20 LASTDRIVE=C DEVICE=IBMCACHE.SYS 64 /E /P4 BREAK =ON

Both the AUTOEXEC.BAT and CONF1G.SYS files should be in the root directory. Load other application programs and files you need. The hard disk should now be ready to use.