Browse Prior Art Database

Boot from PC Fixed Disk with Non-DOS Diskette in Drive A:

IP.com Disclosure Number: IPCOM000105783D
Original Publication Date: 1993-Sep-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 4 page(s) / 99K

Publishing Venue

IBM

Related People

Forder, DA: AUTHOR [+3]

Abstract

Disclosed is a method to allow a Personal Computer (PC) to boot from a fixed disk with a non-DOS diskette in drive A. This bypasses the normal requirement that DOS system files reside on a boot-up disk, reclaiming approximately 100 kilobytes of space for more data on the diskette and shortening power-on time.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 47% of the total text.

Boot from PC Fixed Disk with Non-DOS Diskette in Drive A:

      Disclosed is a method to allow a Personal Computer (PC) to boot
from a fixed disk with a non-DOS diskette in drive A.  This bypasses
the normal requirement that DOS system files reside on a boot-up
disk, reclaiming approximately 100 kilobytes of space for more data
on the diskette and shortening power-on time.

      During a normal power-on sequence, a ROM bootstrap routine
copies a bootstrap routine and information from the first sector
(boot sector) of the first disk present in the machine into memory
and executes it.  Among this information is a table containing data
about the disk type and its format (e.g., sectors, clusters, etc.).
The bootstrap routine checks the drive number in the table to see if
DOS is present on that disk.  If so, it loads DOS from that disk into
memory and passes control to it, making the PC operational.  The
figure shows this process.  Steps 1, 2, and 3 represent a normal
power-on to the point of checking for DOS.

      This sequence can be streamlined by modifying the boot sector
of a blank-formatted diskette so that if the PC powers on with this
diskette in drive A, it will load DOS from the fixed disk.  The
modifications can be grouped as follows:

1.  Shorten the error message ("Non-System disk or disk error.
    Replace and press any key when ready.") in the boot sector to
    create room for additional code.
2.  Copy the characteristics of the fixed disk from the fixed disk's
    boot sector to this new space in the diskette's boot sector.  The
    original diskette characteristics must remain to be used when
    reading or writing on the diskette.
3.  Add a Jump instruction in the original bootstrap code to branch
    to our new code (to be added in step 4).
4.  Add code to copy the fixed disk's information over the diskette's
    information in memory.

      As before, when powering up or restarting the PC, control
transfers to the diskette's bootstrap routine.  The modified code
will be invoked to overlay the disk information after it is in
memory.  In the figure, step 4 represents the execution of the boot
sector modifications.  Control returns to the original program, and
the bootstrap continues.  The bootstrap checks the table in memory,
but it sees the fixed disk when it comes from the diskette.  The
bootstrap then loads DOS from the fixed disk and transfers control to
it to finish the process.

      The offsets of the information and the location of the error
message varies between levels of DOS, so it is important to use the
correct values for the particular level of DOS in use.  Also, the
fixed disk and subject diskette must be formatted using the same
level of DOS.  Since specific fixed disk information resides on the
modified diskette, it can only be used in the PC that altered it
(since fixed disk sizes and types often vary between models and
styles of PCs).

EXAMPLE IMPL...