Q76677: How to Determine if BIOS Reports Floppy Drive Type Correctly
Article: Q76677
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:3.x,4.x,5.x,6.0,6.2,6.21,6.22
Operating System(s):
Keyword(s):
Last Modified: 17-DEC-2000
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The information in this article applies to:
- Microsoft MS-DOS operating system versions 3.1, 3.2, 3.21, 3.3, 3.3a, 4.0, 4.01, 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22
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SUMMARY
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MS-DOS determines the number and type of floppy drives on a system using the ROM
BIOS services, called interrupts. The MS-DOS DEBUG utility can be used to "ask"
the ROM BIOS for this information directly. This is helpful in determining why
MS-DOS may not be accessing a floppy drive or drives correctly.
It is possible that one version of MS-DOS might access a drive and another
version may not. Some hardware manufacturers modify their MS-DOS to ignore the
information returned from the ROM BIOS and rely on vendor-specific information
about the drives.
Early versions of Microsoft MS-DOS make assumptions about the existence of one or
more floppy drives that might, coincidentally, be correct. However, in Microsoft
MS-DOS versions 4.0 and later, MS-DOS correctly relies on the information
reported by the ROM BIOS.
The following procedure uses function 8 of interrupt 13 to determine disk
parameters. The number of sectors per track and tracks is returned, which
correlates between the information returned and the parameters MS-DOS uses for
the FORMAT utility. On AT and PS/2 systems, a drive type code is returned as
well.
If the BIOS is not returning the correct drive information on an AT or PS/2
machine, it may indicate that the drive type has not been set properly in the
CMOS setup. On any machine, it may indicate that the drive is not properly
installed, or that the drive is not supported by the ROM BIOS. For information
about your ROM BIOS and whether you need to upgrade, contact your machine or
BIOS manufacturer.
MORE INFORMATION
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To start the DEBUG utility, type "debug" (without the quotation marks) at the
command prompt and press ENTER. If at any time you wish to exit and start over,
use the Q[uit] command to exit back to MS-DOS. It is recommended that DEBUG not
be used when other programs are active or in a multitasking environment such as
Microsoft Windows.
DEBUG
Displays Enter To
-------- ----- --
- a 100 Begin entering commands at address 100
XXXX:0100 int 13 Do an interrupt 13
XXXX:0102 int 20 An interrupt 20 here (insurance)
XXXX:0104 ENTER Press ENTER here
- rip Display and modify the IP register
IP XXXX
: 0100 Begin executing at 100
- rax Display and modify AX register
AX 0000
: 0800 Function 8 (get drive information)
- rdx Display and modify DX register
DX 0000
: <drive> Enter 0 for first floppy, 1 for
second, and so on
- p Process
At this point, DEBUG will execute the command at 0100, interrupt 13. The ROM BIOS
routine will process interrupt 13, function 8, on the drive you specified, and
return some information in the registers. DEBUG will display something like
this:
AX=0000 BX=0002 CX=4F0F DX=0102 SP=FFEE BP=0000 SI=0000 DI=2115
DS=0E77 ES=F000 SS=0E77 CS=0E77 IP=0102 OV UP EI NG NZ NA PO NC
0E77:0102 CD20 INT 20
The name of each register is displayed, along with its current value. All values
are in hexadecimal. Note that each X register can also be addressed by its
"high" and "low" halves; that is, if CX=4F0F, CH=4F and CL=0F.
The flags register is displayed differently. The status of the flags register is
the series of two-letter codes at the end of the second line. Note the value of
the last flag on the second line. If it is CY (CarrY), the carry flag was set by
the BIOS, which means the interrupt failed. In this case, AX= an error value.
See page 54 of the "IBM ROM BIOS Quick Reference" guide for information about
these errors. If the last flag is NC (no carry), the carry flag was not set,
which indicates that the interrupt worked correctly.
On AT and PS/2 systems, the low byte of the BX register (BL) will contain the
drive type 01 if 360, 02 if 1.2, 03 if 720, and 04 if 1.44.
The maximum value for the last track on the drive is stored in CH: 27 hexadecimal
(39 decimal) if there are 40 tracks maximum, or 4Fh (79d) if there are 80
tracks. The maximum sector number is stored in CL: 9h (9d), Fh (15d), 12h (18d),
or 24h (36d). Finally, the maximum head number is stored in DH; because floppies
have two heads, this is 1. (The ROM BIOS numbers heads and tracks, or cylinders,
from 0, and sectors from 1.)
Finally, DL indicates the number of floppy drives. Note that the value returned
in DL (number of drives) is the number of floppy drives attached to the disk
controller for the specified drive. Normally, there is only one controller, and
thus DL=the total number of floppies. However, if floppies A and B are attached
to different controllers, then DL=1 will be returned for each.
So, from the previous example (values unrelated to the drive type are indicated
by xx):
AX=xxxx BX=xx02 CX=4F0F DX=0102 SP=xxxx BP=xxxx SI=xxxx DI=xxxx
DS=xxxx ES=xxxx SS=xxxx CS=xxxx IP=xxxx xx xx xx xx xx xx xx NC
xxxx:xxxx xxxx xxx xx
NC, so no error; BL = 02, so 1.2 MB; CH=4F, or 80 tracks; CL=0F, or 15 sectors
per track; DH=01, or 2 heads; DL=2, two drives. All of which indicates that
there are two drives on this system, and this particular floppy drive is 1.2 MB.
REFERENCES
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"IBM ROM BIOS Quick Reference," Ray Duncan. Microsoft Press. "QUE DOS
Programmer's Reference," Terry Dettman. QUE Corporation.
Additional query words: 6.22 3.x 4.x 5.00 5.00a 6.00 6.20
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Keywords :
Technology : kbMSDOSSearch kbMSDOS321 kbMSDOS400 kbMSDOS320 kbMSDOS330a kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS310 kbMSDOS500 kbMSDOS330 kbMSDOS401 kbMSDOS500a
Version : MS-DOS:3.x,4.x,5.x,6.0,6.2,6.21,6.22
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