How to Determine if BIOS Reports Floppy Drive Type Correctly

This article was previously published under Q76677
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MS-DOS determines the number and type of floppy drives on a system usingthe ROM BIOS services, called interrupts. The MS-DOS DEBUG utility can beused to "ask" the ROM BIOS for this information directly. This is helpfulin determining why MS-DOS may not be accessing a floppy drive or drivescorrectly.

It is possible that one version of MS-DOS might access a drive and anotherversion may not. Some hardware manufacturers modify their MS-DOS to ignorethe information returned from the ROM BIOS and rely on vendor-specificinformation about the drives.

Early versions of Microsoft MS-DOS make assumptions about theexistence of one or more floppy drives that might, coincidentally, becorrect. However, in Microsoft MS-DOS versions 4.0 and later, MS-DOScorrectly relies on the information reported by the ROM BIOS.

The following procedure uses function 8 of interrupt 13 to determinedisk parameters. The number of sectors per track and tracks isreturned, which correlates between the information returned and theparameters MS-DOS uses for the FORMAT utility. On AT and PS/2 systems, adrive type code is returned as well.

If the BIOS is not returning the correct drive information on an AT orPS/2 machine, it may indicate that the drive type has not been setproperly in the CMOS setup. On any machine, it may indicate that thedrive is not properly installed, or that the drive is not supported by theROM BIOS. For information about your ROM BIOS and whether you need toupgrade, contact your machine or BIOS manufacturer.
More information
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 exitand start over, use the Q[uit] command to exit back to MS-DOS. It isrecommended that DEBUG not be used when other programs are active or in amultitasking environment such as Microsoft Windows.
DEBUGDisplays        Enter           To--------        -----           ---               a 100           Begin entering commands at address 100XXXX:0100       int 13          Do an interrupt 13XXXX:0102       int 20          An interrupt 20 here (insurance)XXXX:0104       ENTER           Press ENTER here-               rip             Display and modify the IP registerIP XXXX:               0100            Begin executing at 100-               rax             Display and modify AX registerAX 0000:               0800            Function 8 (get drive information)-               rdx             Display and modify DX registerDX 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. TheROM BIOS routine will process interrupt 13, function 8, on the drive youspecified, and return some information in the registers. DEBUG will displaysomething like this:
AX=0000  BX=0002  CX=4F0F  DX=0102  SP=FFEE  BP=0000  SI=0000  DI=2115DS=0E77  ES=F000  SS=0E77  CS=0E77  IP=0102   OV UP EI NG NZ NA PO NC0E77:0102 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 beaddressed by its "high" and "low" halves; that is, if CX=4F0F, CH=4Fand CL=0F.

The flags register is displayed differently. The status of the flagsregister is the series of two-letter codes at the end of the secondline. 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 interruptfailed. In this case, AX= an error value. See page 54 of the "IBM ROMBIOS Quick Reference" guide for information about these errors. If thelast flag is NC (no carry), the carry flag was not set, whichindicates that the interrupt worked correctly.

On AT and PS/2 systems, the low byte of the BX register (BL) willcontain the drive type 01 if 360, 02 if 1.2, 03 if 720, and 04 if1.44.

The maximum value for the last track on the drive is stored in CH: 27hexadecimal (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 headnumber is stored in DH; because floppies have two heads, this is 1.(The ROM BIOS numbers heads and tracks, or cylinders, from 0, andsectors from 1.)

Finally, DL indicates the number of floppy drives. Note that the valuereturned in DL (number of drives) is the number of floppy drivesattached to the disk controller for the specified drive. Normally,there is only one controller, and thus DL=the total number offloppies. However, if floppies A and B are attached to differentcontrollers, then DL=1 will be returned for each.

So, from the previous example (values unrelated to the drive type areindicated by xx):
AX=xxxx  BX=xx02  CX=4F0F  DX=0102  SP=xxxx  BP=xxxx  SI=xxxx  DI=xxxxDS=xxxx  ES=xxxx  SS=xxxx  CS=xxxx  IP=xxxx   xx xx xx xx xx xx xx NCxxxx:xxxx xxxx          xxx     xx				
NC, so no error; BL = 02, so 1.2 MB; CH=4F, or 80 tracks; CL=0F, or 15sectors per track; DH=01, or 2 heads; DL=2, two drives. All of whichindicates that there are two drives on this system, and this particularfloppy drive is 1.2 MB.
"IBM ROM BIOS Quick Reference," Ray Duncan. Microsoft Press."QUE DOS Programmer's Reference," Terry Dettman. QUE Corporation.
6.22 3.x 4.x 5.00 5.00a 6.00 6.20

Article ID: 76677 - Last Review: 10/26/2013 13:34:00 - Revision: 3.0

Microsoft MS-DOS 4.0 Standard Edition, Microsoft MS-DOS 5.0 Standard Edition, Microsoft MS-DOS 6.0 Standard Edition, Microsoft MS-DOS 6.2 Standard Edition, Microsoft MS-DOS 6.21 Standard Edition, Microsoft MS-DOS 6.22 Standard Edition

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