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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.
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.
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:
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
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.
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 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):
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.
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
"IBM ROM BIOS Quick Reference," Ray Duncan. Microsoft Press. "QUE DOS Programmer's Reference," Terry Dettman. QUE Corporation.
Article ID: 76677 - Last Review: October 26, 2013 - Revision: 3.0
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