Table of Contents

Note: This API call is for DOS and Win16 personality only. Use Family API for portability.

2018/09/07 05:04 · prokushev · 0 Comments

Int 21H, AH=1BH

Version

1 and higher

Brief

GET ALLOCATION INFORMATION FOR DEFAULT DRIVE

Family API

none

Input

      AH = 1Bh

Return

AL = sectors per cluster (allocation unit) CX = bytes per sector DX = total number of clusters DS:BX → media ID byte (see #01356)

Notes

under DOS 1.x, DS:BX points at an actual copy of the FAT; later versions return a pointer to a copy of the FAT's ID byte

this function may not be properly supported on CD-ROMs and other installable file systems (use AX=4402h“CD-ROM” for CD-ROMs instead)

BUG: DOS 7.10 (Win95B/Win98) reportedly returns incorrect values in AL and DX for FAT12/FAT16 drives, even though it manages correct results for FAT32 drives (DOS windows are not affected, only real-mode DOS)

Values for media ID byte:

FFh floppy, double-sided, 8 sectors per track (320K)
FEh floppy, single-sided, 8 sectors per track (160K)
FDh floppy, double-sided, 9 sectors per track (360K)
FCh floppy, single-sided, 9 sectors per track (180K)
FAh HP 200LX D: ROM disk, 16 sectors per track (995K) HP 200LX E: (Stacker host drive ???)
F9h floppy, double-sided, 15 sectors per track (1.2M) floppy, double-sided, 9 sectors per track (720K,3.5“)
F8h hard disk
F0h other media (e.g. floppy, double-sized, 18 sectors per track – 1.44M,3.5”)

AH=1Ch,AH=36h

Note

Text based on Ralf Brown Interrupt List Release 61

2018/09/04 17:23 · prokushev · 0 Comments