Interrupt 21h Function 440Dh Minor Code 71h

Retrieves the first cluster of the specified file or directory.

mov ax, 440Dh            ; generic IOCTL
mov bx, CharSet          ; see below
mov ch, 08h              ; device category
mov cl, 71h              ; Get First Cluster
mov dx, seg PathName     ; see below
mov ds, dx
mov dx, offset PathName  
int 21h

jc error

Parameters

CharSet

Character set of PathName. This parameter must be one of these values:

BCS_WANSI (0)	Windows ANSI character set
BCS_OEM (1)	Current OEM character set
BCS_UNICODE (2)	Unicode character set

PathName

Address of a null-terminated string containing the path of the file or directory to retrieve the first cluster for.

Return Value

Clears the carry flag and sets DX:AX to the first cluster number if successful. Otherwise, the function sets the carry flag and returns either the ERROR_INVALID_FUNCTION or ERROR_ACCESS_DENIED value in AX.

Remarks

The first cluster of a file is the first cluster of the FAT cluster chain describing the data associated with the file. The first cluster of a directory is the first cluster of the FAT cluster chain associated with the directory. It is the cluster that contains the "." and ".." entries. The function finds any file or directory regardless of attribute (system, hidden, or read-only). It does not find volume labels.

If your application is unable to accommodate a 32-bit cluster number, you must check to see if the value returned in the DX register is greater than zero.

if(MAKELONG(regAX,regDX) > 0x0000FFF8)
    b32BitNum = TRUE;
else
    b32BitNum = FALSE;
 

It is the calling application's responsibility to check to see if the returned cluster number is valid.

if((MAKELONG(regAX,regDX) < 2L) || (MAKELONG(regAX,regDX) > maxClus))
    bInvalidNum = TRUE;
else
    bInvalidNum = FALSE;
 

In the preceding example, the maxClus variable is the maximum legal cluster number, as a DWORD type, computed from the drive parameters.