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:

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.