Function 21h Random Read

mov dx, seg FileFCB

mov ds, dx

mov dx, offset FileFCB ;ds:dx points to FCB

mov ah, 21h ;Random Read

int 21h

cmp al, 0 ;zero means success

jne error_handler

Random Read (Function 21h) reads a record from the file identified by the file control block (FCB). Data read from the file is written to the memory at the current disk transfer address (DTA).

This function has been superseded by Read File or Device (Function 3Fh).

Parameter

FileFCB

Points to an FCB structure that identifies an open file. The structure must have been previously filled by using Open File with FCB (Function 0Fh) or Create File with FCB (Function 16h). Also, the fcbRandomRecNo field must specify the record to read. The FCB structure has the following form:

FCB STRUC

fcbDriveID db ? ;drive no. (0=default, 1=A, etc.)

fcbFileName db '????????' ;filename

fcbExtent db '???' ;file extension

fcbCurBlockNo dw ? ;current block number

fcbRecSize dw ? ;record size

fcbFileSize db 4 dup (?) ;size of file in bytes

fcbFileDate dw ? ;date file last modified

fcbFileTime dw ? ;time file last modified

fcbReserved db 8 dup (?) ;reserved

fcbCurRecNo db ? ;current record number

fcbRandomRecNo db 4 dup (?) ;random record number

FCB ENDS

For a full description of the FCB structure, see Chapter 3, “File System.”

Return Value

If the function is successful, the AL register contains 00h and the memory at the DTA contains the record read from the file. Otherwise, the AL register contains an error value, which may be one of the following:

Value

Meaning

01h	End of file encountered, no data in record
02h	Segment boundary overlapped by DTA, read canceled
03h	End of file encountered, partial record at DTA (rest of record filled with zeros)

Comments

MS-DOS updates the fcbCurBlockNo and fcbCurRecNo fields in the FCB structure to agree with the fcbRandomRecNo field before it attempts to read the record from the disk. No record numbers are incremented; successive calls to the function repeatedly read the same record.

This function can be used to read files on a network drive but only if the network has granted read (or similar) access to the given file or drive.