Data set information block

The data set information block is a control block that contains information about a file that the I/O replaceable routine opens. For an OPENR, OPENW, or OPENX function request, the I/O routine returns the address of the data set information block (DSIB) in parameter 2, and the length of the DSIB in parameter 3. TSO/E provides a mapping macro IRXDSIB you can use to map the block. The mapping macro is in SYS1.MACLIB.

Table 1 shows the format of the control block.

Table 1. Format of the data set information block
Offset (decimal)	Number of bytes	Field name	Description
0	8	ID	An eight character string that identifies the information block. It contains the characters ‘IRXDSIB’.
8	2	LENGTH	The length of the data set information block.
10	2	—	Reserved.
12	8	DDNAME	An eight character string that specifies the ddname for which information is being returned. This is the DD that the I/O routine opened.
20	4	FLAGS	A fullword of bits that are used as flags. Only the first nine bits are used. The remaining bits are reserved. The flag bits indicate whether information is returned in the fields at offset +24 - offset +42. Each flag bit corresponds to one of the remaining fields in the control block. Information about how to use the flag bits and their corresponding fields is provided after the table.
24	2	LRECL	The logical record length (LRECL) of the data set. This field is required. Note: The LRECL field and its corresponding flag bit (at offset +20) are the last required fields to be returned in the data set information block. The remaining fields are not required.
26	2	BLKSZ	The block size (BLKSIZE) of the data set.
28	2	DSORG	The data set organization (DSORG) of the data set. ‘0200’ - Data set is partitioned. ‘0300’ - Data set is partitioned and unmovable. ‘4000’ - Data set is sequential. ‘4100’ - Data set is sequential and unmovable.
30	2	RECFM	The record format (RECFM) of the data set. ‘F ’ - Fixed ‘FB’ - Fixed blocked ‘V ’ - Variable ‘VB’ - Variable blocked 'VS' - Variable spanned 'VX' - Variable blocked spanned 'U' - Undefined.
32	4	GET_CNT	The total number of records read by the GET macro for this DCB.
36	4	PUT_CNT	The total number of records written by the PUT or PUTX macro for this DCB.
40	1	IO_MODE	The mode in which the DCB was opened. ‘R’ - open for READ (uses GET macro) ‘X’ - open for READX (update uses GET and PUTX macros) ‘W’ - open for WRITE (uses PUT macro) ‘L’ - open for exec load (uses READ macro)
41	1	CC	Carriage control information. ‘A’ - ANSI carriage control ‘M’ - machine carriage control ‘ ’ - no carriage control
42	1	TRC	IBM® 3800 Printing Subsystem character set control information. ‘Y’ - character set control characters are present ‘N’ - character set control characters are not present
43	1	—	Reserved.
44	4	—	Reserved.

At offset +20 in the data set information block, there is a fullword of bits that are used as flags. Only the first nine bits are used. The remaining bits are reserved. The bits are used to indicate whether information is returned in each field in the control block starting at offset +24. A bit must be set on if its corresponding field is returning a value. If the bit is set off, its corresponding field is ignored.

The flag bits are:

The LRECL flag. This bit must be on and the logical record length must be returned at offset +24. The logical record length is the only data set attribute that is required. The remaining eight attributes starting at offset +26 in the control block are optional.
The BLKSIZE flag. This bit must be set on if you are returning the block size at offset +26.
The DSORG flag. This bit must be set on if you are returning the data set organization at offset +28.
The RECFM flag. This bit must be set on if you are returning the record format at offset +30.
The GET flag. This bit must be set on if you are returning the total number of records read at offset +32.
The PUT flag. This bit must be set on if you are returning the total number of records written at offset +36.
The MODE flag. This bit must be set on if you are returning the mode in which the DCB was opened at offset +40.
The CC flag. This bit must be set on if you are returning carriage control information at offset +41.
The TRC flag. This bit must be set on if you are returning IBM 3800 Printing Subsystem character set control information at offset +42.