When you read a sequential data set, you can determine the length of the block in one of the following ways, depending on the access method and record format of the data set.

For unblocked and undefined record formats, each block contains one logical record.

1. Fixed-length, unblocked records: The length of all records is the value in the DCBBLKSI field of the DCB without LBI or the DCBEBLKSI field of the DCBE with LBI. You can use this method with BSAM or BPAM.
2. Variable-length records and Format-D records with BUFOFF=L: The block descriptor word in the block contains the length of the block. You can use this method with BSAM or BPAM. Block Descriptor Word (BDW) describes the BDW format.
3. Format-D records without BUFOFF=L: The block length is in DCBLRECL after you issue the CHECK macro. It remains valid until you again issue a CHECK macro.
4. Undefined-length records when using LBI or for fixed-length blocked: The method described in the following paragraphs can be used to calculate the block length. You can use this method with BSAM, BPAM, or BDAM. (It should not be used when using chained scheduling with format-U records. In that case, the length of a record cannot be determined.
   1. After issuing the CHECK macro for the DECB for the READ request, but before issuing any subsequent data management macros that specify the DCB for the READ request, obtain the status area address in the word that is 16 bytes from the start of the DECB.
   2. If you are not using LBI, take the following steps:
      1. Obtain the residual count that has been stored in the status area. The residual count is in the halfword, 14 bytes from the start of the status area.
      2. Subtract this residual count from the number of data bytes requested to be read by the READ macro. If 'S' was coded as the length parameter of the READ macro, the number of bytes requested is the value of DCBBLKSI at the time the READ was issued. If the length was coded in the READ macro, this value is the number of data bytes and it is contained in the halfword 6 bytes from the beginning of the DECB. The result of the subtraction is the length of the block read.
      If you are using LBI for BSAM or BPAM, subtract 12 from the address of the status area. This gives the address of the 4 bytes that contain the length of the block read.
5. Undefined-length records when not using LBI: The actual length of the record that was read is returned in the DCBLRECL field of the DCB. Because of this use of DCBLRECL, you should omit LRECL. Use this method only with BSAM, or BPAM or after issuing a QSAM GET macro.

          ...
          OPEN      (DCB,(INPUT))
          LA        DCBR,DCB
          USING     IHADCB,DCBR
          ...
          READ      DECB1,SF,DCB,AREA1,'S'
          READ      DECB2,SF,DCB,AREA2,50
          ...
          CHECK     DECB1
          LH        WORK1,DCBBLKSI               Block size at time of READ
          L         WORK2,DECB1+16               Status area address
          SH        WORK1,14(WORK2)              WORK1 has block length
          ...
          CHECK     DECB2
          LH        WORK1,DECB2+6                Length requested
          L         WORK2,DECB2+16               Status area address
          SH        WORK1,14(WORK2)              WORK1 has block length
          ...
          MVC       DCBBLKSI,LENGTH3             Length to be read
          READ      DECB3,SF,DCB,AREA3
          ...
          CHECK     DECB3
          LH        WORK1,LENGTH3                Block size at time of READ
          L         WORK2,DECB+16                Status area address
          SH        WORK1,14(WORK2)              WORK1 has block length
          ...
DCB       DCB       ...RECFM=U,NCP=2,...
          DCBD
          ...

When you write a short block to an extended-format data set, the system pads it to full length but retains the value of what your program said is the length. When you read such a block, be aware that the system reads as many bytes as the block can have and is not limited by the length specified for the write. If you know that a particular block is short and you plan to read it to a short data area, then you must decrease DCBBLKSI or DCBEBLKSI with LBI to the length of the short area before the READ.