BTAM - Basic Tape Access Method

BTAM provides the user with an effective and versatile means of storing and retrieving the blocks of sequentially organized tape files. It can be used to process magnetic tapes, tape cartridges and tapes created on non-BS2000 systems.

BTAM can also process tape files which have nonstandard labels or no labels or which were created in accordance with DIN 66029, interchange level 1 (corresponding to LABEL=(STD,1) in the FILE/FCB macro or LABEL-TYPE=*STD(DIN-REVISION-NUMBER=1) in the ADD-FILE-LINK command).

By means of the BTAM macro, the user can position tapes to any desired block or file section. An important feature of BTAM is that a program may read and/or write data blocks as desired and change the processing direction, in both cases without having to close the file beforehand.

Codes supported by BTAM:

• EBCDIC

• ISO 7-bit code

• OWN code

As for SAM processing, the label specification can influence the code and vice versa.

BTAM functions

• Chained input/output
  (CHAINIO operand of the BTAM macro)

  Chained I/O economizes on CPU performance and saves time during data input and output.

• MAV mode
  (multi-job management, BTAMRQS/REQNO operand of the BTAM macro)

  Two or more tape I/O operations can be processed (asynchronously) in parallel. As MAV mode is extremely time-critical, it can be used to maximum advantage only with chained I/O: the greater the chaining factor, the better the device utilization and time behavior. Time behavior is enhanced by the fact that I/O operations can be performed continuously without the tape having to be repositioned for each operation. In the case of corresponding BTAM I/O operations, BTAM does not wait for the operation to be concluded, but returns control to the caller immediately. The caller can obtain the results of the I/O operation later with a WAIT/CHECK call. The BTAM I/O operation to which a subsequently issued WAIT/CHECK call belongs must be indicated in each asynchronous BTAM job by means of a request number (REQNO operand of the BTAM macro). The increase in performance derives from the fact that BTAM can execute several asynchronous operations in parallel. In other words, the next I/O operation does not have to wait until the previous operation has been completed. The highest request number which can be assigned (and thus the maximum number of asynchronous I/O operations which can be executed in parallel for a job) is defined by means of the BTAMRQS operand.

• Buffered input and output
  (TAPEWR operand of the BTAM macro)

  For files on magnetic tape cartridges.

BTAM macros

The following macros are available for processing tape files with the BTAM access method:

• all service macros (OPEN, CLOSE, etc.)

• the BTAM macro with the following functions:

  Function	Meaning
  CHK	Check the processing status of an I/O operation.
  ERG	Generate an interblock gap.
  POS	Position the tape.
  RBID	Determine the current tape position.
  RD/RDWT	Read data into main memory and wait for I/O termination.
  RDBF	Read data from the save area of the tape cartridge buffer.
  REV/REVWT	Read data in the reverse direction and optionally wait for I/O termination.
  RT/RTL	Read with data transfer; with/without message if the length is less than anticipated.
  RNT/RNTL	Read without data transfer; with/without message if the length is less than anticipated.
  SYNC	Synchronize and determine the tape position.
  WRT/WRTWT	Read data from main memory and wait for I/O termination.
  WT	Wait for completion of the I/O operation.

  Table 41: Functions of the BTAM macro

• The following control codes are provided for the positioning of tapes and the writing of tape marks (in the BTAM macro):

  BSF, BSR, FSF, FSR, REW, RUN, WTM.