MAS Technical Note - Info about AWS Tapes

AWS Tape Information

The Mainframe Appliance for Storage (MAS) stores virtual tape volumes on disk in the AWSTAPE format. The AWS format allows the MAS to maintain an exact representation of a physical tape, including variable block sizes, tapemarks, labels, etc.

AWSTAPE File Format

Each AWSTAPE disk file emulates one single, physical tape volume.

Each emulated physical record (block) in the emulated tape volume is represented in the disk file by an AWS Block Header followed by data. An emulated tapemark is represented by a Block Header only.

The structure of the AWSTAPE block header is as follows. Note that Bus-Tech has made several changes to the standard AWSTAPE format to support data compression and large blocks.

/*--------------------------------------------------------------------*/
/* Structure definition for AWSTAPE block header                      */
/*--------------------------------------------------------------------*/
typedef struct _AWSTAPE_BLKHDR {
        HWORD   curblkl;             /* Length of this block          */
        HWORD   prvblkl;             /* Length of previous block      */
        BYTE    flags1;              /* Flags byte 1                  */
        BYTE    flags2;              /* Flags byte 2                  */
    } AWSTAPE_BLKHDR;

/* Definitions for flags1 */
#define AWSTAPE_FLAG1_NEWREC    0x80 /* Start of a data record.       */
#define AWSTAPE_FLAG1_TAPEMARK  0x40 /* Tape mark                     */
#define AWSTAPE_FLAG1_ENDREC    0x20 /* End of a data record record.  */
#define AWSTAPE_FLAG1_SEGMENTED 0x10 /* Segmented record. This bit is */
                                     /* set on, in addition to NEWREC,*/
                                     /* in the first segment of       */
                                     /* segmented data blocks when the*/
                                     /* MAS writes a large (>65535    */
                                     /* bytes) block. When the MAS    */
                                     /* reads blocks that span        */
                                     /* multiple AWS segments, the    */
                                     /* SEGMENTED bit is optional.    */
                                     /* This is a Bus-Tech extension  */
                                     /* to the standard aws format.   */


/* Definitions for flags2 */         
#define AWSTAPE_FLAG2_COMPR    0x80  /* Data compressed with zlib     */ 
                                     /* version 1.1.14. In this case, */
                                     /* curblkl contains the post-    */
                                     /* compression size; the original*/
                                     /* blocksize can only be         */
                                     /* determined by decompressing.  */
                                     /* This is a Bus-Tech extension  */
                                     /* to the standard aws format.   */
#define AWSTAPE_FLAG2_HWCMPRS   0x40 /* Data compressed with Bus-Tech */
                                     /* hardware compression.         */
                                     /* This is a Bus-Tech extension  */
                                     /* to the standard aws format.   */
#define AWSTAPE_FLAG2_ENCRYPTED 0x20 /* Encrypted block               */
                                     /* This is a Bus-Tech extension  */
                                     /* to the standard aws format.   */
/*--------------------------------------------------------------------*/

An HWORD is a two-byte (16 bit) unsigned value. Values in an HWORD are in Intel "little-endian" (low-byte, hi-byte) format.

A BYTE is an unsigned one-byte (8 bit) value.

The block length (current and previous) defines the length of the AWS block itself, not necessarily the length of the original tape block (in cases where a tape block spans multiple AWS blocks, or when the data has been compressed). The length field does not include the length of the AWS block header itself, just the data block. The maximum amount of data that can be stored in a single AWS data block hex FFFF (65535 decimal) bytes.

The first block header in an AWSTAPE file has a previous block length (prvblkl) of zero.

A tapemark has a current block length (curblkl) of zero, and AWSTAPE_FLAG1_TAPEMARK flag set in flags1. NEWREC and ENDREC are both off in all tapemark block headers.

The NEWREC bit in flags1 signifies the start of a tape data block. The ENDREC bit in flags1 signifies the end of a tape data block. If a tape data blocks fits into a single AWS block, both NEWREC and ENDREC will be set.

A tape block can span multiple AWS blocks. Typically, this would be done to store a tape block that was larger then 65535 bytes. In this case, NEWREC will be set in the first AWS block of the chain, and ENDREC will be set in the last. If a tape data block spans more than two AWS blocks, the AWS blocks inbetween the first and last will have neither NEWREC nor ENDREC set.

Undefined bits in flags1 and flags2 should all be off. These bits are reserved for future expansion, and may cause unpredictable results if used.

No special record, block header or flag signifies the end of the AWS virtual tape volume; the end of the disk file simply signifies the end of the tape volume. Typically, the last data in the file will be a block header for a tapemark, but that is not a requirement; the file could end with the end of a data record.

Data in an AWSTAPE data record is stored unchanged from how the mainframe sent it. Normally, mainframe data is in EBCDIC format. But if the mainframe application writes the data in ASCII, then the AWSTAPE data record will contain ASCII data.

More Information

You can find more information, utilities, and links about AWS tapes at this location. (Note: This link is provided as a convenience. It takes you outside of the Bus-Tech support web; Bus-Tech is not responsible for any information that you may find at this location.)