MARC 21 Specifications for Record Structure, Character Sets, and Exchange Media

EXCHANGE MEDIA: Part 4

Tape Transfer (Pre-1977)

January 2000

CONTENTS

INTRODUCTION

The specifications described below were valid prior to 1977. The primary difference between the obsolete and current tape specifications is that the pre-1977 specifications did not allow for records to be spanned. These specifications became obsolete when the new specifications were adopted by MARC 21 users at the end of 1977.

STANDARD

TAPE FORMAT

Tapes were available in two formats: nine channel tapes (odd parity) written at 800 bpi and seven channel tapes (odd parity) written at 556 bpi. Tapes contained internal labels written in ASCII. Each label was an 80-byte record, the byte positions of which were numbered, starting with the leftmost byte position, 0 to 79. Each file of records was terminated by a tape mark (character 13(hex) for nine channel, character 17 (octal) for seven channel).

The total length of a record was given in the first five bytes of each new record. Records were stored in physical units which could contain a minimum of 12 characters up to a maximum of 2048 characters. Each record began at the first of a physical unit. If a record contained less than 2048 characters, then the size of the physical unit matched the size of the record and also contained less than 2048 characters. If the record contained more than 2048 characters, then the first 2048 characters of the record composed the first physical unit, the next 2048 characters the next physical unit, etc. The last physical unit containing the record could be less than 2048 characters in length. However, if the remainder of the record contained less than 12 characters, the physical unit containing this remainder was padded with blanks out to 12 characters.

TAPE FILE ORGANIZATION

Volume and file organization were as follows:

                     Volume Header Label (VOL1)
                     File Header Label (HDR1)
                       Tape Mark
                     File of data records
                       Tape Mark
                     End of File Label (EOF1)
                       Tape Mark
                       Tape Mark

TAPE LABELS

The structure and contents of the internal tape labels were as follows. Where numeric characters are indicated in Content, if the numeric value is shorter than the length allotted for the element, then the value is right justified and unused positions filled with zeros. Where alphanumeric characters are indicated in Content, if the alphanumeric value is shorter than the length allotted for the element, then the value is left justified and unused positions filled with blanks (ASCII character 20(hex)).

Volume Header Label (VOL1) Nine Channel Tapes:
    Element Name              Bytes  Length  Content (Description)
    Label Identifier          0-2    3       VOL
    Label Number              3      1       1
    Volume Serial             4-9    6       (alphanumeric characters)
    Accessibility             10     1       (blank)
    Unused                    11-36  26      (blanks)
    Owner Identifier          37-50  14      (alphanumeric characters)
    Unused                    51-78  28      (blanks)
    Label Standard Version    79     1       1
File Header Label (HDR1) Nine Channel Tapes:
    Element Name              Bytes  Length  Content (Description)
    Label Identifier          0-2    3       HDR
    Label Number              3      1       1
    File Identifier           4-20   17      (alphanumeric characters)
    File Set Identifier       21-26  6       (alphanumeric characters)
    File Section Number       27-30  4       0001
    File Sequence Number      31-34  4       0001
    Unused                    35-40  6       (blanks)
    Creation Date             41-46  6       (blank and 5 numeric characters*)
    Expiration Date           47-52  6       (blanks)
    Accessibility             53     1       (blank)
    Block Count               54-59  6       000000
    System Code               60-72  13      (alphanumeric characters)
    Unused                    73-79  7       (blanks)

* The date is formatted yyddd, where yy = last 2 digits of the year and ddd = Julian day.

End of File Label (EOF1) Nine Channel Tapes:
    Element Name              Bytes  Length  Content (Description)
    Label Identifier          0-2    3       EOF
    Label Number              3      1       1
    Same as File Header Label 4-53   50      (same as File Header Label)
    Block Count               54-59  6       (numeric characters)
    Same as File Header Label 60-79  20      (same as File Header Label)

TAPE LABELS -- SEVEN CHANNEL TAPES

Seven channel tapes also contained the above labels, but all alphabetic characters were lowercase.

TAPE MARK

The tape mark was a special block consisting of a gap of approximately 3.5 inches of tape followed by a single byte containing the character 13(hex) for nine channel tapes and 17 (octal) for seven channel tapes.


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