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.

Standards

American National Standard Magnetic Tape Labels for Information Interchange (ANSI X3.27-1969)

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.

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

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)

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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