RCA 301, Programmer'S Reference Manual

Page: 15 / 235

are recorded on tape serially so that the characters making up an item follow one another in sequence from most to
least significant. Interblock gaps on the Hi-Data Tape Group, Model 381, are nominally .34 inch. Interblock gaps
on the Hi-Data Tape Group, Model 382, are nominally .54 inch. All data on 33KC and 66KCTrtiagnetic tape is dually
recorded. Interblock gaps on 33KC and 66KC tape are nominally .45 inch and .55 inch respectively.
Variable Item and Record Length
Data storage in the RCA 301 system incorporates true variable item length. This concept may be more fully
appreciated if prefaced with a definition of fixed and fixed variable word and block lengths.
"Word" is generally defined as a fixed number of consecutive characters or character locations, and "block"
as a fixed number of consecutive words. These terms, word and block, are more aptly used with respect to fixed
and fixed variable systems.
In a computer system using fixed word length, the number of characters for each word, or the number of words
for each block, cannot be changed. In order to store data of a varying nature, redundant zeros or spaces are used to
fill out the incomplete words, with uneconomical use of space.
In a fixed variable system, the number of character positions for each item in a record is assigned in accordance
with the anticipated maximum length for that .item. These lengths may be individually predetermined for each file,
but remain constant for each item within the file. Fixed variable word length does provide greater flexibility than
does fixed word length.
Data storage in a true variable length system does not have the limitations imposed by fixed or fixed variable
systems. In the RCA 301, the use of control symbols and the ability to address each character location individually
permits the length of any item in any record to be in strict accordance with that item's actual character count. This
allows for total variability of item and record length but does not preclude the use of fixed or fixed variable lengths
when the programmer finds this expedient.
In each of these categories—fixed, fixed variable, and variable, the method of internal storage is extended to
the external storage. Therefore, if redundant zeros of the fixed and fixed variable systems are used in the computer,
they would also appear on magnetic tape or other storage devices. By utilization of true variable length in the RCA 301
system, a characteristic business file requires less space in the Data Disc and Data Record File and less tape footage,
and can be read and written out in less time than a fixed or fixed variable system would permit.
1-9