For Direct Access Data Files - IBM 5110 User Manual

110
When the 5110 APL language is used to read a general exchange
format data file (see
To Read a Sequential Access Data File
later in
this chapter), the following actions are taken by the 5110 if a cursor
return character (hex 9C) or end-of-block character (hex FF) was
embedded in a character vector that was written to the data file:
• If a cursor return character was embedded in the character vector,
the data will be read from tape in a different sequence than it was
written to tape. This condition occurs because as the interchange
data is written to tape, the system writes an end-of-record
character (hex 9C) after each character vector (record) that was
written to tape. The end-of-record character and the cursor return
character are the same. When used on tape, this character
separates the data (records) so that it can be read from tape in the
same sequence as it was written to tape. However, if a cursor
return character is embedded in the data that was written to tape,
the system will recognize it as an end-of-record character when the
data is read from tape.
• If an end-of-block character was embedded in the character vector,
any data from the embedded end-of-block character to the next
physical record is not read from tape. This condition occurs
because the system looks at the tape in 512-byte segments (one
physical record). A physical record can be terminated by an
end-of-block character (hex FF). When the system is reading data
from the tape and an end-of-block character is encountered, the
system skips to the next physical record and continues reading data.
Therefore, if an hex FF character is embedded in the data that was
written to tape, the system recognizes it as an end-of-block
character when the data is read from tape and skips ahead to the
next physical record.
For Direct Access Data Files
There are five types of data file formats for direct access data files.
These formats are:
• APL internal format
• General exchange format
• Unblocked and unspanned format
•• Mixed format
• Nontranslated format
Following is a description of these data file formats.