DIGITAL RESEARCH(r) CP/M Plus TM (CP/M Version 3) Operating ...

6.3 ED Operation CP/M 3 User's Guide
6.3 ED Operation
With ED, you change portions of a file that pass through a memory buffer. When
you start ED with one of the preceding commands, this memory buffer is empty. At
your command, ED reads segments of the source file, for example MYFILE.TEX,
into the memory buffer for you to edit. If the file is new, you must insert text into
the file before you can edit. During the edit, ED writes the edited text onto a temporary
work file, MYFILE.$$$.
When you end the edit, ED writes the memory buffer contents to the temporary
file, followed by any remaining text in the source file. ED then changes the name of
the source file from MYFILE.TEX to MYFILE.BAK, so you can reclaim this original
material from the back-up file if necessary. ED then renames the temporary file,
MYFILE.$$$, to MYFILE.TEX, the new edited file. The following figure illustrates
the relationship between the source file, the temporary work file, and the new file.
Note: when you invoke ED with two filespecs, an input file and an output file, ED
does not rename the input file to type BAK; therefore, the input file can be Read-
Only or on a write-protected disk if the output file is written to another disk.
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