Commodore 1541-II - Save with Replace a; Verifying Programs

SAVE WITH REPLACE OPTION
If a file already exists, it can't be saved again because the disk only allows one copy
of any given file name per diskette. It is possible to get around this problem using the
Rename and Scratch commands described later. However, if all you wish to do is replace
a program or data file with a revised version, another command is more convenient.
Known as Save-with-replace , or ©Save, this option tells the disk to replace any file it
finds in the directory with the same name, substituting the new file for it.
FORMAT FOR SAVE WITH REPLACE: FORMAT FOR SAVE WITH REPLACE:
BASIC 2 BASIC 3.5
SAVE "©Drive #:file name", device # DSAVE 4<@file name",Ddrive #,
Udevice #
where all the parameters are as usual except for adding a leading "at1' sign (@.) The
"0:" in the Basic 2 version, though a holdover from earlier dual drives is required here.
EXAMPLES:
SAVE"@0:REVISED PROGRAM",8 DSAVE "©REVISED PROGRAM"
The actual procedure is that the new version is saved completely, then the old version
is scratched, and its directory entry altered to point to the new version. Because it works
this way, there is little, if any, danger that a disaster such as having the power going off
midway through the process would destroy both the old and new copies of the file.
Nothing happens to the old copy until after the new copy is saved properly.
However, we do offer one caution—do not use ©Save on an almost-full diskette.
Only use it when you have enough room on the diskette to hold a second complete copy of
the program being replaced. Due to the way @Save works, both the old and new versions
of the file are on disk simultaneously at one point, as a way of safeguarding against loss of
the program. If there is not enough room left on diskette to hold that second copy, only as
much of the new version will be saved on the 1541 as there is still room for. After the
command completes, a look at a directory will show the new version is present, but
doesn't occupy enough blocks to match the copy in memory. Unfortunately, the Verify
command (see next section) will not detect this problem, because however much did get
saved will have been saved properly.
VERIFYING PROGRAMS
Although not as necessary with a disk drive as with a cassette, Basic's Verify
command can be used to make doubly certain that a program file was properly saved to
disk% It works much like the Load command, except that it only compares each
character in the program against the equivalent character in the computer's mem
ory, instead of actually being copied into memory.
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