Lsdos X

I recently purchased PFS.Files, and noted In the user's manual that it is limited to making 5 backup copies. At first glance this seemed a rather small number, and I was a little concerned in case I ran into trouble. (You never know!)

Being both nosey and cautious, I decided to enter into a little research project to determine as much as possible about operation with a copy-limited program.

The first step was to install a write-protect tab on the original disk. Sort of like AIDS protection. Next, I used one of the aftermarket copying utilities to make a verbatim copy without using up any of the five official copies.

This first copy I identified as disk #0. Disk 0 is a bootable disk, which turned out to have TRSDOS V6.2.0 on it.

I then proceeded to make copy #1, #2, #3, #4, and #5 from disk #0.

When I attempted copy #6, the DOS reported, "Disk contains protected files. Backup reconstruct invoked.", and then went on to make a copy.

I then attempted to boot all 7 disks, #0 through #6.

The first 6 booted properly and AUTO started PFS.File.

The last disk, #6, would bootup OK, and issue the AUTO command. But the DOS then responded with "File not found." A DIR check of the disks showed that #0 through #5 contained all the same files, but #6 was lacking two files; PFS/CMD and FILES/CMD. Obviously these are the protected files.

So far, the "5 permitted copies" seemed valid. BUT!

I next booted up with disk #1 and attempted to back it up to the freshly reformatted #6 disk. No squawks, it backed up just fine. And now #6 boots up PFS just fine!

So I continued with backing up from #1, and ended up with bootable PFS disks #6, #7, #8, and #9. These all booted PFS correctly.

#10 however, lacked the two protected files again.

Next I reformatted #10. I now went to disk #2 and repeated the process. This time I got three working disks, #10, #11, and #12.

I won't bore you with the repetitions, but instead will refer you to the chart below which reveals what is going on.

There is a counter on the original disk #0, initially set at 5. As each backup is done from this disk, the counter is first checked, and if not zero, decremented and a new backup made. This is a perfectly good, bootable disk with all files on it. The copy's counter matches the decremented value of the disk it is being copied from. Only when the source disk has decremented to zero does it complain and refuse to backup the protected files.

The table is constructed in the following fashion: Row A has only the original disk #0 in it. This disk is initially good for 5 copies, after which the counter is zeroed out. (Actually an FF is placed in the counter at that point, 0 never appears). The next row B has 5 working copies, #1 through #5. #1 is initially good for 4 copies, #2 is good for 3 copies, etc. Row C has good disks #6 through #15. Row D has good disks #16 through #25. Row E has good disks #26 through #30. The last row F has only one good disk, #31, for a grand total of 32 working disks, including the starting disk #0! As you descend down the table, each disk has one less permitted copy than the level above; the same is true as you move from left to right in the table.

It turns out that the number of disks, including the 0 disk is given by the equation Q=2~N, where N is the number of "permitted copies". The limitation of "5" copies is really not so limiting after all, when you understand what is going on. Of course to make maximum use of the limited disk, you must make all of the 31 permitted copies initially before any are lost through inadvertance, bad machines, spilled coffee, etc. You need not be as systematic as the chart implies. Just keep backing up from each disk until the "Disk contains protected files." message appears.

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