Pack Rat 64 is a unique utility program that analyzes and corrects the "blocks free" on any 1541-formatted disk. Many commercial disks only use a fraction of the total space available, leaving ample room for other programs or information, which many people would like to use. However, since many of these disks are copy-protected, you can't safely use this free space. If you try to, without using Pack Rat 64 first, you risk losing the information already on that disk.
By using Pack Rat 64 to analyze and correct your disks, you can safely put more than one program on most of your commercial disks. With a bit of creativity, you can even put a menu program on the disk as well, to let you select which program you want to use! Pack Rat 64 was made to be used with disk-menu programs, such as the author's "Auto- Menu" or "Generic DiskMenu", available with "AutoBoot Factory" on the November/December 1991 ReRun disk.
You can also use Pack Rat 64 on your own disks, either right after you've formatted them, or when they start to develop bad spots, such as read errors in a sector or two. One of Pack Rat's main functions is to find and identify read errors in a block, and mark it as a block that has already been used. This isolates the read error, and makes the rest of the disk safe to use again.
Using Pack Rat 64 to analyze your disks
Pack Rat 64, as any good program should, thinks for itself where it can, saving you hassle. You just select it from Menu 64, or a work disk, and follow the on-screen prompts.
Insert the disk you want to correct when prompted, press a key, and wait while the program scans and analyzes the disk. It's a relatively fast process, considering it has to analyze your entire disk. The whole operation normally only takes a few minutes per disk.
Although Pack Rat 64 works with commercial disks, it is always safest to modify only your archival backup disks, keeping your valued originals safely stored away. With many titles becoming rarer and hard-to-find, treat your original disks with all the care you would for a valued music recording collection, or out-of-print books. With reasonable care, your C64 disk collection should give you years of enjoyment!
What makes Pack Rat 64 work?
Liberal use of machine language in the program's data statements keep analysis time reasonable. The actual routine that analyzes each sector is tucked inside a small data buffer in the disk drive. Your disk drive has a small computer of its own, built-in, with RAM, ROM, and a microprocessor similar to the C-64's.
Your Commodore's DOS (Disk Operating System) includes a Validate command to fix the blocks free. Validate works fine for non-damaged, non-protected disks. But it wasn't made for use with protected disks. If you use it to fix a protected disk, you risk losing vital, hidden information the protection scheme needs to work properly.
Validate assumes all blocks are empty of information unless they belong to a program or file listed in the directory. Hidden information, off by itself -- the nature of any protection -- will be marked as a free block, to be reused the next time you save information to that disk.
Pack Rat 64 improves on this by looking at each block or sector, to see if it contains a read error. If it does, Pack Rat 64 marks that block as used, or "not free" in the BAM (Block Allocation Map). The BAM is a special place on each disk, where each sector has a one-bit code to mark whether or not it is free. Read errors are always marked as "used" whether or not they are part of a copy-protection scheme.
Pack Rat 64 checks each block's information against a pattern that's present only if the block was formatted, but never used. Any variation from this format-only coding, even one byte, will tell Pack Rat 64 that information of some kind exists there.
If the sector contained data besides format information, it is automatically marked as used. Otherwise, it is marked as a free block. This insures that no information is accidentally overwritten, when you save other programs to that disk. Only truly empty blocks will be marked as such, which is why the disk is then safe for more data.
The program itself contains a few tricks. For instance, it checks that your disk's write-protect notch is uncovered, so it can write to the disk at the end. That prevents having to wait through its analysis, just to find it can't correct the disk.
By putting the speed-important routines in the disk drive, where the information is, all the time that would normally be spent reading each block's info into your computer is eliminated. This is why the entire disk can be analyzed and corrected in a reasonable time, even though most of the program is written in Basic 2.0.
