Background

One of the ways to get software for the Commodore 64 in the 1980s was to type in programs from magazines. These magazines would publish the source code for the programs and the reader would type them in. This was a great way to learn how to program and to get new software. They were usually written in BASIC, but sometimes they were written in assembly language. The programs were typically submitted by readers for publication by the magazine.

As cool as that was, it was also sometimes frustrating since there were often many lines of code, some with special graphical characters, and it was easy to make mistakes. Obviously, even a small typo could lead to hours of debugging to get things to work. A fairly common experience for people at the time was simply to give up after hours of typing and debugging. To address this, the magazines would publish the programs with checksums so that the code could be checked and problematic lines could be identified for focused debugging attention. The checksums were usually a two character code that was the result of some algorithm combining the line number and the contents of the line.

This article describes one of the utilities that was published in Ahoy! magazine in the 1980s, named Bug Repellent, that would calculate the checksums for their type-in programs and the steps I took to recreate it for modern use.

The Original C64 Bug Repellent Programs and Magazine Article Excerpts

Excerpts directly from the magazines below:

C-64 BUG REPELLENT (Version 1 - Apr84-May84)

By Michael Kleinert and David Barron

If you’re an Ahoy! reader, chances are that from time to time you type in programs listed in the back. Unfortunately, man is not perfect and you’ve probably made errors typing them in. Another sad but true fact is that you then had to check the nonworking programs line by line, hoping to spot your errors. This has never been fun, and never will be. In order to alleviate this situation, we’ve developed a system for the Commodore 64 that allows for a quick check of your program. Each line is represented by two letters. You check your code against the code listed in Ahoy!, and if there difference, an error exists in that line. This system allows for a fast and easy error check before running the program.

In the program listings section you’ll find the program we’re speaking of, titled C-64 Bug Repellent. It is a basic loader for an assembly language program that resides at $C000 or 49152 (all you non-assemblers stop worrying and keep reading!). Before you start to type in a program from Ahoy!, run Bug Repellent. It will take care of what it has to and then disappear into a cloud of floppy disks. Once it’s run, type NEW and proceed to type in the Ahoy! program you’ve selected. When that’s done, save your program (do not run it!) and type:

SYS 49152 [RETURN]

You will be asked if you want the line value codes displayed on the screen or dumped to the printer. If you select printer, make sure it’s on. A table of codes will be sent to your printer. If you select screen, the table will appear there. This table will move quickly, too quickly for most mortals. In order to satisfy its human operators the program allows you to pause the listing. This is done by depressing and holding the SHIFT key. To pause for an extended period of time, depress SHIFT LOCK. As long as it’s locked, the display remains frozen.

Compare the table your machine generates to the table in Ahoy! that follows the program you’re entering. If you spot a difference, jot down the number of the line where the contradiction occurs. When you’re done with the comparison, go back and LIST each line where an error occurred, spot the error, and correct it.

Another nifty feature of the program is its ability to inform you as to how many lines your program is. This is another quick way to check whether you entered every line, or missed a few.

We hope our program makes computing a bit more enjoyable for you. Don’t put up with disgusting bugs in your programs any longer!

C-64 BUG REPELLENT (Version 2 - Jun84-Apr87)

By Michael Kleinert and David Barron

The program listed below will allow you to quickly debug any Ahoy! program up until May 1987 you type in on your C-64. Starting with the May 1987 issue, a different Bug Repellent version was used.

Type in, SAVE, and RUN the Bug Repellent. Type NEW, then type in or LOAD the Ahoy! program you wish to check. When it’s done, SAVE your program (don’t RUN it!) and type SYS 49152 [RETURN]. You’ll be asked if you want the line value codes displayed on the screen or dumped to the printer. If you select screen, it will appear there.

The table will move quickly, too quickly for most mortals to follow. To pause the listing, depress and hold the SHIFT key. To pause for an extended period, depress SHIFT LOCK. As long as it is locked, the display will remain frozen.

Compare the table your machine generates to the table in Ahoy! that follows the program you’re entering. If you spot a difference, an error exists in the line. Jot down the numbers of lines where contradictions occur, LIST each line, spot the errors, and correct them.

BUG REPELLENT FOR THE 64 & 128 (Version 3 - May87 onwards)

By Buck Childress

Please note: the Bug Repellent programs listed here are for Ahoy! program published from the May 1987 issue onward! For older programs, use the older version.

Type in, save, and run Bug Repellent. You’ll be asked if you want automatic saves to take place. If so, you’re prompted for the device, DISK (D) or TAPE (T). You then pick a starting file number, 0 through 99. Next, you enter a name, up to 14 characters long. At this point, Bug Repellent verifies your entries and gives you a chance to change them if you want. If no changes are needed, Bug Repellent activates itself. (Pressing RETURN without answering the prompts defaults to disk drive and begins your files with “OOBACKUP”).

As you enter program lines and press RETURN, a Bug Repellent code appears at the top of your screen. If it doesn’t match the code in the program listing, an error exists. Correct the line and the codes will match.

If used, automatic saves take place every 15 minutes. When the RETURN key is pressed on a program line, the screen changes color to let you know that a save will begin in about three seconds. You may cancel the save by pressing the RUN STOP key. The file number increments after cache save. It resets to 00 if 99 is surpassed. After saving, or cancelling, the screen returns to its original color and the timer resets for 15 minutes.

When you’ve finished using Bug Repellent, deactivate it by typing SYS 49152 [RETURN] for the Commodore 64 or SYS 4864 [RETURN] for the Commodore 128.

Link to my Github repo for the original C64 Bug Repellent versions.