Ah yes, I have fond memories of writing TSR pranks that hooked common DOS interrupts (not technically viruses since they didn't infect/replicate) that did things like:
1) buffer 1 keystroke in memory (so typing diw<backspace>r would leave "di" on screen - then they'd type "r" and get an "r" and then hit enter and get another "r", then they'd hit backspace to correct the "dirr" and the command would execute, etc)
2) hooked the printer interrupt to introduce typos in printouts
3) randomly swap letters on the screen periodically
Fun times. It's probably for the best that I grew up in the era that I did since pulling pranks like these has a high risk of getting slapped with felony charges in this day and age.
BBS lists, Turbo Debugger and that giant purple & white Microsoft Press book with most of the Int 21h calls. Undocumented DOS, Undoc PC and Abrash's books were also helpful. Later, there were good free resources.
Yeah, DOS viruses were bad: booting off an infected floppy, infected HD or running untrusted code was all it took. In Windows, it was worse because autorun would run untrusted code automatically without warning.
There were some darn useful TSR's like Super PC-kwik Power Pak, QEMM and Desqview. Stacker and Doublespace/DriveSpace were kinda slow but did (mostly) work. Stacker had an accelerator card IIRC. And a commercial white bag/gray box mod for Welltris, Mouse Commander, added mouse support to Welltris for DOS by doing some mouse driver to keyboard translation in a TSR that was loaded and unload in a wrapper .BAT file. Many trainers, no-cd/runtime cracks and cheats also used TSR tricks to run.
Qemm was essential. Not only did it hook CTRL+ALT+DEL to skip the POST BIOS check, making reboots faster, but it made it possible to shunt stuff out of convential memory into extended memory, making it possible to run games whilst having various drivers and so forth loaded.
Qemm was the single best thing that ever happened to DOS-using me. The second best was Windows 95.
When I was young I build small TSR .com (DOS exec extension) thing to instantly switch screen from game playing mode to serious work-BS mode with hotkey - at a first sign of boss approaching.
Ah, the good old "boss key" present in so many games! I remember it from Leisure Suit Larry [1], and a few more. At the time it was more like a "mom key" :)
The open office was probably invented as a countermeasure for the boss key.
Writing TSRs were the holy grail for me back in the day. I cut my teeth on Turbo C and even to this day I fondly remember hacking around with them.
The first thing I did when I got internet access in 1996 was look up information on how how to write TSRs (oh, and stuff about X-Files) as I did not have access to any books on the subject.
Guilty. In 1988 or so I made a virus that attached to .com files, which would spread it to others when executed. It was never released into the open. On each infection, it incremented a generation counter. Upon reaching a certain value, the payload would trigger, causing a message to be printed.
What the heck was I using? MASM? TASM? I cannot remember.
IIRC, looking for other .com files was just using DOS's FindFirst and FindNext functions at int 21h. (I'm now naming them in terms of their Win32 counterparts.)
I think I didn't have a sanity check against duplicate infection in the early versions and added that during testing. 17 year old's programmer's "doh" moment.
I remember that I ran the experiments in a subdirectory called "petridir". :)
I always thought the Casino DOS virus was pretty inventive. If an infected file is run on the payload date, it moves the FAT to RAM and warns the user not to shut down their computer. The user plays a game with IIRC a 1/17 chance of winning. If they win, the FAT is restored, else it's gone.
I think it's an interesting way of holding the PC hostage.
From the perspective of the user, MS-DOS was a lousy OS. However, as a tinkerer, it was super fun. To this day, my best programming memories are about using Turbo Assembler and the information from Interrup.lst to do weird things with the graphics card's text mode (unusual resolutions, borders, 18 bit color palettes, smooth scrolling...).
1) buffer 1 keystroke in memory (so typing diw<backspace>r would leave "di" on screen - then they'd type "r" and get an "r" and then hit enter and get another "r", then they'd hit backspace to correct the "dirr" and the command would execute, etc)
2) hooked the printer interrupt to introduce typos in printouts
3) randomly swap letters on the screen periodically
Fun times. It's probably for the best that I grew up in the era that I did since pulling pranks like these has a high risk of getting slapped with felony charges in this day and age.