3 ms·

Same here - I remember when you'd get games as .COM file with a nag screen/message etc that you could just skip over by flipping a jump instruction... I rememb

Same here - I remember when you'd get games as .COM file with a nag screen/message etc that you could just skip over by flipping a jump instruction... I remember being super puzzled when I tried that with a .exe and it didn't work

bicolao 6y ago
It was a whole new world to create a .com file with just debug.com and some assembly (most of the work is already in interrupt 21h). Good times. I looked at DOS .exe (NE? or was that the format for Win 3.1...) format too. Still relatively simple. Then came the PE which was super complicated.
- decasteve 6y ago
  RE: debug.com As a teenager who didn't know any better, it was empowering to gain that level of access to a computer, to realize a certain level of control over it that was previously abstracted away. It felt like some insider knowledge that I was becoming aware of.
  - pajko 6y ago
    At 14, I wrote a functional virus on paper, hand-calculated the jump offsets and entered the "code" in debug.com. The first version was 273 bytes long. It did not do much, just printing the BELL character, then looking for non-infected COM files in the current directory and infecting one of them. F-Prot identified it as a generic virus and killed it. That made me a little mad at that time, so added some polymorphism to it by decoding and encoding the virus code with a different key every time (using XOR). The code size has increased by about 60 bytes, and only around 10 bytes were unencrypted, which did the in-place decryption in memory. F-Prot identified it as a generic polymorph virus and killed it again... After that tried to make it resident, but intercepting the int 21h calls turned out to be a tough nut to crack, so tried to do something with EXE files instead. A basic variant was nearly ready, when I have fucked up my 40MB hard drive and lost everything... But that's an other story of my early self-education :)
AnIdiotOnTheNet 6y ago
There's a whole fun game to played yet today trying to defeat copy protection on old DOS games. Programmers of the time resorted to a lot of complicated tricks to keep what you're describing from working properly, but assembly of the era is pretty easy to follow and having a 30 year advantage in tooling helps a lot. Hyperspeed was an interesting one. The crack that comes with the Steam version doesn't actually work since it triggers a lockup the first time you use the spindrive, so players just disable the crack and play the game normally with a PDF manual. I spent a long time trying to bypass that copy protection screen and all the checks to see if I'd bypassed it, but every time I thought I'd finally nailed it I ran into a new place the game would intentionally lock up. Ultimately I just truncated the word match table to a single entry so the word would always be the same. That was a single-byte change.

bicolao 6y ago

It was a whole new world to create a .com file with just debug.com and some assembly (most of the work is already in interrupt 21h). Good times. I looked at DOS .exe (NE? or was that the format for Win 3.1...) format too. Still relatively simple. Then came the PE which was super complicated.

decasteve 6y ago
RE: debug.com As a teenager who didn't know any better, it was empowering to gain that level of access to a computer, to realize a certain level of control over it that was previously abstracted away. It felt like some insider knowledge that I was becoming aware of.
- pajko 6y ago
  At 14, I wrote a functional virus on paper, hand-calculated the jump offsets and entered the "code" in debug.com. The first version was 273 bytes long. It did not do much, just printing the BELL character, then looking for non-infected COM files in the current directory and infecting one of them. F-Prot identified it as a generic virus and killed it. That made me a little mad at that time, so added some polymorphism to it by decoding and encoding the virus code with a different key every time (using XOR). The code size has increased by about 60 bytes, and only around 10 bytes were unencrypted, which did the in-place decryption in memory. F-Prot identified it as a generic polymorph virus and killed it again... After that tried to make it resident, but intercepting the int 21h calls turned out to be a tough nut to crack, so tried to do something with EXE files instead. A basic variant was nearly ready, when I have fucked up my 40MB hard drive and lost everything... But that's an other story of my early self-education :)

AnIdiotOnTheNet 6y ago

There's a whole fun game to played yet today trying to defeat copy protection on old DOS games. Programmers of the time resorted to a lot of complicated tricks to keep what you're describing from working properly, but assembly of the era is pretty easy to follow and having a 30 year advantage in tooling helps a lot. Hyperspeed was an interesting one. The crack that comes with the Steam version doesn't actually work since it triggers a lockup the first time you use the spindrive, so players just disable the crack and play the game normally with a PDF manual. I spent a long time trying to bypass that copy protection screen and all the checks to see if I'd bypassed it, but every time I thought I'd finally nailed it I ran into a new place the game would intentionally lock up. Ultimately I just truncated the word match table to a single entry so the word would always be the same. That was a single-byte change.