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This is what I was trying to do: c:\> echo ABCDEFG > file.txt c:\> ren file.txt file.exe c:\> file.exe DOSBOX: Exit to error: Illegal GRP4 Cal
by timonoko 6y ago
This is what I was trying to do:
c:\> echo ABCDEFG > file.txt
c:\> ren file.txt file.exe
c:\> file.exe
DOSBOX: Exit to error: Illegal GRP4 Call 5
Now. Imagine that you study very hard raw machine code and find a new string (replacing ABCDEFG), which constitute valid EXE-header and a piece of code, which does not cause error and does something useful.
And useful thing here was to interpret letter-pairs in the string as hexadecimal numbers, so you then can write any program you want.
Did this help any?
- em-bee 6y agoyes, you are confirming what i guessed. so you were able to type out the initial program only with the keys on the keyboard? how did you find what to type? and then once you had the hex-reader, where did you get the hexdump to type in? which programs did you create this way?
- timonoko 6y agoNo thank you kindly. I give up. This was 31 years ago and it was once-only issue. Who cares. Remembered however that COM-files were the simple ones. EXE-headers were more complicated. But quite obviously I made a program that read stuff from serial port. And once you had ZMODEM you could copy rest of standard MSDOS command set from anywhere. The machine was truly empty, the was only hardwired COMMAND.COM running.
- em-bee 6y agowell, that wasn't obvious to me. thank you. so you did have access to the right executables, just no direct way to get them into the machine. i only dabbled with old unix workstations. the oldest one being a machine from 1989 running apollo domain OS. and i remember booting an AIX workstation into some kind of rescue mode and using ed to fix config files (fstab probably) to allow it to boot.