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indirect far jmp (Developers)

posted by Arjay, 12.05.2012, 11:09

> That doesn't seem to have any advantages over just executing the
> corresponding int opcode?
Under normal circumstances I agree. There are times when it can be useful, e.g. see Art Of Assembly - Chapter Seventeen: Interrupts Traps and Exeptions for more info / better examples to use. Also can be useful for obscuration / basic anti-disassembler trickery.

> where a jmp far is) sequence is useful when calling an interrupt handler
> that isn't actually stored in the IVT.)
 Yup. e.g. under runcom etc.

> Hahah~ (You really shouldn't use the stack at 10000h though.)
 Indeed. That particular quick and dirty demo was to give a few friends a little puzzle a number of years ago; with a normal version of the same thing.

Interestingly last night whilst quickly digging that out I did note a DOS compatibility bug in the versions of both DOSEmu and DOSBox that are on this little box. One of several Easter eggs that I expect you spotted in that code is to print my initials over the PS if the .COM program is typed from the DOS prompt, "TYPE MSG2MARK.COM" however I noted it fails on both of the old versions of DOSEmu and DOSBox that are on this box. e.g. under DOSBox v0.73 EOF (1Ah) is ignored. Likewise the DOSEmu ignores the CR (0Dh) - as both are older versions (due to this box) I will at some point test to see if bugs present in newer versions. Obviously not critical bugs however there are a number of programs/data that use character tricks (inc. 08h) as "type" easter eggs.

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