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Comment by int_19h | Hacker News Reader

If I remember correctly, the lowest DOS interrupt was 20h. But I don't know if all interrupts below that were actually used. The highest that I remember is 1Ch for the timer interrupt (for which the stock BIOS interrupt handler just returns immediately, but which was commonly overriden by TSRs to simulate multitasking).

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Also there were two groups of interrupts, IIRC: DOS interrupts and BIOS interrupts. BIOS ones were lower level and I guess some of the DOS interrupts used some BIOS ones. I remember a friend using BIOS interrupts to write a program to format a floppy disk and we sat there watching the drive make sounds as it did it.

> using BIOS interrupts to write a program to format a floppy disk and we sat there watching the drive make sounds...

There used to be a virus/trojan that would execute the floppy drive head re-calibration interrupt (BIOS) in an infinite loop to cause the head alignment to go out of whack, rendering the drive defunct.

The things people will do ...

There were four broad groups:

CPU interrupts - handlers for CPU errors like division by zero (0h) or overflow (4h).

IRQ interrupts - handlers for hardware events; e.g. hardware timer (8h) or keyboard (9h).

BIOS interrupts - for BIOS-implemented syscalls; e.g. video output (10h) or sector-oriented disk I/O (13h). Also callbacks to be invoked by BIOS, like the aforementioned 1Ch for configurable timer.

DOS interrupts - for DOS-implemented syscalls; mostly this was 21h with actual syscall ID in the register. This is where you got things like file-oriented I/O, and some basic memory management.

And then there was "everything else", meaning things installed by third party TSRs and drivers. For example, 33h for the Microsoft mouse driver (which all other mouse drivers promptly copied, so it became the de facto standard mouse API in DOS), or 3Eh for Borland's software floating point emulator.

Of these, only the first two are actually "special" as far as hardware is concerned - the rest is just convention.

Interesting, didn't know that there were 4 groups. I do remember INT 21H and setting a value in a register to specify what syscall you want run. Turbo Pascal had some support for assembly and even register handling, IIRC, so you could write such low-level programs in it. Also had absolute memory addressing which could be used for anything where you need to manipulate fixed, known memory addresses, such as video RAM.

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