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Comment by vram22 | Hacker News Reader

>a .com created this way would instantly reboot the computer. Even more weirdly, this only worked for the specific command that happened to enter in my first attempt

Interesting that you did that by chance, since the probability was somewhat low. but possible, of course.

I used to assemble a tiny .COM program on new computers sometimes, using just DEBUG.COM (the built-in rudimentary debugger that came with DOS), using something like the following (the dash is the DEBUG prompt):

DEBUG reboot.com

-A

JMP F000:FFF0

-W

-Q

Meaning of the above: A to enter Assembly mode - to enter assembly language instructions, the JMP jumps to that address (Intel x86 segment:offset format), which was the location of the instruction in ROM that would reboot the PC, the dot to signal end of assembly input, W to write the code to reboot.com, Q to quit DEBUG.

Then if you typed "reboot", it would.

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This!! And as I recall, if we replace -W with -G (for 'go') and hit Enter, it would immediately reboot. Then, there also used to be a flag (boolean) that could be used to effect either a warm or a cold boot. Nostalgic. :-)

I had forgotten about -G, and knew about warm and cold boot, but did not know about the flag. Cool, and nostalgic indeed :-)

Or you could just use

INT 19 ; (CD 19)

On my Windows 9x box it was the fastest way to soft reboot when made to be an MSDOS mode shortcut

Didn't know, thanks. And nice appropriate HN handle :)

I'll one-up that.

In the context of the current thread, your handle is cool too!! As I recall, the code for up to INT 19H (or 20?) was stored in the ROM BIOS and for INT 21H and up, in the hidden system files (io.sys/msdos.sys) which would get loaded just before command.com at boot.

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).

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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