4 ms·

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

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.

int_19h 9y ago
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).
- vram22 9y ago
  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.
  - webtechgal 9y ago
    > 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.
    - vram22 9y ago
      The things people will do ...
  - int_19h 9y ago
    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.
    - vram22 9y ago
      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.

int_19h 9y ago

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).

vram22 9y ago
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.
- webtechgal 9y ago
  > 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.
  - vram22 9y ago
    The things people will do ...
- int_19h 9y ago
  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.
  - vram22 9y ago
    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.