operating system - What is INT 21h?

Question

Inspired by this question

How can I force GDB to disassemble?

I wondered about the INT 21h as a concept. Now, I have some very rusty knowledge of the internals, but not so many details. I remember that in C64 you had regular Interrupts and Non Maskable Interrupts, but my knowledge stops here. Could you please give me some clue ? Is it a DOS related strategy ?

Answer 1

From here: A multipurpose DOS interrupt used for various functions including reading the keyboard and writing to the console and printer. It was also used to read and write disks using the earlier File Control Block (FCB) method.