What do you mean by int 21h?

In assembly language programming for DOS, "int 21h" refers to a software interrupt that provides access to various DOS services. The "21h" indicates the hexadecimal value of the interrupt, which allows programs to perform functions such as file management, input/output operations, and other system-level tasks. Each function is specified by setting a register (typically AH) to a specific value before calling the interrupt. This mechanism is essential for interacting with the operating system at a low level.