In x86 architecture, the interrupt instruction takes two bytes, but an interrupt with number 3 has an alternative one-byte version. Why was this necessary?

In x86 architecture, interrupts are usually encoded with two bytes: the first is the opcode (for example, 0xCD), and the second is the interrupt number. However, interrupt number 3 (INT 3) has a one-byte alternative (0xCC).

This is done for debugging convenience. INT 3 is a software breakpoint. The one-byte instruction allows inserting breakpoints into code without changing the instruction length, simplifying debugging and minimizing impact on code layout.

Thus, the one-byte version of INT 3 serves for efficient and convenient management of breakpoints in debuggers.