INT 3
The INT 3 instruction is defined for use by debuggers to temporarily replace an instruction in a running program, in order to set a breakpoint. Other INT instructions are encoded using two bytes. This makes them unsuitable for use in patching instructions (which can be one byte long). (see SIGTRAP)
The opcode for INT 3 is 0xCC, as opposed to the opcode for INT immediate', which is 0xCD imm8. According to Intel documentation: "Intel and Microsoft assemblers will not generate the CD03 opcode from any mnemonic" and 0xCC has some special features, which are not shared by "the normal 2-byte opcode for INT 3 (CD03)"
Read more about this topic: INT (x86 Instruction)