Int3 is a special single-byte (CC, if I recall correctly) form of the INT instruction (which is CD imm8, I think) to raise an interrupt. Interrupt #3 is the debugging interrupt, so by overwriting any instruction with CC, you place a breakpoint there.
Int3 is a special single-byte (CC, if I recall correctly) form of the INT instruction (which is CD imm8, I think) to raise an interrupt. Interrupt #3 is the debugging interrupt, so by overwriting any instruction with CC, you place a breakpoint there.