4 The Real Mode Vector Table

The 8086 family of processors can respond to 256 different interrupts. Starting at location 0000:0000 is a table of addresses for each interrupt. Each entry in this interrupt vector table is four bytes long, enough for a segment and an offset. So, we have 1024 bytes or 1K reserved for the Real Mode Interrupt vector table. If the processor receives an interrupt 2 (INT 2), for example, it saves the flags and current values of CS and IP on the stack. It then obtains the address stored at location 0000:0008 and executes the interrupt service routine (ISR) at that address. An IRET instruction in the ISR signals the end of the interrupt¹ and causes the processor to resume what it was doing before the interrupt.