The bottom 1024 bytes of system memory are called the interrupt-vector table. Each 4-byte position in the table corresponds to an interrupt type (0 through 0FFH) and contains the segment and offset of the interrupt handler for that level. Interrupts 0 through 1FH (the lowest levels) are used for internal hardware interrupts; MS-DOS uses Interrupts 20H through 3FH; all the other interrupts are available for use by either external hardware devices or system drivers and application software.
When an 8259A PIC or other device interrupts the CPU by means of the INTR pin, it must also place the interrupt type as an 8-bit number (0 through 0FFH) on the system bus, where the CPU can find it. The CPU then multiplies this number by 4 to find the memory address of the interrupt vector to be used.