Table 5-1, IRQ5 is assigned to parallel port 2; this port is generally available in most microcomputers and is commonly   used   by   most   sound   cards.   When microprocessors expanded from 8-bit to 16-bit proces- sors, the amount of hardware supported also grew. This required the addition of more IRQ channels. Manufac- turers  added  an  additional  8-channel  processor  and  cas- caded them by connecting IRQ2 on processor to IRQ9. The latest development in microcomputer technol- ogy concerning interrupt processing is the Plug-n-Play feature. A true plug and play system requires three components to work together; the hardware, the BIOS, and the operating system. During the power-on cycle of computers that are Plug-n-Play capable, the firmware contained in the basic input/output system (BIOS) interrogates  each  component  in  the  system  to  determine the type of board, IRQ channel requirements, DMA channel  requirements,  and  ROM  requirements.  The board responds with the specifications it requires, then the BIOS assigns IRQs, DMA, ROM resources, etc., to all the boards, ensuring that there are no conflicts. The functions of the BIOS are covered in detail later in this chapter.  This  process  is  repeated  every  time  the computer  is  turned  on. Controllers  that  are  not Plug-n-Play compatible can be installed by using the standard  configuration  program  and  locking  the resource to those unique settings. MINI  AND  MAINFRAME  INTERRUPT TYPES.— Within larger computers, interrupts are divided  into  a  number  of  separate  classes.  Multiple classes of interrupts are needed because there are several  levels  of  processing  within  these  computers  and many different types of operations and conditions that have to be monitored. Some operations and conditions are more important than others. There  are  generally  three  or  four  classes  of interrupts, which we designate class I, II, III, and IV. Interrupts are prioritized by these classes and by the types of interrupts within a class. Class I interrupts are the highest priority or most important interrupt class as far as the computer is concerned. The other classes (II, III, and IV) are in turn lower in priority than Class I. Class I Interrupts.— Class I interrupts function during all computer operations; in other words, they will  interrupt  any  computer  program  or  instruction. These  are  the  highest  priority  interrupts.  Known  as fault  and  hardware  or  hardware  error  interrupts, these  interrupts  indicate  there  is  a  serious  hardware problem with the computer, or more accurately within the CPU or its communication buses. The following are some of the more common class I interrupts: Power fault or power tolerance Memory parity errors Memory resume errors Bus communication errors The most common class I interrupt is the  power fault or power tolerance interrupt. This interrupt indicates that the power supply voltage has fallen below a certain tolerance level and that the computer should execute  its  power  failure  processing  routines  before there is a total loss of power. The actual routines will vary from computer to computer based on the device’s automatic   restart   and   backup   storage   power capabilities. Class II Interrupts.— Class II interrupts are used to identify faults and errors within the CPU or IOC instruction execution and program timing processes. These software interrupts can indicate the following conditions: Execution of illegal instruction operation (op) codes (CPU or IOC instructions) Execution of privileged instructions in the task mode Floating-point math underflow or overflow conditions Real-time clock (RTC) overflow Monitor clock timeouts Class  III  Interrupts.—  Class  III  interrupts  are primarily  I/O  operation  interrupts.  They  indicate such  functions  as  the  following: External interrupts Input or output chain interrupts Intercomputer timeouts Input data ready or output data ready interrupts Class IV Interrupts.— In some computers, there is a class IV interrupt that indicates  executive  state entrance. In others, the executive state entrance is a class II interrupt. A limited number of instructions can be executed only in the executive states. Among them are  privileged  instructions. MINI   AND   MAINFRAME   INTERRUPT LOCKOUT  OF  CLASS  I,  II,  III,  AND  IV TYPES.—  Computers that operate with different levels of interrupts are equipped with the logic circuitry to 5-12