Supply and Demand - One way of looking at interrupt requests is from a supply and demand perspective. While; having the CPU interrupted all the time may at first glance appear to be an inefficient way to run a computer, when examined closer reality proves otherwise. A large part of the reason is that the modern CPU is literally light years ahead; in terms of speed, when compared to the vast majority of devices conducting transactions with it. To put it into perspective let us consider the following scenario:
Let us suppose that a typist is typing at a rate of 120 words per minute and that on average there are five letters per word. This equates to the typist producing 600 characters of keyboard input per minute. This means that an old 200 MHz Pentium class CPU will process 20,000,000 instructions between each keystroke.
This is why having the processor spend a lot of time asking the keyboard if it needs any help would be wasteful, especially since the typist may at any time you might stop for a minute or two to review the copy, or do something else like make a cup of coffee. In fact; even while handling a full-bandwidth transfer from a 28,800 Kb/sec modem, which of course moves data much faster than the typist's fingers, the processor has over 60,000 instruction cycles between the bytes it needs to process.
Hardware Interrupts - Hardware interrupts such as those mentioned above (typing/keyboard input and modem transfers) are distinguishable from software interrupts by the fact that they generally originate from a device outside the CPU. Table 1 below shows the default IRQs in order of descending priority.
Software Interrupts - There is another type of interrupt that occurs in modern PCs; known as software interrupts. These are generated by the operating system, programs and various other software applications and are used by various software programs in response to different events that occur as the operating system and applications run.
In essence a software interrupt represents the CPU interrupting itself (as the interrupt originates from within the CPU) and is part of the reason of how the CPU is able to do many things at once.
Basic Input/Output System (BIOS) - A computer's BIOS provides various software routines (subprograms) that can be called by higher-level software such as DOS, Windows, or their applications, to perform different tasks. This includes actions like reading and writing from the hard disk, processing information received from devices, etc.
BIOS Access - Another performance enhancing functionality that software interrupts make possible is facilitating system software (DOS or the operating system), applications and other software to directly access one another via BIOS routines without having to know where the target application resides in memory.
Interrupt Vector Table - Normally, to call a software routine you need to know its address (location) in memory. However; with interrupt requests a table called an interrupt vector table is created and this is used to bypass the need to explicitly know the correct address in memory. Now every time the system is booted, the BIOS puts addresses representing where its routines are located for each interrupt that it is configured to respond to.
So whenever DOS, the operating system or other applications want to use a BIOS routine, it generates a software interrupt. The system processes the interrupt, looks up the value in the interrupt vector table and then automatically jumps directly to the appropriate BIOS routine. Modern operating systems and applications as well as DOS can also directly use this interrupt vector table.
BIOS Bypass - More recently newer operating systems bypass the BIOS totally as this does help improve system performance.
BIOS Assigned Logical Names - The BIOS assigns logical names to devices based on their IRQ number and memory address. For example:
IRQ 3 and I/O address 2F8-2FFh becomes Com2.
Legacy Devices Default Settings - For older systems; such as those based around the ISA bus, it is often the case that the default IRQ assignments must be preserved. This necessity is most common with older software and games that require the ISA SoundBlaster sound card to be accessible using the following configurations:
IRQ 5, I/O Address 220h and DMA 1
Failure to comply will result in; at best no in game sound and at worst no game at all.
Interrupt Summary - In short; interrupt requests are a mechanism that allows the various devices and software that make up a computer to jump the processing queue in order to have tasks of a predetermined “higher” priority to be executed “out of turn”. BIOS services are accessed using software interrupts, which are similar to the hardware interrupts except that they are generated inside the CPU by programs instead of being generated outside the CPU by hardware devices.
