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Interrupt Handlers When the CPU gets an interrupt, it must execute a program to handle
the interrupt. (For example, when the user presses a key on the
keyboard, the CPU must execute a special program that copies the
keystroke value into memory, and displays the value on the monitor).
These special programs are called interrupt handlers, and may
original from several different sources:
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When the PC is turned on, or "booted", the programs in the BIOS ROM
chip are copied into upper memory (called shadowing). These programs
are relatively small (between 1 - 8 MB), since they are meant for only a
small number of standard devices, such as the keyboard, monitor and
storage devices.
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Many devices, such as sound cards and network cards have so many
variations in their configurations that it would be impossible for
any BIOS to include all of the programs needed to handle them. As a
result, the programs for these devices are often found in a separate
BIOS chip on the expansion card itself. These "external" programs
are sometimes called BIOS extensions. Like the standard BIOS
programs, they are copied into upper memory when the computer is
booted.
The small programs found in the BIOS and BIOS Extensions are
sometimes known as firmware, which refers to the fact that they have
a closer relationship to the PC's hardware than normal software
programs.
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Device Drivers are low-level programs that are similar to the
firmware programs found in the BIOS and BIOS extensions. Like
firmware programs, they are needed to make a hardware device work,
improve the performance of a hardware device, or add functionality
to a hardware device. However, unlike firmware programs, device
drivers are packaged (usually in CD-ROM format) and distributed with
the particular device. After the device is physically installed, an
installation program is used to copy the device drivers to a
permanent location on the hard disk. When the computer is booted,
the device drivers are then loaded into memory along with the
firmware programs found in the standard BIOS and the BIOS
extensions.
Typically, device drivers are used to enhance or improve the BIOS
extensions on the adapter card for that device, or they are use for
devices that do not require an expansion card (such as a printer).
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Interrupt Vectors
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Since firmware programs and device drivers are loaded into memory
when the PC is booted, they may not always be in the same memory
location. How does the CPU know where to find the correct program
when a particular interrupt occurs? The memory location of each program is then stored in an interrupt
vector table in lower memory. When a hardware or software interrupt
occurs, the CPU responds by searching the interrupt vector table to
find the location of the interrupt handler, then locates and
executes it.
Each time the PC is booted, a special table is created in memory
that stores the starting location of each interrupt handler program,
along with the IRQ that it is associated with. This table is called
an interrupt vector table. When an interrupt occurs, the CPU looks
in the table to find the address of the interrupt handler that it
needs. Then it goes to that address, and executes the interrupt
handler.
The figure on the right shows the relationship between interrupt vectors
and interrupt handlers:
When the user the presses a key on the keyboard, the following
sequence occurs:
- An interrupt signal travels from the keyboard, along IRQ1 to the
CPU.
- The CPU copies all of its registers to a special area called the
“stack”.
- The CPU knows that the signal came from IRQ1, so it looks in the
Interrupt Vector Table for the IRQ1 record.
- The IRQ1 record says that the required interrupt handler program
is found in memory at address C.
- The CPU goes to address C and executes the interrupt handler
program. The program is very simple - it simply retrieves the
keystroke value from the keyboard buffer, copies it into a
corresponding I/O address in memory, and (depending on the
application) displays the character on the screen (which
may actually involve another interrupt).
- The program has now successfully handled the keystroke. The CPU
retrieves its previous contents from the stack, and continues where
it left off.
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