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2/3 Keyboards
• Roll call
• Video: keyboard
• Step-by-step
lecture
Watch the Video
1: A key is pressed
When a key is
pressed, the
current changes.
The current
changes back
when the key is
released.
circuit
To keyboard
processor
2: the processor watches
circuit
Keyboard
Processor
The processor is
constantly scanning
the keys' circuits
for changes.
To insure accuracy,
the scan is repeated
hundreds of times
each second.
2A: the processor watches
circuit
Keyboard
Processor
The processor
registers both the
downstroke and
upstroke of a key.
Every key has a
separate circuit
and register.
3: the processor processes
circuit
Keyboard
Processor Memory
Buffer
To port
The processor
translates the
voltage change
into a “Scan
code”, and holds
that info in its
memory buffer.
3A: the processor asks for attention
Interrupt
Controller
CPU
The processor
sends an “Interrupt
signal” to the
computer’s CPU
via the “Interrupt
Controller” that
has precedence
over almost
anything.
4: the CPU listens
BIOS
CPU
The Interrupt
Controller tells the
CPU that the
keyboard needs
attention.
The CPU tells the
BIOS to handle it.
4A: the BIOS acts
To keyboard
processor
BIOS
The BIOS reads
the code from the
keyboard’s
processor, and
then tells the
keyboard to dump
it.
5: the BIOS uses the RAM
BIOS
RAM
The BIOS uses the
RAM to convert
the “Scan code”
into ASCII code.
If any special keys
(shift, etc.) were
pressed, that code
goes in a special
place in RAM.
5: the ASCII code is ready.
>C:/ RUN
The ASCII code
can be used by
other programs
and components of
the computer.