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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.