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Hello,

Im stuck on a question I was asked.


Id like to know the process taken by the PC to display a character from the keyboard to the word processing package say.

press H
sent to the CPU
sent to memory
word SW reads memory
tells the cpu where to display on screen (x,y)
sends the command tot he display device
display device o/p the visual data to the monitor

that kind of thing is what im after.

Any help would be most appreciated,
Kind regards.

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Last Post by jbennet
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Look up something called interrupts and interrupt handling.

When an interrupt is generated (e.g key pressed), the processor saves its current execution state (onto the stack) and begins execution of the interrupt service routine (ISR) code (it gets the address for this in memory using a process called vectored interrupt handling) which then does whatever it needs to do to respond to the event. Once its handled, it pops what it was last doing off of the stack and loads it into the registers on the CPU, continuing whatever it was doing before the interrupt occured.

Generally the ISR will get the value of the key pressed and then store its ascii code in a buffer (thats why sometimes if you push keys when the pc has crashed it beeps, generally it means the keyboard buffer is full, because the kernel has crashed and is thereofre unable to handle the interrupts)

Your program will then check whether there is anything in the input buffer, and if so, will do something with it. e.g. write it to the video memory in the correct way so it will be displayed graphically on the screen.

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