Question

This question was in my job interview.. I just to see whether I gave all the details…

Answer 1

• mechanical switch short-circuits pull up R1 resistor end to the ground
• a special multiplexor translates it into a message, to reduce the number of wires
• the message interpreted by a CPU embedded in the keyboard
• message translated to a USB protocol message, and modulated as a series of electric impulses of alternating voltage between zero and 5 volts
• USB receiving hub measures samples line voltage periodically
• host hub controller translates the message to data
• data enters PC thru USB bus controller, connected to PCIE bus, thru a combination of IRQ notificaitons and a DMA transfer, issued by the bus driver
• Bus driver interprets the message and forwards it along the driver stack, ultimately to an HID driver
• HID driver talks to windows, ultimately resulting in a window message sent to a window belonging to msword process
• WM_KEYDOWN is translated to WM_CHAR by DefWindowProc(). While key is down, multiple WM_CHARs may be created.
• Word application catches WM_CHAR to add another character to the document model and issue re-rendering of UI
• UI rendering engine translates unicode codepoint to graphical image by loading respective font
• graphics engine computes the new image of the whole area to avoid flicker, and puts it pixel-by-pixel to the screen