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The Keyboard Controller

Learn the essential steps for implementing an interrupt service routine for the Keyboard Controller in a mini x86 operating system that supports console input/output, device interrupts, and graceful application exit. Includes controller commands, interrupt handler actions, and demo.

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The Keyboard Controller

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  1. The Keyboard Controller Essential steps for implementing an interrupt service routine for the Keyboard Controller

  2. Our near-term goal • To build a ‘mini’ x86 operating system • It should be able run a Linux application • It needs to support: • basic console input/output (keyboard, screen) • handling of device-interrupts (timer, keyboard) • ‘exit’ gracefully when application is finished • utilize x86 privilege restrictions and 32bit code • Certain building-blocks are already done

  3. Keyboard and its controller PIC PIC CPU RAM IRQ2 IRQ0 IRQ1 0x64 0x64 0x60 0x60 bus TIMER Keyboard Controller control reg status reg input buffer output buffer output port input port PS/2 MOUSE KEYBOARD

  4. KB Controller Status 7 6 5 4 3 2 1 0 PARE TIM AUXB KEYL C / D SYSF INPB OUTB LEGEND OUTB: Output-Buffer Full (1=yes, 0=no) INPB: Input-Buffer Full (1=yes, 0=no) SYSF: System-Flag (1=self-test successful, 0=power-on reset) C/D: Command/Data was written to (1=port 0x64, 0=port 0x60) KEYL: Keyboard-Lock status (1=keyboard available, 0=locked) AUXB: Output-Buffer’s data is (1=for mouse, 0=for keyboard) TIM: General Timeout-Error has occurred (1=yes, 0=no) PARE: Parity-Error on last byte from keyboard/mouse (1=yes, 0=no)

  5. A few Controller Commands • 0xAD: Disable Keyboard • 0xAE: Enable Keyboard • 0xA7: Disable PS/2 Mouse • 0xA8: Enable PS/2 Mouse • 0xC0: Read input-port (to output buffer) • 0XD0: Read output-port (to output buffer)

  6. Command to set LEDs • 0xED: Turns on/off the keyboard’s LEDs • # • # Algorithm to change the keyboard’s Light Emitting Diodes • # • Wait until keyboard controller’s output buffer is not full • Output command-byte 0xED to port 0x60 • Wait until keyboard controller’s output buffer is not full • Output octal value 0..7 to port 0x60 2 1 0 CAPS LOCK NUM LOCK SCROLL LOCK Octal-value = 1=on 0=off 1=on 0=off 1=on 0=off

  7. Interrupt-Handler actions • When the keyboard-controller issues an interrupt, these actions should be taken: • Save values in the working CPU registers • Read keyboard-controller’s status-register • If the output-buffer has new data, read it • If a “special” key (shift/toggle), adjust kb_flags • For a normal key, translate scancode to ascii • Insert the code-pair at tail-end of kb-queue • Send EOI-command to Interrupt Controller • Restore the saved values from CPU registers • Resume the interrupted procedure (with ‘iret’)

  8. Keyboard ‘scancode’ format 7 6 5 4 3 2 1 0 BRK Key identifier (bits 6..0) BRK (bit 7) 1 = ’Break’ (the key was released) 0 = ‘Make’ (the key was pressed)

  9. Keyboard Queue KBHEAD KBTAIL

  10. BIOS DATA AREA • KBFLAGS is word at address 0x40:0x17 • KBHEAD is word at address 0x40:0x1A • KBTAIL is word at address 0x40:0x1C • KBQUEUE is array of 16 words array base-address at 0x40:0x1E • KBBASE is word at 0x40:0x80 • KBTAIL is word at 0x40:0x82

  11. Format of KBFLAGS LSB 7 6 5 4 3 2 1 0 Insert-mode is active (1=yes, 0=no) Caps-Lock is active (1=yes, 0=no) Num-Lock is active (1=yes, 0=no) Scroll-Lock is active (1=yes, 0=no) Alt-Key is pressed (1=yes, 0=no) Ctrl-Key is pressed (1=yes, 0=no) Right-Shift key is pressed (1=yes, 0=no) Left-Shift key is pressed (1=yes, 0=no)

  12. Demo ‘minikybd.s’ • It shows a minimal implementation for the keyboard-controller’s interrupt-handler • It runs in Real-Mode • It ‘echos’ a user’s keystrokes IN-CLASS EXERCISE: modify this demo to run in Protected-Mode

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