The Keyboard Controller

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
• Most needed building-blocks already done

3
Keyboard and its controller
CPU
PIC
RAM
PIC
0x64
0x64
0x60
0x60
bus
Keyboard Controller
TIMER
output buffer
status reg
control reg
input buffer
output port
input port
KEYBOARD
PS/2 MOUSE
4
KB Controller Status
6
7
5
4
3
2
1
0
PARE
TIM
AUXB
KEYL
C / D
SYSF
INPB
OUTB
LEGEND OUTB Output-Buffer Full (1yes, 0no)
INPB Input-Buffer Full (1yes, 0no) SYSF
System-Flag (1self-test successful, 0power-on
reset) C/D Command/Data was written to
(1port 0x64, 0port 0x60) KEYL Keyboard-Lock
status (1keyboard available, 0locked) AUXB
Output-Buffers data is (1for mouse, 0for
keyboard) TIM General Timeout-Error has
occurred (1yes, 0no) PARE Parity-Error on
last byte from keyboard/mouse (1yes, 0no)
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
Interrupt-Handler actions

• When the keyboard-controller issues an interrupt,
  these actions should be taken
• Save values in the working CPU registers
• Read keyboard-controllers 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)

7
Keyboard Queue
KBHEAD
KBTAIL
8
BIOS DATA AREA

• KBFLAGS is word at address 0x400x17
• KBHEAD is word at address 0x400x1A
• KBTAIL is word at address 0x400x1C
• KBQUEUE is array of 16 words
• array base-address at 0x400x1E
• KBBASE is word at 0x400x80
• KBTAIL is word at 0x400x82

9
Format of KBFLAGS LSB
0
1
2
3
4
5
6
7
Insert-mode is active (1yes, 0no)
Caps-Lock is active (1yes, 0no)
Num-Lock is active (1yes, 0no)
Scroll-Lock is active (1yes, 0no)
Alt-Key is pressed (1yes, 0no)
Ctrl-Key is pressed (1yes, 0no)
Right-Shift key is pressed (1yes, 0no)
Left-Shift key is pressed (1yes, 0no)
10
Demo minikybd.s

• It shows a minimal implementation for the
  keyboard-controllers interrupt-handler
• It runs in Real-Mode
• It echos a users keystrokes
• IN-CLASS EXERCISE
• modify this demo to run in Protected-Mode