PC AT keyboard

For PC AT or newer:

- 8048 chip in the keyboard

- 8042 chip on the PC motherboard



Both are tiny 8-bit microprocessors, programmed to understand

simple one- or two-byte commands.



Features of the 8042 chip:

- I/O ports:

    60h read/write      Data register

    64h write           Command register

    64h read            Status register

- Can be in "AT mode" or "PS/2 mode", depending on the

  motherboard type. If your PC has a PS/2 mouse port,

  the 8042 uses PS/2 mode.

- "Output Port"; used for controlling the A20 gate, resetting

  the PC, etc. This port is written by 8042 command byte D1h,

  and read by command byte D0h

- "Input Port"; used for reading the status of the keyboard

  lock keyswitch (located on the front of many PCs). This port

  is read by 8042 command byte C0h

- The poorly-named "Command Byte register"; various functions.

  This register is written by 8042 command byte 60h, and

  read by 8042 command byte 20h



================================================================

8042 REGISTERS IN "AT MODE"

================================================================

Bits in 8042 Status register

    b7  PERR    parity error in data received from keyboard

    b6  RxTO    receive (Rx) timeout

    b5  TxTO    transmit (Tx) timeout

    b4  INH     keyboard communications inhibited

    b3  A2      0=60h was the port last written to, 1=64h was last

    b2  SYS     distinguishes reset types: 0=cold reboot, 1=warm reboot

    b1  IBF     input buffer full (data from host to keyboard)

    b0  OBF     output buffer full (data from keyboard to host)



Bits in 8042 Output Port (Table P0383 in PORTS.A)

    b7          1=keyboard data line pulled low (inhibited)

    b6          1=keyboard clock line pulled low (inhibited)

    b5          output buffer empty (xxx - input buffer NOT full?)

    b4          input buffer full (xxx - output buffer NOT empty?)

    b3          (undefined)

    b2          (undefined)

    b1          A20 gate on/off

    b0          reset the PC (THIS BIT SHOULD ALWAYS BE SET TO 1)



Bits in 8042 Input Port

    b7          1=keyboard inhibit switch is activated (locked)

    b6          display type switch status (1=color, 0=monochrome)

    b5          manufacturing setting (?)

    b4          external RAM (?)

    b3          (undefined)

    b2          (undefined)

    b1          (undefined)

    b0          (undefined)



Bits in Command Byte register (Table P0404 in PORTS.A)

    b7          (reserved)

    b6  XLAT    convert set 2 scancodes to set 1 ("IBM PC compatibility mode")

    b5  PC      ?

    b4  _EN     disables keyboard when set

    b3  OVR     1=override inhibit keyswitch

    b2  SYS     System Flag (same as b2 in Status register)

    b1          (reserved)

    b0  INT     enables IRQ 1 interrupt on keyboard IBF



================================================================

8042 REGISTERS IN "PS/2 MODE"

================================================================

Bits in 8042 Status register

    b7  PERR    parity error in data received from keyboard

    b6  TO      general timeout (Rx or Tx)

    b5  MOBF    mouse output buffer full

    b4  INH     keyboard communications inhibited

    b3  A2      0=60h was the port last written to, 1=64h was last

    b2  SYS     distinguishes reset types: 0=cold reboot, 1=warm reboot

    b1  IBF     input buffer full (data from host to keyboard)

    b0  OBF     output buffer full (data from keyboard to host)



Bits in 8042 Output Port

    b7          1=keyboard data line pulled low (inhibited)

    b6          1=keyboard clock line pulled low (inhibited)

    b5          enables IRQ 12 interrupt on mouse IBF

    b4          enables IRQ 1 interrupt on keyboard IBF

    b3          1=mouse clock line pulled low (inhibited)

    b2          1=mouse data line pulled low (inhibited)

    b1          A20 gate on/off

    b0          reset the PC (THIS BIT SHOULD ALWAYS BE SET TO 1)



Bits in 8042 Input Port

    b7          1=keyboard inhibit switch is activated (locked)

    b6          display type switch status (1=color, 0=monochrome)

    b5          manufacturing setting (?)

    b4          external RAM (?)

    b3          (undefined)

    b2          (undefined)

    b1          mouse data line

    b0          keyboard data line



Bits in Command Byte register

    b7          (reserved)

    b6  XLAT    convert set 2 scancodes to set 1 ("IBM PC compatibility mode")

    b5  _EN2    disables mouse when set

    b4  _EN     disables keyboard when set

    b3          (reserved)

    b2  SYS     System Flag (same as b2 in Status register)

    b1  INT2    enables IRQ 12 interrupt on mouse IBF

    b0  INT     enables IRQ 1 interrupt on keyboard IBF



================================================================

8042 CONTROLLER COMMANDS (from Table P0401 of PORTS.A)

================================================================

Before writing each byte of these commands to port 64h,

poll the status register (port 64h) until bit b1=0.



20h-3Fh         reads byte from 8042 RAM at address=lower 5 bits of command

                  The byte at address 0 is the "Command Byte".

60h-7Fh nn      writes byte nn to 8042 RAM at address=lower 5 bits of command

                  The byte at address 0 is the "Command Byte".



(xxx - It may be that only address 0 [the "Command Byte"] can be

read and written. In that case, only command bytes 20h and 60h

will work.)



90h-9Fh         write bottom 4 bits of command to bottom 4 bits of Output Port

A7h             disables PS/2 mouse port

A8h             enables PS/2 mouse port

A9h             self-test mouse interface, returns Result Byte (see below)

AAh             self-test 8042; returns 55h if success, FCh if failure

ABh             self-test keyboard interface, returns Result Byte (see below)

ADh             disables keyboard (sets b4 of Command Byte register)

AEh             enables keyboard (clears b4 of Command Byte register)

C0h         	reads Input Port

D0h         	reads Output Port

D1h nn      	writes Output Port

                  Important: bit 0 (system reset) should always be set here,

		  as the system may hang constantly. To reset the PC,

		  pulse b0 of the Output Port with command FEh instead.

D4h nn          send command byte to PS/2 mouse

DDh             disable A20 ("Vectra" method; see

                        http://www.execpc.com/~geezer/osd/boot/a20.asm)

DFh             enable A20 ("Vectra" method)

F0h-FFh     	pulse bit(s) of Output Port low for 6 microseconds.

                  If b0-b3 of the command is low, the corresponding bit

                  in the Output Port will be pulsed low. b0=system reset,

                  and should ALWAYS be PULSED low, never set low constantly.



================================================================

8048 KEYBOARD COMMANDS (from Table P0386 of PORTS.A)

================================================================

Before writing each byte of these commands to port 60h,

poll the status register (port 64h) until bit b1=0.

Unless otherwise noted: each command responds with FAh (ACKnowledge)

or FEh (Resend) after receiving each byte of the command.



EDh nn      	write LEDs. nn=

		    b2      Caps Lock

		    b1      Num Lock

		    b0      Scroll Lock

EEh             Echo, keyboard responds with EEh

F0h nn          Selects scancode set nn=1-3 or 0 to return current set.

                Only scancode set 2 is widely (and correctly) supported.

F2h             Read ID. Keyboard responds with ACK (FAh) and two optional

		ID bytes:

		    (none)	AT keyboard

		    83h ABh   	(?)

		    ABh 41h   	MF2, translation mode

		    ABh 83h   	MF2, pass-through mode

F3h nn          Set typematic (auto-repeat) rate/delay. nn=

		    b7      unused

		    b6..5   Repeat delay (00=250 msec ... 11=1000msec)

                    b4..0   Repeat rate (00000=30 Hz ... 11111=2 Hz).

F4h             Clears output buffer, enables keyboard

F5h             Disables keyboard, resets to defaults

F6h             Sets keyboard default typematic rate/delay and

                selects scancode set 2

F7h             [*] Make all keys make/typematic (auto-repeat; no break)

F8h             [*] Make all keys make/break (no typematic)

F9h             [*] Make all keys make-only (no typematic or break)

FAh             [*] Make all keys typematic/make/break

FBh nn          [*] Make one key make/typematic

                    nn is the key's Set 3 scancode

FCh nn          [*] Make one key make/break

FDh nn          [*] Make one key make-only

FEh             Resend previous byte (unless previous byte is FEh)

FFh             Reset 8048, do power-on self-test, return test result byte



[*] These functions work only when the keyboard is using scancode Set 3



Result Byte for interface self-tests (Table P0406 in PORTS.A)

Returned by controller commands A9h or ABh

    0           no error

    1           clock line stuck low

    2           clock line stuck high

    3           data line stuck low

    4           data line stuck high



non-key status bytes

--------------------

00h    Key detection error or buffer full.

AAh    Power-on/reset diagnostics successful.

E0h    (scancode sets 1 and 2) Prefix byte for "gray" keys

	(keys not on original 83-/84-key keyboard)

EEh    Sent in response to ECHO command.

F0h    (scancode sets 2 and 3) Prefix byte for break codes.

FAh    ACKnowledge; response to most commands.

FCh    Diagnostics failed (MF keyboard).

FDh    Diagnostics failed (AT keyboard).

	The keyboard stops scanning and waits for next command

	after returning code FCh or FDh

FEh    Last command was invalid or had parity error; resend it.

FFh    Key detection error or buffer full.