A user program can talk to the keyboard controller on the motherboard. +The keyboard controller can again talk to the keyboard. +
When a key is pressed the keyboard sends the corresponding +keyboard scancode to the keyboard controller, and the keyboard controller +translates that and interrupts the CPU, allowing the CPU to read the result. +
More detailed: when a key is pressed, the keyboard sends +a start bit (low), followed by 8 data bits for the keyboard scancode +of the key (least significant first), followed by an odd parity bit, +followed by a stop bit (high). +The keyboard controller reads the data and checks the parity. +If incorrect, retransmission is requested. If incorrect again +a parity error is reported. +If the time between request to send and start of transmission is greater +than 15 ms, or if the eleven bits are not received within 2ms, +a timeout is reported. +In both cases (parity error or timeout), the data byte is set to 0xff. +
The keyboard controller has three 8-bit registers involved in +communication with the CPU: its input buffer, that can be written +by the CPU by writing port 0x60 or port 0x64; its output buffer, +that can be read by the CPU by reading from port 0x60; and the +status register, that can be read by the CPU by reading from port 0x64. +
If the CPU writes to port 0x64, the byte is interpreted as a command byte. +If the CPU writes to port 0x60, the byte is interpreted as a data byte. +
The keyboard controller has two 8-bit I/O ports involved in +communication with the keyboard: the +input port P1 (receiving input from the keyboard) +and the +output port P2 (for sending output +to the keyboard). +
+
The keyboard controller has an 8-bit status register. +It can be inspected by the CPU by reading port 0x64. +
(Typically, it has the value 0x14: keyboard not locked, self-test completed.) +
+
| +PARE | +TIM | +AUXB | +KEYL | +C/D | +SYSF | +INPB | +OUTB |
| + + |
+
+0: OK. +1: Parity error with last byte. ++
+
+0: OK. +1: Timeout. +On PS/2 systems: General timeout. +On AT systems: Timeout on transmission from keyboard to keyboard controller. +Possibly parity error (in which case both bits 6 and 7 are set). ++
Bit 5: + Auxiliary output buffer full +
+
+On PS/2 systems: +Bit 0 tells whether a read from port 0x60 will be valid. +If it is valid, this bit 5 tells what data will be read from port 0x60. +0: Keyboard data. 1: Mouse data. ++On AT systems: +0: OK. +1: Timeout on transmission from keyboard controller to keyboard. +This may indicate that no keyboard is present. +
+
+0: Locked. +1: Not locked. ++
+
+0: Last write to input buffer was data (written via port 0x60). +1: Last write to input buffer was a command (written via port 0x64). +(This bit is also referred to as Address Line A2.) ++
+
+Set to 0 after power on reset. +Set to 1 after successful completion of the keyboard controller self-test +(Basic Assurance Test, BAT). +Can also be set by command (see +below). ++
Bit 1: + Input buffer status +
+
+0: Input buffer empty, can be written. +1: Input buffer full, don't write yet. ++
Bit 0: + Output buffer status +
+
+0: Output buffer empty, don't read yet. +1: Output buffer full, can be read. +(In the PS/2 situation bit 5 tells whether the available data is +from keyboard or mouse.) +This bit is cleared when port 0x60 is read. ++
+
The keyboard controller is provided with some RAM, for example 32 bytes, +that can be accessed by the CPU. The most important part of this RAM is +byte 0, the Controller Command Byte (CCB). It can be read/written by +writing 0x20/0x60 to port 0x64 and then reading/writing a data byte +from/to port 0x60. +
This byte has the following layout. +
+
| +0 | +XLATE | +ME | +KE | +IGNLK | +SYSF | +MIE | +KIE |
| + + |
+
+Always 0. ++
+
+0: No translation. +1: Translate keyboard scancodes, using the +translation table given above. +MCA type 2 controllers cannot set this bit to 1. In this case +scan code conversion is set using keyboard command 0xf0 to port 0x60. ++
+
+On an EISA or PS/2 system: 0: Enable mouse. 1: Disable mouse +by driving the clock line low. +On an ISA system: "PC Mode": 0: use 11-bit codes, check parity and do +scan conversion. +1: use 8086 codes, don't check parity and don't do scan conversion. ++
+
+0: Enable keyboard. 1: Disable keyboard +by driving the clock line low. ++
Bit 3: + Ignore keyboard lock +
+
+For PS/2: Unused, always 0. +For AT: +0: No action. 1: Force +bit 4 of the status register +to 1, "not locked". This is used for keyboard testing after power on. +Maybe only on older motherboards. ++
+
+This bit is shown in +bit 2 of the status register. +A "cold reboot" is one with this bit set to zero. +A "warm reboot" is one with this bit set to one (BAT already completed). +This will influence the tests and initializations done by the POST. ++
Bit 1: + Mouse interrupt enable +
+
+On an ISA system: unused, always 0. On an EISA or PS/2 system: +0: Do not use mouse interrupts. +1: Send interrupt request IRQ12 when the mouse output buffer is full. ++
Bit 0: + Keyboard interrupt enable +
+
+0: Do not use keyboard interrupts. +1: Send interrupt request IRQ1 when the keyboard output buffer is full. ++When no interrupts are used, the CPU has to poll bits 0 (and 5) +of the status register. +
+
The CPU can command the keyboard controller by writing port 0x64. +Useful, generally available, keyboard commands are: +
+
+
| + +20 | Read keyboard controller command byte |
| + +60 | Write keyboard controller command byte |
| + +aa | Self test |
| + +ab | Interface test |
| + +ad | Disable keyboard |
| + +ae | Enable keyboard |
| + +c0 | Read input port |
| + +d0 | Read output port |
| + +d1 | Write output port |
| + +e0 | Read test inputs |
| + +fe | System reset |
| + + |
Useful, generally available, mouse commands are: +
+
+
| + +a7 | Disable mouse port |
| + +a8 | Enable mouse port |
| + +a9 | Test mouse port |
| + +d4 | Write to mouse |
| + + |
Obscure, probably obsolete, commands: +
+
+
| + +00-1f | Read keyboard controller RAM |
| + +20-3f | Read keyboard controller RAM |
| + +40-5f | Write keyboard controller RAM |
| + +60-7f | Write keyboard controller RAM |
| + +90-93 | Synaptics multiplexer prefix |
| + +90-9f | Write Port13-Port10 |
| + +a0 | Read copyright |
| + +a1 | Read firmware version |
| + +a2 | Switch speed |
| + +a3 | Switch speed |
| + +a4 | Check if password installed |
| + +a5 | Load password |
| + +a6 | Check password |
| + +ac | Diagnostic dump |
| + +af | Read keyboard version |
| + +b0-b5 | Reset keyboard controller line |
| + +b8-bd | Set keyboard controller line |
| + +c1 | Continuous input port poll, low |
| + +c2 | Continuous input port poll, high |
| + +c8 | Unblock lines P22 and P23 |
| + +c9 | Block lines P22 and P23 |
| + +ca | Read keyboard controller mode |
| + +cb | Write keyboard controller mode |
| + +d2 | Write keyboard output buffer |
| + +d3 | Write mouse output buffer |
| + +dd | Disable A20 address line |
| + +df | Enable A20 address line |
| + +f0-ff | Pulse output bit |
| + + |
Command 0x00-0x1f: + Read keyboard controller RAM +
+
+(AMIBIOS only) Aliases for 0x20-0x3f. ++
Command 0x20-0x3f: + Read keyboard controller RAM +
+
+The last six bits of the command specify the RAM address to read. +The read data is placed into the output buffer, and can be read +by reading port 0x60. +On MCA systems, type 1 controllers can access all 32 locations; +type 2 controllers can only access locations 0, 0x13-0x17, 0x1d, 0x1f. ++Location 0 is the +Command byte, see above. +
Location 0x13 (on MCA) is nonzero when a password is enabled. +
Location 0x14 (on MCA) is nonzero when the password was matched. +
Locations 0x16-0x17 (on MCA) give two make codes to be discarded +during password matching. +
Command 0x40-0x5f: + Write keyboard controller RAM +
+
+(AMIBIOS only) Aliases for 0x40-0x5f. ++
Command 0x60-0x7f: + Write keyboard controller RAM +
+
Command 0x90-0x93: + Synaptics routing prefixes +
+
+Prefix a PS/2 mouse command with one of these to talk to one of at most four +multiplexed devices. See also the +multiplexing handshake below. ++Unfortunately, VIA also uses this command: +
Command 0x90-0x9f: + Write Port13-Port10 +
+(VIA VT82C42) Write low nibble to Port13-Port10. ++
Command 0xa0: + Read copyright +
+
+On some keyboard controllers: an ASCIZ copyright string +(possibly just NUL) is made available for reading via port 0x60. +On other systems: no effect, the command is ignored. ++
Command 0xa1: + Read controller firmware version +
+
+On some keyboard controllers: a single ASCII byte is made available +for reading via port 0x60. +On other systems: no effect, the command is ignored. ++
Command 0xa2: + Switch speed +
+
+(On ISA/EISA systems with AMI BIOS) +Reset keyboard controller lines P22 and P23 low. +These lines can be used for speed switching via the keyboard controller. +When done, the keyboard controller sends one garbage byte to the system. ++
Command 0xa3: + Switch speed +
+
+(On ISA/EISA systems with AMI BIOS) +Set keyboard controller lines P22 and P23 high. +These lines can be used for speed switching via the keyboard controller. +When done, the keyboard controller sends one garbage byte to the system. ++(Compaq BIOS: Enable system speed control.) +
Command 0xa4: + Check if password installed +
+
+On MCA systems: +Return 0xf1 (via port 0x60) when no password is installed, +return 0xfa when a password has been installed. +Some systems without password facility always return 0xf1. ++(On ISA/EISA systems with AMI BIOS) +Write Clock = Low. +
(Compaq BIOS: toggle speed.) +
Command 0xa5: + Load password +
+
+On MCA systems: +Load a password by writing a NUL-terminated string to port 0x60. +The string is in scancode format. ++(On ISA/EISA systems with AMI BIOS) +Write Clock = High. +
(Compaq BIOS: special read of P2, with bits 4 and 5 replaced: +Bit 5: 0: 9-bit keyboard, 1: 11-bit keyboard. +Bit 4: 0: outp-buff-full interrupt disabled, 1: enabled.) +
Command 0xa6: + Check password +
+
+On MCA systems: +When a password is installed: +Check password by matching keystrokes with the stored password. +Enable keyboard upon successful match. ++(On ISA/EISA systems with AMI BIOS) +Read Clock. 0: Low. 1: High. +
Command 0xa7: + Disable mouse port +
+
+On MCA systems: disable the mouse (auxiliary device) +by setting its clock line low, and set +bit 5 +of the +Command byte. Now P23 = 1. ++(On ISA/EISA systems with AMI BIOS) +Write Cache Bad. +
Command 0xa8: + Enable mouse port +
+
+On MCA systems: enable the mouse (auxiliary device), +clear +bit 5 of the +Command byte. Now P23 = 0. ++(On ISA/EISA systems with AMI BIOS) +Write Cache Good. +
Command 0xa9: + Test mouse port +
+
+On MCA and other systems: test the serial link between +keyboard controller and mouse. The result can be read from port 0x60. +0: OK. +1: Mouse clock line stuck low. +2: Mouse clock line stuck high. +3: Mouse data line stuck low. +4: Mouse data line stuck high. +0xff: No mouse. ++(On ISA/EISA systems with AMI BIOS) +Read Cache Bad or Good. 0: Bad. 1: Good. +
+
+Perform self-test. Return 0x55 if OK, 0xfc if NOK. ++
Command 0xab: + Interface test +
+
+Test the serial link between keyboard controller and keyboard. +The result can be read from port 0x60. +0: OK. +1: Keyboard clock line stuck low. +2: Keyboard clock line stuck high. +3: Keyboard data line stuck low. +4: Keyboard data line stuck high. +0xff: General error. ++
Command 0xac: + Diagnostic dump +
+
+(On some systems) +Read from port 0x60 sixteen bytes of keyboard controller RAM, +and the output and input ports and the controller's program status word. ++
Command 0xad: + Disable keyboard +
+
+Disable the keyboard clock line and set +bit 4 +of the +Command byte. +Any keyboard command enables the keyboard again. ++
Command 0xae: + Enable keyboard +
+
+Enable the keyboard clock line and clear +bit 4 +of the +Command byte. ++
Command 0xaf: + Read keyboard version +
+
+(Award BIOS, VIA) ++
Command 0xb0-0xb5,0xb8-0xbd: + Reset/set keyboard controller line +
+
+AMI BIOS: +Commands 0xb0-0xb5 reset a keyboard controller line low. +Commands 0xb8-0xbd set the corresponding keyboard controller line high. +The lines are P10, P11, P12, P13, P22 and P23, respectively. +(In the case of the lines P10, P11, P22, P23 this is on ISA/EISA systems only.) +When done, the keyboard controller sends one garbage byte to the system. ++VIA BIOS: +Commands 0xb0-0xb7 write 0 to lines P10, P11, P12, P13, P22, P23, P14, P15. +Commands 0xb8-0xbf write 1 to lines P10, P11, P12, P13, P22, P23, P14, P15. +
Command 0xc0: + Read input port +
+
+Read the +input port (P1), +and make the resulting byte available to be read from port 0x60. ++
Command 0xc1: + Continuous input port poll, low +
+
+(MCA systems with type 1 controller only) +Continuously copy bits 3-0 of the input port to be read from bits 7-4 +of port 0x64, until another keyboard controller command is received. ++
Command 0xc2: + Continuous input port poll, high +
+
+(MCA systems with type 1 controller only) +Continuously copy bits 7-4 of the input port to be read from bits 7-4 +of port 0x64, until another keyboard controller command is received. ++
Command 0xc8: + Unblock keyboard controller lines P22 and P23 +
+
+(On ISA/EISA systems with AMI BIOS) +After this command, the system can make lines P22 and P23 low/high +using +command 0xd1. ++
Command 0xc9: + Block keyboard controller lines P22 and P23 +
+
+(On ISA/EISA systems with AMI BIOS) +After this command, the system cannot make lines P22 and P23 low/high +using +command 0xd1. ++
Command 0xca: + Read keyboard controller mode +
+
+(AMI BIOS, VIA) +Read keyboard controller mode to bit 0 of port 0x60. +0: ISA (AT) interface. +1: PS/2 (MCA)interface. ++
Command 0xcb: + Write keyboard controller mode +
+
+(AMI BIOS) +Write keyboard controller mode to bit 0 of port 0x60. +0: ISA (AT) interface. +1: PS/2 (MCA)interface. +(First read the mode using command 0xca, then modify only +the last bit, then write the mode using this command.) ++
Command 0xd0: + Read output port +
+
+Read the +output port (P2) +and place the result in the output buffer. +Use only when output buffer is empty. ++
Command 0xd1: + Write output port +
+
+Write the +output port (P2). +Note that writing a 0 in bit 0 will cause a hardware reset. ++(Compaq: the system speed bits are not set. Use commands 0xa1-0xa6 for that.) +
Command 0xd2: + Write keyboard output buffer +
+
+(MCA) +Write the keyboard controllers output buffer with the byte +next written to port 0x60, and act as if this was keyboard data. +(In particular, raise IRQ1 when +bit 0 +of the +Command byte says so.) ++
Command 0xd3: + Write mouse output buffer +
+
+(MCA) +Write the keyboard controllers output buffer with the byte +next written to port 0x60, and act as if this was mouse data. +(In particular, raise IRQ12 when +bit 1 +of the +Command byte says so.) ++Not all systems support this. +
+ Synaptics multiplexing +On the other hand, Synaptics (see +ps2-mux.PDF) +uses this command as a handshake between driver and controller: +if the driver gives this command three times, with data bytes +0xf0, 0x56, 0xa4 respectively, and reads 0xf0, 0x56, but not 0xa4 +back from the mouse output buffer, then the driver knows that the +controller supports Synaptics AUX port multiplexing, and the controller +knows that it does not have to do the usual data faking and goes +into multiplexed mode. The third byte read is the version of the +Synaptics standard. +
There is a corresponding deactivation sequence, namely +0xf0, 0x56, 0xa5. (And again the last byte is changed to the +version number of the standard supported.) +This latter sequence works both in multiplexed mode and in legacy mode +and can thus be used to determine whether this feature is present +without activating it. +
See also the multiplexer commands +0x90-0x93. +
For some laptops it has been reported that bit 3 of every third +mouse byte is forced to 1 (as it would be with the standard +3-byte mouse packets). This may turn 0xf0, 0x56, 0xa4 into +0xf0, 0x56, 0xac and cause misdetection of Synaptics multiplexing +(for version 10.12). +
Command 0xd4: + Write to mouse +
+
+(MCA) +The byte next written to port 0x60 is transmitted to the mouse. ++
Command 0xdd: + Disable A20 address line +
+
+(HP Vectra) ++
Command 0xdf: + Enable A20 address line +
+
+(HP Vectra) ++
Command 0xe0: + Read test inputs +
+
+This command makes the status of the +Test inputs T0 and T1 available +to be read via port 0x60 in bits 0 and 1, respectively. +Use only when the output port is empty. ++
+
Command 0xf0-0xff: + Pulse output bit +
+
+Bits 3-0 of the +output port P2 +of the keyboard controller may be pulsed low for approximately 6 µseconds. +Bits 3-0 of this command specify the output port bits to be pulsed. +0: Bit should be pulsed. +1: Bit should not be modified. +The only useful version of this command is Command 0xfe. +(For MCA, replace 3-0 by 1-0 in the above.) ++
Command 0xfe: + System reset +
+
+Pulse bit 0 of the +output port P2 +of the keyboard controller. This will reset the CPU. ++
+
This has the following layout. +
+
| +bit 7 | Keyboard lock | 0: locked, 1: not locked |
| +bit 6 | Display | 0: CGA, 1: MDA |
| +bit 5 | Manufacturing jumper | 0: installed, 1: not installed |
| + | with jumper the BIOS runs an infinite diagnostic loop | |
| +bit 4 | RAM on motherboard | 0: 512 KB, 1: 256 KB |
| +bit 3 | Unused in ISA, EISA, PS/2 systems | |
| + | Can be configured for clock switching | |
| +bit 2 | Unused in ISA, EISA, PS/2 systems | |
| + | Can be configured for clock switching | |
| + | Keyboard power | PS/2 MCA: 0: keyboard power normal, 1: no power |
| +bit 1 | Mouse data in | Unused in ISA |
| +bit 0 | Keyboard data in | Unused in ISA |
| + + |
Clearly only bits 1-0 are input bits. +Of the above, the original IBM AT used bits 7-4, while PS/2 MCA systems +use only bits 2-0. +
Where in the above lines P10, P11, etc are used, these refer to the pins +corresponding to bit 0, bit 1, etc of port P1. +
+
This has the following layout. +
+
| +bit 7 | Keyboard data | data to keyboard |
| +bit 6 | Keyboard clock | |
| +bit 5 | IRQ12 | 0: IRQ12 not active, 1: active |
| +bit 4 | IRQ1 | 0: IRQ1 not active, 1: active |
| +bit 3 | Mouse clock | Unused in ISA |
| +bit 2 | Mouse data | Unused in ISA. Data to mouse |
| +bit 1 | A20 | 0: A20 line is forced 0, 1: A20 enabled |
| +bit 0 | Reset | 0: reset CPU, 1: normal |
| + + |
Where in the above lines P20, P21, etc are used, these refer to the pins +corresponding to bit 0, bit 1, etc of port P2. +
+
bit 0 +
+
+Keyboard clock (input). ++
bit 1 +
+
+(AT) Keyboard data (input). +(PS/2) Mouse clock (input). ++
+