Section 2: Connection Path

The Big Picture: Three Ways to Reach the PC

The keyboard has worked out which key is down (Section 1). Now it must tell the PC. There are three common roads:

The big surprise: the keyboard and mouse look the same to Windows whichever road they use. Windows describes them with the same simple idea, called HID (Human Interface Device).

2.1 USB HID (Human Interface Device) Class

What it is: USB is the common wired connection (see Chapter 4, Section 2). USB groups devices into classes. A class is a ready-made rulebook, so one built-in driver can work with many makers' devices. The HID class is the rulebook for things humans use: keyboards, mice, game controllers, and also buttons, dials and touch screens.

Step 1: The device describes itself. When you plug the keyboard in, Windows asks it "what are you?" (This is the plug-in steps in Chapter 13, Section 16: the USB hub notices the new device, the system gives it an address, and Windows picks a driver for its class.) A HID device answers with a report descriptor: a small table that says "my messages look like this: first byte is the modifier keys, second byte is reserved, then six key numbers." Because the device describes its own messages, one driver can read keyboards, mice and odd gadgets alike.

Step 2: The PC polls. USB is not interrupt-driven the way PS/2 is. A USB device cannot speak first. The USB host controller (the chip, usually called xHCI for USB 3, that runs the USB ports) polls each device: it asks "anything new?" at a fixed interval. A basic keyboard or mouse is often polled about every 8 milliseconds, which is about 125 times a second (125 Hz). Gaming devices may ask for 1 millisecond (1000 Hz), and some go faster. The exact interval is requested by the device and may differ by model.

   time ───────────────────────────────────────────────────►
   PC:     "news?"    "news?"    "news?"    "news?"
              │          │          │          │
   Device:  "no"       "no"   "yes: key A!"  "no change"
                                     ▲
                      the report is sent only when asked

Name warning. USB calls the endpoint used for this an interrupt endpoint. That does not mean the device raises a hardware interrupt like PS/2. It only means "small messages, delivered at a regular guaranteed interval". The host controller does the polling by itself, and the CPU is told only when a report arrives.

Step 3: The report. Each answer is a HID report: a few bytes. For a keyboard, the common "boot protocol" format is 8 bytes. (Boot protocol is a simple fixed format that BIOS/UEFI can understand before any real driver exists.)

Byte:   0          1          2    3    4    5    6    7
      ┌─────────┬──────────┬────┬────┬────┬────┬────┬────┐
      │Modifiers│ Reserved │Key1│Key2│Key3│Key4│Key5│Key6│
      └─────────┴──────────┴────┴────┴────┴────┴────┴────┘

• Byte 0, modifiers: one bit each for Left Ctrl, Left Shift, Left Alt, Left Windows key, and the four right-hand ones.
• Byte 1: reserved (always 0).
• Bytes 2 to 7: up to six key codes currently held down. This is where the 6-key rollover from Section 1.5 comes from. An empty slot holds 0.

Example: you hold Shift and press A. HID code numbers ("usages") are not the same as the old scan codes. In HID, A is 04, and the Shift bit is 02.

Shift+A held:    02 00 04 00 00 00 00 00
Everything up:   00 00 00 00 00 00 00 00

Notice: there is no "break code". The report shows what is down right now. The PC finds out a key was released because it has disappeared from the next report. Many keyboards use other, longer reports as well (extra media keys, N-key rollover), described by their descriptor.

LED lights go the other way. For Caps Lock, the PC sends the keyboard a small output report with bits for the lamps.

Auto-repeat. HID reports do not repeat a held key by themselves. For USB keyboards, Windows (the driver stack) generally creates the repeating when you hold a key, using your delay and rate settings (Practical Lab).

2.2 Bluetooth HID and 2.4 GHz Wireless

What it is: Wireless keyboards and mice use the same HID idea over radio. The reports look the same; only the carrier changes.

Points to remember:

• Pairing means the two sides swap secret keys once, so later traffic is encrypted and a stranger nearby cannot just read your typing. (How strong this is depends on the Bluetooth version and the maker. Keep devices updated.)
• A radio link has more delay and more chance of missed messages than a cable, so gaming mice often use a special 2.4 GHz link rather than plain Bluetooth.
• To save battery, wireless devices sleep when idle. The first key press after a pause may feel slightly slow while the device wakes and reconnects.
• On the PC side, Windows treats a paired device as a HID device; a different driver carries the bytes (the Bluetooth stack instead of the USB one). Above that, the chain is the same as the one in Section 3.

2.3 The Old Way: PS/2

What it is: PS/2 is the older connection named after an IBM PC model. It uses a small round socket, usually purple for the keyboard and green for the mouse. You can see both in the photo.

Figure 31.2 — Back of a PC. Find the two round PS/2 sockets (purple and green) and compare them with the rectangular USB sockets beside them. Photo: Hitachi-Train, public domain, via Wikimedia Commons.

How PS/2 works. It is a simple serial link: bits go one after another over a data wire, with a separate clock wire to keep time. The keyboard sends its scan codes (Section 1.2) by itself the moment a key changes. A small chip on the board (the keyboard controller, an "8042" type, today usually part of the chipset) catches each byte and raises an interrupt.

This ties back to Chapter 4, Section 3: the classic interrupt lines are IRQ 1 for the keyboard and IRQ 12 for the PS/2 mouse. When you press a key, the controller taps the CPU's shoulder, the CPU runs a small piece of the keyboard driver, and the byte is saved.

The PS/2 idea that "the keyboard is a separate little computer sending codes" lives on in USB. Only the road is different.

2.4 The Path to Windows (Short Version)

PS/2 :  keyboard ──► keyboard controller (IRQ 1) ──► i8042prt driver ─┐
                                                                       ├─► keyboard class driver ─► Windows input
USB  :  keyboard ──► USB port ──► xHCI polls it ──► HID drivers ──────┤
BT   :  keyboard ──► radio ──► Bluetooth stack ──► HID drivers ───────┘

All three roads join into the same class driver and the same Windows input system. Names of the drivers are in Section 3. If you want to see the USB path on your own PC, the Practical Lab shows how to use Device Manager's Devices by connection view.

Summary