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In my answer here I infer that the unusual scancode for the Pause/Break key emulates the user pressing and then releasing CtrlNumLock. Obviously that key combination did something specific, back when this key was introduced to PC keyboards.
What did CtrlNumLock do on the early IBM PC or in common software around that time?
In the IBM PC 5150, CtrlNumLock paused the system until another key (not NumLock) was pressed. As described in page 5-21 of the relevant Technical Reference:
The combination of the Ctrl and Num Lock keys will cause the keyboard interrupt routine to loop, waiting for any key except the Num Lock key to be pressed. This provides a system- or application-transparent method of temporarily suspending list, print, and so on, and then resuming the operation. The “unpause” key is thrown away. Pause is handled within the keyboard routine.
When the Pause key was introduced, with the 101/102-key keyboard, handling was similar. As described in page 4-72 of the relevant Technical Reference:
The Pause key causes the keyboard interrupt routine to loop, waiting for any character or function key to be pressed. This provides a method of temporarily suspending an operation, such as listing or printing, and then resuming the operation. The method is not apparent to either the system or the application program. The key stroke used to resume operation is discarded. Pause is handled internal to the keyboard routine.
CTRL+NUMLOCK was Pause, CTRL+SCROLLOCK was Break.
It was not until later that the keyboard was updated with new buttons for newer machines, and got a separate key for Pause/Break, which then still needs to send the same combination of scan codes to simulate CTRL+NUMLOCK and CTRL+SCROLLOCK.
So a more modern Enhanced Keyboard with Pause/Break key sends Pause key as events for pushing Left Ctrl and NumLock, but prefixed with E1, and immediately sends events for releasing Left Ctrl and NumLock, prefixed with E1 to indicate a two key extended sequence. A program ignoring E1 will just see Left Ctrl and NumLock.
And that's why Break is invoked by pushing Ctrl and Pause/Break keys, because it already forces you to manually send Ctrl down event and then the Pause/Break key just needs to send key down event for Scroll Lock, immediately followed by release event for Scroll Lock. The Scroll Lock press and release events in this case are prefixed with E0 to mark them as extended keys, but a program ignoring E0 will see just Scroll Lock with Ctrl held down.
Please note that these scancodes are the original scancodes used by BIOS, not scancodes communicated on keyboard wire, as the later keyboards use different scancodes that are translated to original scancodes by the motherboard keyboard controller.
In my answer here I infer that the unusual scancode for the Pause/Break key emulates the user pressing and then releasing Ctrl+NumLock.
No it does not.
1DE0 1DE1 14 7714 is the alpha key TE0 14 is not a legal escape code.1D 77E0 1D 77E1 is a special escape code only for keys that will send the break code right away. So far that is no other than the Pause/Break. E1 does not work like the standard E0 code. It wei always be followed by the appropriate break code.
