Extended Control-Break processing note v1.0a:

Ctrl-Break clears keyboard buffer, places word 0000h in buffer,
 invokes INT 1B, and sets flag 80 hex at 0040h:0071h

Interrupt vector 1B may be trapped, set to empty IRET, and restored
 using any assembly vector redirection program.

Buffer value returned is double-nul byte, 80 hex is 1000 0000 binary.

The assembly program Keytrap.asm is included for trapping Control-Break.

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