INT 0x15 - 0x4f - Keyboard Intercept (BIOS date specific)

Request/Call:

reg	descr
AH	80h, CF Set (PC, PCjr) 86h, CF set (XT BIOS 11/08/82, AT BIOS 1/10/84)
AL	CF set, new scan code CF clear, original scan code

available with XT BIOS after 11/8/82, AT BIOS after 1/10/84
called by INT 9, makes allowance for keyboard translation
normally returns the scan code in AL, with CF set
if function returns with CF clear, INT 9 ignores keystroke
do not rely on this function being called for each INT 9 since any user INT 9 handler can exit prematurely and circumvent this function.