INT 15,4F - Keyboard Intercept (BIOS date specific)

	AH = 4F
	AL = scan code
	CF = set to 1 (via STC instruction)

	on return

	AH = 80h, CF set  (PC, PCjr)
	   = 86h, CF set  (XT BIOS 11/8/82, AT BIOS 1/10/84)
	AL = CF set, new scan code
	   = CF clear, original scancode

	- 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