INT 15,F - Format Periodic Interrupt (PS/2 only)

	AH = 0F
	AL = phase code
	   = 00  reserved
	   = 01  surface analysis
	   = 02  formatting

	on return:
	CF = 0 continue formatting/scanning
	   = 1 end of format/scanning

	- after formatting/scanning each cylinder the formatting
	  routine calls this interrupt
	- may be used by a program to gain control after formatting
	  a cylinder
	- if invoked from a non-PS/2 CF=1 and AH contains the following
	  AH = 80h  for PC and PCjr
	     = 86h  on most other machines