INT 15 - SYSTEM - SWITCH TO PROTECTED MODE                                      
	AH = 89h
	BL = interrupt number of IRQ0 (IRQ1-7 use next 7 interrupts)
	BH = interrupt number of IRQ8 (IRQ9-F use next 7 interrupts)
	ES:SI -> GDT for protected mode (see #00500)
Return: CF set on error
	   AH = FFh  error enabling address line 20
	CF clear if successful
	   AH = 00h
	   in protected mode at specified address
	   BP may be destroyed; all segment registers change
Notes:	BL and BH must be multiples of 8
	the protected-mode CS must reference the same memory as the CS this
	  function is called from because execution continues with the address
	  following the interrupt call
SeeAlso: AH=87h,AH=88h,INT 67/AX=DE0Ch


Format of BIOS(Basic Input/Output System) A set of standardized calls giving low-level access to the hardware.  The BIOS is the lowest software layer above the actual hardware and serves to insulate programs (and operating systems) which use it from the details of accessing the hardware directly. switch-to-protected-mode Global Descriptor Table:
Offset	Size	Description	(Table 00500)
 00h  8 BYTEs	null descriptor (initialize to zeros)
 08h  8 BYTEs	GDT descriptor (see #00501)
 10h  8 BYTEs	IDTsee Interrupt Descriptor Table descriptor
 18h  8 BYTEs	DS descriptor
 20h  8 BYTEs	ES
 28h  8 BYTEs	SS
 30h  8 BYTEs	CS
 38h  8 BYTEs	uninitialized, used to build descriptor for BIOS(Basic Input/Output System) A set of standardized calls giving low-level access to the hardware.  The BIOS is the lowest software layer above the actual hardware and serves to insulate programs (and operating systems) which use it from the details of accessing the hardware directly. CS


Format of segment descriptor table entry:
Offset	Size	Description	(Table 00501)
 00h	WORD	segment limit, low word
 02h  3 BYTEs	segment base address, low 24 bits
 05h	BYTE	access mode (see #00502)
 06h	BYTE	386+ extended access mode (see #00505)
 07h	BYTE	386+ segment base address, high 8 bits
SeeAlso: #00500,INT 2C/AX=0002h,INT 31/AX=0009h


Bitfields for segment descriptor table access mode field:
Bit(s)	Description	(Table 00502)
 3-0	segment type (see #00503,#00504)
 4	descriptor type (1 = application, 0 = system)
 6-5	descriptor privilege level
 7	segment is present in RAM(Random Access Memory)	See also DRAM, SRAM.
SeeAlso: #00501,#00505


(Table 00503)
Values for system segment descriptor type:
 0	reserved
 1	available 16-bit TSS
 2	LDT
 3	busy 16-bit TSS
 4	16-bit call gate
 5	task gate
 6	16-bit interrupt gate
 7	16-bit trap gate
 8	reserved
 9	available 32-bit TSS
 10	reserved
 11	busy 32-bit TSS
 12	32-bit call gate
 13	reserved
 14	32-bit interrupt gate
 15	32-bit trap gate
SeeAlso: #00502,#00504


Bitfields for application segment descriptor type:
Bit(s)	Description	(Table 00504)
 3	code/data
	0 date
	1 code
---data segments---
 2	expand down
 1	writeable
---code segments---
 2	conforming
 1	readable
------
 0	accessed
SeeAlso: #00502,#00503


Bitfields for 386+ segment descriptor table extended access mode field:
Bit(s)	Description	(Table 00505)
 3-0	high 4 bits of segment limit
 4	available
 5	reserved (0)
 6	default operation size (1 = 32 bits, 0 = 16 bits)
 7	granularity (1 = 4K, 0 = byte)
SeeAlso: #00501,#00502,#02557