Int 15 Fn 89  - System - Switch To Protected Mode                          [B]

   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 #0426)

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

See Also: AH=87h,AH=88h,INT 67/AX=DE0Ch

Format of BIOS switch-to-protected-mode Global Descriptor Table:
Offset Size    Description (Table 0426)
 00h  8 BYTEs  null descriptor (initialize to zeros)
 08h  8 BYTEs  GDT descriptor (see #0427)
 10h  8 BYTEs  IDT 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 CS

Format of segment descriptor table entry:
Offset Size    Description (Table 0427)
 00h   WORD    segment limit, low word
 02h  3 BYTEs  segment base address, low 24 bits
 05h   BYTE    access mode (see #0428)
 06h   BYTE    386+ extended access mode (see #0429)
 07h   BYTE    386+ segment base address, high 8 bits

See Also: #0426,INT 2C/AX=0002h,INT 31/AX=0009h

Bitfields for segment descriptor table access mode field:
Bit(s) Description (Table 0428)
 3-0   segment type (see #3724,#3725)
 4 descriptor type (1 = application, 0 = system)
 6-5   descriptor privilege level
 7 segment is present in RAM

See Also: #0427,#0429

(Table 3724)
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

See Also: #0428,#3725

Bitfields for application segment descriptor type:
Bit(s) Description (Table 3725)
 3 code/data
   0 date
   1 code
---data segments---
 2 expand down
 1 writeable
---code segments---
 2 conforming
 1 readable