Real mode BIOS routine and Protected Mode

Question

I am doing some OS experiment. Until now, all my code utilized the real mode BIOS interrupt to manipulate hard disk and floppy. But once my code enabled the Protect Mode of the

Answer 1

V86: yes the way to go, but if setting up an OS:

try this(designed for long mode but should work.I havent tested this YET, I see no reason it wont work yet.Issue is not nasm, its ld.)

LD H8s 16-bit ELF/aout references.This is standard to load from GRUB.

I kow the 32-bit CS is off, I need to double check its location. Otherwise it looks ok.

This is hard to find code.

-- ;modify for 32bit??

;this code is placed somewhere after 10000h
;-----we're in LONG MODE-----
  mov          dword [.stckptr], esp   ;first of all save stack
  sgdt         [.gdtv32]               ;save your gdt pointer
  lgdt         [.gdtv16]               ;load a new one
  sidt         [.idt32]                ;save your idt pointer
  lidt         [.idt16]                ;load real mode idt
  ;far jump in long mode is not possible, do a trick
  push         DESC_REAL
  push         @f-10000h               ;this is CS*10h, modify if needed!
  retfd
.stckptr:
  dd           0
  align        16
.gdtv32:
  dw           0
  dd           0
  align        16
.gdtv16:
  dw           .gdtend-.gdt-1
  dd           .gdt,0
  align        16
.gdt:
  dd           0,0                      ;null descriptor
DESC_DATA=8                                 ;descriptor in YOUR GDT (modify)
DESC_LONG=$-.gdt
  dd           00000000h,00209800h      ;32 bit  mode cs -MOD ME
DESC_REAL=$-.gdt
  dd           0000FFFFh,00009801h      ;16 bit real mode cs (modify base if needed!)
.gdtend:
  align        16
.idt32:
  dw           0
  dd           0
  align        16
.idt16:
  dw           3FFh
  dd           0
  USE16

;-----we're in COMPATIBLITY MODE-----
  ;disable paging and protmode at once
@@:   mov          eax, cr0
  and          eax, 7FFFFFFEh   
  mov          cr0, eax

  ;set up real mode segment registers and stack
  mov          esp, realmode_stack_top          ;modify it to your needs!
  xor          ax, ax
  mov          ds, ax
  mov          es, ax
  mov          fs, ax
  mov          gs, ax
  mov          ss, ax
  ;convert long mode rip to real mode cs:ip
  ;jmp CS:(pmode address)-CS*10h

  jmp          1000h:@f-10000h                  ;modify if needed!
;-----we're in REAL MODE-----
@@:   ;***********call some BIOS interrupt here**********
  mov          ax, 3
  int          10h


  ;switch back to long mode
  mov          eax, cr0
  or           eax, 80000001h
  mov          cr0, eax                         ;enable protmode and paging

  ;jmp         DESC_LONG:@f
  db           66h
  db           0EAh
  dd           @f
  dw           DESC_LONG
  USE32
;-----we're in protected MODE-----
@@:   lgdt         [cs:.gdtv32]                    ;restore gdt
  mov          ax, DESC_DATA                   ;read YOUR DATA descriptor to selectors
  mov          ds, ax
  mov          es, ax
  mov          fs, ax
  mov          gs, ax
  mov          ss, ax
  lidt         [.idt32]                        ;restore idt
  mov          rsp, qword [.stckptr]           ;restore stack
  ;must be a non rip-relative jump
  mov          eax, @f
  jmp          eax
@@:

  ;AS WE WERE!