bioscall.asm

.intel_syntax noprefix
# GDT:
# 0x00 NULL
# 0x10 32 bit code
# 0x18 32 bit data
# 0x20 16 bit code ; 64kb limit
# 0x28 16 bit data ; 64kb limit

.extern BootDrive
.extern halt

.code32
# void do_bios_call_13(u16 sector=si)
do_bios_call_13:


  jmp 0x20:.real

.real:
.code16


  mov ax, 0x28
  mov ds, ax
  mov es, ax
  #mov fs, ax
  #mov gs, ax
  mov ss, ax
  
  #mov eax, cr0
  #and eax, ~2#~CRO_PE
  xor eax, eax
  mov cr0, eax


# AH = 02h
# AL = number of sectors to read (must be nonzero)
# CH = low eight bits of cylinder number
# CL = sector number 1-63 (bits 0-5)
# high two bits of cylinder (bits 6-7, hard disk only)
# DH = head number
# DL = drive number (bit 7 set for hard disk)
# ES:BX -> data buffer

  mov ax, 0x0201
  xor ch, ch

  mov cl, [esp-4]
  mov bx, 0x7c00 + 0x200 + 0x200

  xor dh, dh
  mov dl, [BootDrive]
  sti
  int 0x13
  cli
  jc halt
.did_call:
  jmp 0x10:.protected
.protected:
.code32

  #mov eax, cr0
  #or  eax, 2#CRO_PE
  mov eax, 1
  mov cr0, eax

  mov ax, 0x18
  mov ds, ax
  mov es, ax
  #mov fs, ax
  #mov gs, ax
  mov ss, ax
  ret