/*
 *  GRUB  --  GRand Unified Bootloader
 *  Copyright (C) 1999,2000,2001,2002,2003,2005,2006,2007,2008 Free Software Foundation, Inc.
 *
 *  GRUB is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  GRUB is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
 */


/*
 * Note: These functions defined in this file may be called from C.
 *       Be careful of that you must not modify some registers. Quote
 *       from gcc-2.95.2/gcc/config/i386/i386.h:

   1 for registers not available across function calls.
   These must include the FIXED_REGISTERS and also any
   registers that can be used without being saved.
   The latter must include the registers where values are returned
   and the register where structure-value addresses are passed.
   Aside from that, you can include as many other registers as you like.

  ax,dx,cx,bx,si,di,bp,sp,st,st1,st2,st3,st4,st5,st6,st7,arg
{  1, 1, 1, 0, 0, 0, 0, 1, 1,  1,  1,  1,  1,  1,  1,  1,  1 }
 */

/*
 * Note: GRUB is compiled with the options -mrtd and -mregparm=3.
 *       So the first three arguments are passed in %eax, %edx, and %ecx,
 *       respectively, and if a function has a fixed number of arguments
 *       and the number if greater than three, the function must return
 *       with "ret $N" where N is ((the number of arguments) - 3) * 4.
 */

#include <grub/symbol.h>

.intel_syntax noprefix

#define GRUB_DRIVEMAP_INT13H_OFFSET(x) ((x) - grub_drivemap_int13_handler_base)

/* Copy starts here.  When deployed, this label must be segment-aligned.  */
VARIABLE(grub_drivemap_int13_handler_base)

/* Far pointer to the old handler.  Stored as a CS:IP in the style of real-mode
   IVT entries (thus PI:SC in mem).  */
VARIABLE(grub_drivemap_int13_oldhandler)
  .long 0

/* Drivemap module bundle - INT 13h handler - BIOS HD map */
/* We need to use relative addressing, and with CS to top it all, since we
   must make as few changes to the registers as possible.  IP-relative
   addressing like on amd64 would make life way easier here. */
.code16
FUNCTION(grub_drivemap_int13_handler)
  push bp
  mov bp, sp
  push ax  /* We'll need it later to determine the used BIOS function.  */

  /* Map the drive number (always in DL?).  */
  push ax
  push bx
  push si
  mov bx, offset GRUB_DRIVEMAP_INT13H_OFFSET(grub_drivemap_int13_mapstart)
  xor si, si
1:mov ax, word ptr cs:[bx + si]
  cmp al, ah
  jz 3f /* DRV=DST => map end - drive not remapped, leave DL as-is.  */
  cmp al, dl
  jz 2f /* Found - drive remapped, modify DL.  */
  add si, 2
  jmp 1b /* Not found, but more remaining, loop.  */
2:mov dl, ah
3:pop si
  pop bx
  xchg ax, [bp - 4] /* Recover the old AX and save the map entry for later.  */
  
  push bp
  /* Simulate interrupt call: push flags and do a far call in order to set
     the stack the way the old handler expects it so that its iret works.  */
  push [bp + 6]
  mov bp, word ptr [bp]  /* Restore the caller BP (is this needed and/or sensible?).  */
  call dword ptr cs:[GRUB_DRIVEMAP_INT13H_OFFSET(grub_drivemap_int13_oldhandler)]
  pop bp /* The pushed flags were removed by iret.  */
  /* Set the saved flags to what the int13h handler returned.  */
  push ax
  pushf
  pop ax
  mov word ptr [bp + 6], ax
  pop ax

  /* Reverse map any returned drive number if the data returned includes it.  
     The only func that does this seems to be origAH = 0x08, but many BIOS
     refs say retDL = # of drives connected.  However, the GRUB Legacy code
     treats this as the _drive number_ and "undoes" the remapping.  Thus,
     this section has been disabled for testing if it's required.  */
#  cmp byte ptr [bp - 1], 0x08 /* Caller's AH.  */
#  jne 4f
#  xchg ax, [bp - 4] /* Map entry used.  */
#  cmp al, ah  /* DRV=DST => drive not remapped.  */
#  je 4f
#  mov dl, ah  /* Undo remap.  */

4:mov sp, bp
  pop bp
  iret
/* This label MUST be at the end of the copied block, since the installer code
   reserves additional space for mappings at runtime and copies them over it.  */
.align 2
VARIABLE(grub_drivemap_int13_mapstart)
/* Copy stops here.  */
.code32
VARIABLE(grub_drivemap_int13_size)
  .word GRUB_DRIVEMAP_INT13H_OFFSET(grub_drivemap_int13_size)