;For the moment we will only support FAT12 and FAT16 loading.
virtual at 0
struc FATXX
{
      .jmp rb 3
   .oemname rb 8
       .BytesPerSec rw 1
   .SecPerClus rb 1
    .ReservedSec rw 1
   .NumFATs rb 1
       .RootEntry rw 1
     .TotalSec16 rw 1
    .Media rb 1
 .FATSz16 rw 1
       .SecPerTrack rw 1
   .NumHeads rw 1
      .HiddenSec rd 1
     .TotalSec32 rd 1
    .Drv_Num rb 1
       .Res1 rb 1
  .BootSig rb 1
       .VolID rd 1
 .VolLabel rb 11
     .FileSys rb 8
}
end virtual

;macro struct name  
;{ virtual at 0  
;   name name  
;   sizeof.#name = $ - name  
;   name equ sizeof.#name  
;   end virtual } 
;
;struct FATXX

;Valid Filesystems
Unknown equ -1
FAT12 equ 01
FAT16 equ 02

;expects esi = disk buffer.
FS_Check_Valid_FS:

;See we only handle FAT12 or FAT16 filesystems, this is going to be pretty easy.
;The correct way of determining the type of FAT, is by using the total cluster count.
;

 movzx eax, word [esi+FATXX.RootEntry]
       movzx ebx, word [esi+FATXX.BytesPerSec]
     shl eax, 5              ;mul RootEntry by 32
        mov ecx, ebx
        dec ebx         ;BytesPerSec - 1
    add eax, ebx    ;add them together
  xor edx, edx
        div ecx         ;divide eax / ecx
   ;eax = RootDirSectors
       push eax                ;save it
    cmp word [esi+FATXX.FATSz16], 0
     jne .not_supported
  movzx eax, word [esi+FATXX.FATSz16]
 movzx edx, byte [esi+FATXX.NumFATs]
 mul edx         ;eax * edx = edx:eax
    movzx edx, word [esi+FATXX.TotalSec16]
      movzx ebx, word [esi+FATXX.ReservedSec]
     sub edx, eax
        sub edx, ebx
        pop eax
     sub edx, eax    ;edx = Count of Clusters!
   
    cmp edx, 4085
       ja .FAT16
   ;if here we are FAT12
       mov dword [FS_Type], FAT12
  clc
 ret
 
.FAT16:
     cmp edx, 65525
      ja .not_supported
   mov dword [FS_Type], FAT16
  clc
 ret
 
.not_supported:
     mov dword [FS_Type], Unknown
        stc
 ret
 
;Data
align 4
FS_Type dd 0    

struc FATXX
{
        .jmp rb 3
        .oemname rb 8
        .BytesPerSec rw 1
        .SecPerClus rb 1
        .ReservedSec rw 1
        .NumFATs rb 1
        .RootEntry rw 1
        .TotalSec16 rw 1
        .Media rb 1
        .FATSz16 rw 1
        .SecPerTrack rw 1
        .NumHeads rw 1
        .HiddenSec rd 1
        .TotalSec32 rd 1
        .Drv_Num rb 1
        .Res1 rb 1
        .BootSig rb 1
        .VolID rd 1
        .VolLabel rb 11
        .FileSys rb 8
}
virtual at 0
  FATXX FATXX
end virtual