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; boot12.asm  FAT12 bootstrap for real mode image or loader
; Version 1.0, Jul 5, 1999
; Sample code
; by John S. Fine  [email protected]
; I do not place any restrictions on your use of this source code
; I do not provide any warranty of the correctness of this source code
;_____________________________________________________________________________
;
; Documentation:
;
; I)    BASIC features
; II)   Compiling and installing
; III)  Detailed features and limits
; IV)   Customization
;_____________________________________________________________________________
;
; I)    BASIC features
;
;    This boot sector will load and start a real mode image from a file in the
; root directory of a FAT12 formatted floppy or partition.
;
;    Inputs:
; DL = drive number
;
;    Outputs:
; The boot record is left in memory at 7C00 and the drive number is patched
; into the boot record at 7C24.
; SS = DS = 0
; BP = 7C00
;_____________________________________________________________________________
;
; II)   Compiling and installing
;
;  To compile, use NASM
;
;  nasm boot12.asm -o boot12.bin
;
;  Then you must copy the first three bytes of BOOT12.BIN to the first three
;  bytes of the volume and copy bytes 0x3E through 0x1FF of BOOT12.BIN to
;  bytes 0x3E through 0x1FF of the volume.  Bytes 0x3 through 0x3D of the
;  volume should be set by a FAT12 format program and should not be modified
;  when copying boot12.bin to the volume.
;
;  If you use my PARTCOPY program to install BOOT12.BIN on A:, the
;  commands are:
;
;  partcopy boot12.bin 0 3 -f0
;  partcopy boot12.bin 3e 1c2 -f0 3e
;
;  PARTCOPY can also install to a partition on a hard drive.  Please read
;  partcopy documentation and use it carefully.  Careless use could overwrite
;  important parts of your hard drive.
;
;  You can find PARTCOPY and links to NASM on my web page at
;  http://www.erols.com/johnfine/
;_____________________________________________________________________________
;
; III)  Detailed features and limits
;
;   Most of the limits are stable characteristics of the volume.  If you are
; using boot12 in a personal project, you should check the limits before
; installing boot12.  If you are using boot12 in a project for general
; distribution, you should include an installation program which checks the
; limits automatically.
;
; CPU:  Supports any 8088+ CPU.
;
; Volume format:  Supports only FAT12.
;
; Sector size:  Supports only 512 bytes per sector.
;
; Drive/Partition:  Supports whole drive or any partition of any drive number
; supported by INT 13h.
;
; Diskette parameter table:  This code does not patch the diskette parameter
; table.  If you boot this code from a diskette that has more sectors per
; track than the default initialized by the BIOS then the failure to patch
; that table may be a problem.  Because this code splits at track boundaries
; a diskette with fewer sectors per track should not be a problem.
;
; File position:  The file name may be anywhere in the root directory and the
; file may be any collection of clusters on the volume.  There are no
; contiguity requirements.  (But see track limit).
;
; Track boundaries:  Transfers are split on track boundaries.  Many BIOS's
; require that the caller split floppy transfers on track boundaries.
;
; 64Kb boundaries:  Transfers are split on 64Kb boundaries.  Many BIOS's
; require that the caller split floppy transfers on track boundaries.
;
; Cluster boundaries:  Transfers are merged across cluster boundaries whenever
; possible.  On some systems, this significantly reduces load time.
;
; Cluster 2 limit:  Cluster 2 must start before sector 65536 of the volume.
; This is very likely because only the reserved sectors (usually 1) and
; the FAT's (two of up to 12 sectors each) and the root directory (usually
; either 15 or 32 sectors) precede cluster 2.
;
; Track limit:  The entire image file must reside before track 32768 of the
; entire volume.  This is true on most media up to 1GB in size.  If it is a
; problem it is easy to fix (see boot16.asm).  I didn't expect many people
; to put FAT12 partitions beyond the first GB of a large hard drive.
;
; Memory boundaries:  The FAT, Root directory, and Image must all be loaded
; starting at addresses that are multiples of 512 bytes (32 paragraphs).
;
; Memory use:  The FAT and Root directory must each fit entirely in the
; first 64Kb of RAM.  They may overlap.
;
; Root directory size:  As released, it supports up to 928 entries in the
; root directory.  If ROOT_SEG were changed to 0x7E0 it would support up
; to 1040.  Most FAT12 volumes have either 240 or 512 root directory
; entries.
;_____________________________________________________________________________
;
; IV)   Customization
;
;   The memory usage can be customized by changing the _SEG variables (see
; directly below).
;
;   The file name to be loaded and the message displayed in case of error
; may be customized (see end of this file).
;
;   The ouput values may be customized.  For example, many loaders expect the
; bootsector to leave the drive number in DL.  You could add "mov dl,[drive]"
; at the label "eof:".
;
;   Some limits (like maximum track) may be removed.  See boot16.asm for
; comparison.
;
;   Change whatever else you like.  The above are just likely possibilities.
;_____________________________________________________________________________


; Change the _SEG values to customize memory use during the boot.
; When planning memory use, remember:
;
; *)  Each of ROOT_SEG, FAT_SEG, and IMAGE_SEG must be divisible by 0x20
;
; *)  None of ROOT, FAT or IMAGE should overlap the boot code itself, or
;     its stack.  That means: avoid paragraphs 0x7B0 to 0x7DF.
;
; *)  The FAT area must not overlap the IMAGE area.  Either may overlap
;     the ROOT area;  But, if they do then the root will not remain in
;     memory for possible reuse by the next stage.
;
; *)  The FAT area and the root area must each fit within the first 64Kb
;     excluding BIOS area (paragraphs 0x60 to 0xFFF).
;
; *)  A FAT12 FAT can be up to 6Kb (0x180 paragraphs).
;
; *)  A FAT12 root directory is typically either 0x1E0 or 0x400 paragraphs
;     long, but larger sizes are possible.
;
; *)  The code will be two bytes shorter when FAT_SEG is 0x800 than when it
;     is another value.  (If you reach the point of caring about two bytes).
;
%define ROOT_SEG   0x60
%define FAT_SEG      0x800
%define IMAGE_SEG   0x1000

%if ROOT_SEG & 31
  %error "ROOT_SEG must be divisible by 0x20"
%endif
%if ROOT_SEG > 0xC00
  %error "Root directory must fit within first 64Kb"
%endif
%if FAT_SEG & 31
  %error "FAT_SEG must be divisible by 0x20"
%endif
%if FAT_SEG > 0xE80
  %error "FAT must fit within first 64Kb"
%endif
%if IMAGE_SEG & 31
  %error "IMAGE_SEG must be divisible by 0x20"
%endif

; The following %define directives declare the parts of the FAT12 "DOS BOOT
; RECORD" that are used by this code, based on BP being set to 7C00.
;
%define   sc_p_clu   bp+0Dh      ;byte  Sectors per cluster
%define   sc_b4_fat   bp+0Eh      ;word  Sectors (in partition) before FAT
%define   fats      bp+10h      ;byte  Number of FATs
%define dir_ent      bp+11h      ;word  Number of root directory entries
%define   sc_p_fat   bp+16h      ;word  Sectors per FAT
%define sc_p_trk   bp+18h      ;word  Sectors per track
%define heads      bp+1Ah      ;word  Number of heads
%define sc_b4_prt   bp+1Ch      ;dword Sectors before partition
%define drive      bp+24h      ;byte  Drive number

   org   0x7C00

entry:
   jmp   short begin

; --------------------------------------------------
; data portion of the "DOS BOOT RECORD"
; ----------------------------------------------------------------------
brINT13Flag     DB      90H             ; 0002h - 0EH for INT13 AH=42 READ
brOEM           DB      'MSDOS5.0'      ; 0003h - OEM ID - Windows 95B
brBPS           DW      512             ; 000Bh - Bytes per sector
brSPC           DB      1               ; 000Dh - Sector per cluster
brSc_b4_fat   DW      1               ; 000Eh - Reserved sectors
brFATs          DB      2               ; 0010h - FAT copies
brRootEntries   DW      0E0H      ; 0011h - Root directory entries
brSectorCount   DW      2880      ; 0013h - Sectors in volume, < 32MB
brMedia         DB      240      ; 0015h - Media descriptor
brSPF           DW      9               ; 0016h - Sectors per FAT
brSc_p_trk   DW      18              ; 0018h - Sectors per head/track
brHPC           DW      2      ; 001Ah - Heads per cylinder
brSc_b4_prt   DD      0               ; 001Ch - Hidden sectors
brSectors       DD      0           ; 0020h - Total number of sectors
brDrive      DB      0               ; 0024h - Physical drive no.
      DB      0               ; 0025h - Reserved (FAT32)
      DB      29H             ; 0026h - Extended boot record sig (FAT32)
brSerialNum     DD      404418EAH       ; 0027h - Volume serial number
brLabel         DB      'Joels disk '   ; 002Bh - Volume label
brFSID          DB      'FAT12   '      ; 0036h - File System ID
;------------------------------------------------------------------------


begin:
   xor   ax, ax
   mov   ds, ax
   mov   ss, ax
   mov   sp, 0x7C00
   mov   bp, sp
   mov   [drive], dl   ;Drive number

   mov   al, [fats]   ;Number of FATs
   mul   word [sc_p_fat]   ; * Sectors per FAT
   add   ax, [sc_b4_fat]   ; + Sectors before FAT
            ;AX = Sector of Root directory

   mov   si, [dir_ent]   ;Max root directory entries
   mov   cl, 4
   dec   si
   shr   si, cl
   inc   si      ;SI = Length of root in sectors

   mov   di, ROOT_SEG/32   ;Buffer (paragraph / 32)
   call   read_16      ;Read root directory
   push   ax      ;Sector of cluster two
%define sc_clu2 bp-2      ;Later access to the word just pushed is via bp

   mov   dx, [dir_ent]   ;Number of directory entries
   push   ds
   pop   es
   mov   di, ROOT_SEG*16

search:
   dec   dx      ;Any more directory entries?
   js   error      ;No
   mov   si, filename   ;Name we are searching for
   mov   cx, 11      ;11 characters long
   lea   ax, [di+0x20]   ;Precompute next entry address
   push   ax
   repe cmpsb      ;Compare
   pop   di
   jnz   search      ;Repeat until match

   push word [di-6]   ;Starting cluster number

   mov   ax, [sc_b4_fat]   ;Sector number of FAT
   mov   si, [sc_p_fat]   ;Length of FAT
   mov   di, FAT_SEG/32   ;Buffer (paragraph / 32)
   call   read_16      ;Read FAT

next:
   pop   bx      ;Cluster number
   mov   si, bx      ;First cluster in this sequence
   mov   ax, bx      ;Last cluster in this sequence

.0:
   cmp   bx, 0xFF8   ;End of file?
   jae   .2      ; Yes
   inc   ax      ;Last cluster plus one in sequence

   ;Look in FAT for next cluster
   mov   di, bx      ;Cluster number
   rcr   bx, 1      ;1.5 byte entry per cluster
            ;bx = 0x8000 + cluster/2
            ;c-bit set for odd clusters

   mov   bx, [bx+di+FAT_SEG*16-0x8000]
   jnc   .1
   shr   bx, 1
   shr   bx, 1
   shr   bx, 1
   shr   bx, 1
.1:   and   bh, 0xF

   cmp   ax, bx      ;Is the next one contiguous?
   je   .0      ;Yes: look further ahead
.2:   sub   ax, si      ;How many contiguous in this sequence?
   jz   eof      ;None, must be done.

   push   bx      ;Save next (eof or discontiguous) cluster
   
   mov   bl, [sc_p_clu]   ;Sectors per cluster
   mov   bh, 0      ;  as a word
   mul   bx      ;Length of sequence in sectors
.3:   mov   di, IMAGE_SEG/32 ;Destination (paragraph / 32)
   add   [.3+1], ax    ;Precompute next destination
   xchg   ax, si      ;AX = starting cluster ;SI = length in sectors
   dec   ax
   dec   ax      ;Starting cluster minus two
   mul   bx      ; * sectors per cluster
   add   ax, [sc_clu2]   ; + sector number of cluster two
   adc   dl, dh      ;Allow 24-bit result

   call   read_32      ;Read it
   jmp   short next   ;Look for more

eof:
   jmp   IMAGE_SEG:0

error:   mov   si, errmsg   ;Same message for all detected errors
   mov   ax, 0xE0D   ;Start message with CR
   mov   bx, 7
.1:   int   10h
   lodsb
   test   al, al
   jnz   .1
   xor   ah, ah
   int   16h      ;Wait for a key
   int   19h      ;Try to reboot

read_16:
   xor   dx, dx

read_32:
;
; Input:
;    dx:ax = sector within partition
;    si    = sector count
;    di    = destination segment / 32
;
; The sector number is converted from a partition-relative to a whole-disk
; (LBN) value, and then converted to CHS form, and then the sectors are read
; into (di*32):0.
;
; Output:
;    dx:ax  updated (sector count added)
;    di     updated (sector count added)
;    si = 0
;    bp, ds preserved
;    bx, cx, es modified

.1:   push   dx         ;(high) relative sector
   push   ax         ;(low) relative sector

   add   ax, [sc_b4_prt]      ;Convert to LBN
   adc   dx, [sc_b4_prt+2]

   mov   bx, [sc_p_trk]      ;Sectors per track
   div   bx         ;AX = track ;DX = sector-1
   sub   bx, dx         ;Sectors remaining, this track
   cmp   bx, si         ;More than we want?
   jbe   .2         ;No
   mov   bx, si         ;Yes: Transfer just what we want
.2:   inc   dx         ;Sector number
   mov   cx, dx         ;CL = sector ;CH = 0
   cwd            ;(This supports up to 32767 tracks
   div   word [heads]      ;Track number / Number of heads
   mov   dh, dl         ;DH = head

   xchg   ch, al         ;CH = (low) cylinder  ;AL=0
   ror   ah, 1         ;rotate (high) cylinder
   ror   ah, 1
   add   cl, ah         ;CL = combine: sector, (high) cylinder

   sub   ax, di
   and   ax, byte 0x7F      ;AX = sectors to next 64Kb boundary
   jz   .3         ;On a 64Kb boundary already
   cmp   ax, bx         ;More than we want?
   jbe   .4         ;No
.3:   xchg   ax, bx         ;Yes: Transfer just what we want
.4:   push   ax         ;Save length
   mov   bx, di         ;Compute destination seg
   push   cx
   mov   cl, 5
   shl   bx, cl
   pop   cx
   mov   es, bx
   xor   bx, bx         ;ES:BX = address
   mov   dl, [drive]      ;DL = Drive number
   mov   ah, 2         ;AH = Read command
   int   13h         ;Do it
   jc   error
   pop   bx         ;Length
   pop   ax         ;(low) relative sector
   pop   dx         ;(high) relative sector
   add   ax, bx         ;Update relative sector
   adc   dl, dh
   add   di, bx         ;Update destination
   sub   si, bx         ;Update count
   jnz   .1         ;Read some more
   ret

errmsg   db   10,"Error Executing FAT12 bootsector",13
   db   10,"Press any key to reboot",13,10,0

size   equ   $ - entry
%if size+11+2 > 512
  %error "code is too large for boot sector"
%endif
   times   (512 - size - 11 - 2) db 0

filename db   "LOADER  BIN"      ;11 byte name
   db   0x55, 0xAA      ;2  byte boot signature