Int 26H

1 and higher

ABSOLUTE DISK WRITE (except partitions > 32M)

AL = drive number (00h = A:, 01h = B:, etc)
CX = number of sectors to write (not FFFFh)
DX = starting logical sector number (0000h - highest sector on drive)
DS:BX -> data to write

 CF clear if successful
CF set on error
    AH = status (see #02547)
    AL = error code (same as passed to INT 24 in DI)
    AX = 0207h if more than 64K sectors on drive -- use new-style call
may destroy all other registers except segment registers

   original flags are left on stack, and must be popped by caller
this call bypasses the DOS filesystem, though DOS 5+ invalidates any
  disk buffers referencing sectors which are written with this call
examination of CPWIN386.CPL indicates that if this call fails with
  error 0408h on an old-style (<32M) call, one should retry the
  call with the high bit of the drive number in AL set
Novell DOS 7 decides whether the old-style or new-style (>32M) version
  of INT 26 must be used solely on the basis of the partition's size,
  thus forcing use of the new-style call even for data in the first
  32M of the partition
Windows98 will display an error message and deliberately hang the
  system on attempted write to any hard disk if neither bit 7 of the
  Extended Drive Info byte nor bit 6 of "DOS_FLAG" (List-of-Lists+60h)
  is set
Although all registers except segment registers may be destroyed
 some software depends on some of the registers being preserved.
 For example some Flash disk drivers requires that DX is not trashed.
 DR-DOS 7.03 takes care of this.
BUGS:	DOS 3.1 through 3.3 set the word at ES:[BP+1Eh] to FFFFh if AL is an
  invalid drive number
DR DOS 3.41 will return with a jump instead of RETF, leaving the
  wrong number of bytes on the stack; use the huge-partition version
  (INT 26/CX=FFFFh) for all partition sizes under DR DOS 3.41
DR DOS 6.0 original releases 05/1991 & 08/1991 reported wrong error
  codes for "drive not ready" and "write protect". This was fixed
  with the DR DOS BDOS patch "PAT321" (1992/02/19, XDIR /C: 947Bh)
  and later "full" rebuilds (see INT21/AX=4452h for details).

INT 13/AH=03h,INT 25,INT 26/CX=FFFFh,INT 21/AX=7305h,INT 21/AH=91h"PTS"

Text based on Ralf Brown Interrupt List Release 61