EXE File Header Layout

  Offset Size Contents
  ▀▀▀▀▀▀ ▀▀▀▀ ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
   +0      2  wSignature   5a4dH .EXE file signature ('MZ')
   +2      2  wPartPage    length of partial page at end (generally ignored)
   +4      2  wPageCnt     length of image in 512-byte pages, incl. header
   +6      2  wReloCnt     number of items in relocation table
   +8      2  wHdrSize     size of header in 16-byte paragraphs
  +0aH     2  wMinAlloc    minimum RAM needed above end of prog (paragraphs)
  +0cH     2  wMaxAlloc    maximum RAM needed above end of prog (paragraphs)
  +0eH     2  wInitSS      segment offset of stack segment (for setting SS)
  +10H     2  wInitSP      value for SP register when started
  +12H     2  wChkSum      file checksum (negative sum of all words in file)
  +14H     2  wInitIP      value for IP register when started
  +16H     2  wInitCS      segment offset of code segment (for setting CS)
  +18H     2  wTablOff     file-offset of first relo item (often 001cH)
  +1aH     2  wOverlayNo   overlay number (0 for base module)
          28               size of formatted portion of EXE header

  +?      4*? alReloTbl    variable-length relocation table
  +?       ?  abFiller     filler to paragraph boundary
  +?       ?  abImage      start of program image

 Since an EXE file may be loaded on any segment, all absolute segment
 references (such as FAR CALLs, long address pointers, and references such
 as MOV AX,data_seg) must be adjusted to work for the memory location in
 which they are loaded.  Here are the steps used by the DOS program loader
 (fn 4bH ) to load an EXE file:

       1. Create a PSP via DOS Fn 26H.

       2. Read 1cH bytes of the EXE file (the formatted portion of the EXE
          header) into a local memory area.

       3. Determine the load module size:

       4. Determine file offset of load module = (wHdrSize * 16)

       5. Select a segment addr, START_SEG, for loading (usually PSP+10H)

       6. Allocate enough memory to hold the load module, honoring the
          settings in wMaxMem  and wMinMem.

       7. Read the load module into memory starting at START_SEG:0000

       8. LSEEK to the start of the relocation table (wTablOff)

       9. For each relocation item (wReloCnt):
          a. read the item as two 16-bit words (I_OFF,I_SEG)
          b. add RELO_SEG=START_SEG+I_SEG      (find the address of
             relocation ref)
          c. fetch the word at RELO_SEG:I_OFF  (read current value)
          d. add START_SEG to that word        (perform the segment fixup)
          e. store the sum back at its original address (RELO_SEG:I_OFF)

      10. Allocate memory for the program according to wMaxMem and wMinMem

      11. Initialize registers and execute the program:
          a. ES = DS = PSP
          b. Set AX to indicate the validity of drive IDs in command line
          c. SS = START_SEG+ReloSS, SP = ExeSP
          d. CS = START_SEG+ReloCS, IP = ExeIP

    Note: Recent additions to the EXE format, specifically the CodeView,
          OS/2, and Windows versions of the EXE file contain additional
          information embedded in the executable file.  Sorry, but these
          additions are not covered here.

See Also: Program Startup & Exit
          Process Control Functions
          PSP: Program Segment Prefix
          DOS Environment