http://www.nondot.org/sabre/os/files/Executables/EXE.txt
offset length description comments
----------------------------------------------------------------------
0 word exe file signature usually 4d5a
2 word length of last used sector in file modulo 512
4 word size of file, incl. header in 512-pages
6 word number of relocation table items
8 word size of header in 16-byte paragraphs
a word min. paragraphs needed above program in 16-byte paragraphs
c word max. paragraphs needen above program in 16-byte paragraphs
e word displacement of stack segment in module rel. to start of prog.
10 word contents of SP reg. at entry
12 word checksum 2's complement
14 word contents of IP reg. at entry
16 word displacement of code module rel. to start of prog.
18 word offset to first relocation item in file rel. to start of prog.
1a word overlay number 0 for resident prog.
1c varies variable RESERVED place
varies varies relocation table
varies varies variable RESERVED place
varies varies program and data space
varies varies stack segment
The relocation table is a set of far pointers (eg: 1234:5678h) and it appears
you just add the relocation factor to the value at that address. The relocation
factor is the start segment of where the program is loaded.
Example:
------------------------------------------------
code segment
start:
mov ax,seg _myseg
code ends
_myseg segment
_myseg ends
end start
-------------------------------------------------
Start Stop Length Name Class
00000H 00002H 00003H CODE
00010H 00010H 00000H _MYSEG
-------------------------------------------------
Note that _MYSEG is exactly one segment above CODE.
Generated output is B8 01 00; which is "mov ax,0001"
The fixup table for this file has a single entry, 0000:0001. Thus if the start
of the program begins at segment 3562 then the "mov ax,0001" gets converted to
"mov ax,3563".