| amitophia |
05-20-2003 08:49 |
The algorithm is very simple.
It should be taken into consideration that DOS only addresses 1Mb of memory (20-bit addressing).
So the adress 041Ch in memory will have many SEG:OFFSET representations, for example:
0000:0041h, 0041,000Ch, 0020:021Ch adresses the same memory location
The addressed linear memory value is (SEG*10h)+OFF.
The EXE-module loads in memory just after PSP. So The EXE image (EXE without header) starts always at PSP:0100 or, as was said above, it's the same as (PSP+10h):0000 (this to variants points to the same physical memory location = (PSP*10h)+100h = (PSP+10h)*10h+0).
So let me name PSP+10h as BASE (e.g. BASE=PSP+10h).
MZ-header has two fields: InitCS and InitIP. When program loads CS is set to BASE+InitCS and IP is set to InitIP (but IP isn't meaningful for us).
This means that the phisycal address in EXE-file equals
The Algorithm
PhysOffset = (((SEG-BASE)*10h) + OFFSET) + Sizeof(MZheader)
Sizeof(MZheader) is calculated as appropriate field in the MZ-header (WORD at offset 8h) multiplied by 10h
BASE is the value that equals PSP+10h. PSP calculated when DOS loader loads the program into memory.
Additionally I'm postisng here some information about EXE-header. Mainly, you may need only the 9-th item. It shows how relocations are calculeted and how loader patches the program on load (that is why some offsets in memory cannot be found in EXE-file even it hasn't been packed - loader has patched it)
Code:
ExeHeaderRec
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:
size=((wPageCnt*512)-(wHdrSize*16))-wPartPage
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
|
