I was playing around with some code to work under Linux AND DOS originally written in FASM. FASM seemed to want to dictate using 32bit jumps even for the 16bit code so I tried porting it to NASM 2.01 to see how it would compile. NASM did the job 'as wanted' and surprisingly without too many changes :) Changing a few '=' to equates and not being able to use '@@' as a reusable label was about it. Nice one NASM.
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;Example of program that runs under ;;
;;Linux and DOS (sort of) ;;
;;Note :- as AX is not guaranteed to ;;
;;be 0 on entry of .com executable ;;
;;AFAIK, use at own risk. ;;
;;19/01/2008 ;;
;;Adapted for NASM V2.01 ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
use16
org 08048000h ;Seems to be the norm for ELF ?
ComOff equ $$-100h ;Org 100h displacement
;----------------------------------------------------------
ehdr: ;Elf header
db 0x7F, 'ELF' ;Elf ident, might jump to Com if flag is ok (7F=JG)
db 1 ;32 bit architecture
db 1 ;little Endian
db 1 ;ELF version
Pad9: ;Start of padding, 9 bytes
db 1 ;Padding, 9 bytes. Keep it as add [bx+di],ax
;rather than adding [bx+si] if 0
;the less that is changed,
;the less to go wrong, perhaps...
inc sp ;Undo what was done by 7fELF111, don't worry about SI
sub [bx+di],ax ;ax seems to be 0 on entry but, just in case
sub [bx+di],ax ;if we got this far we should be ok
jmp Com
times 9-($-Pad9) db 0 ;Rest of the padding, if any!
dw 2 ;Executable
dw 3 ;Intel 386+
dd 1 ;Version
dd start ;Program start
dd phdr - $$ ;Program header offset
dd 0 ;Section header offset
dd 0 ;Flags
dw ehdrsize ;Elf header size
dw phdrsize ;Program header size
dw 1 ;Number of program header entries
dw 0 ;Section header size
dw 0 ;Number of section header entries
dw 0 ;String table (undefined)
ehdrsize equ $-ehdr
;----------------------------------------------------------
times 71-ehdrsize db 0 ;Could use this wasted space for something...
Com: ;Where '7fE' JG +47h jumps too
mov eax,'DOS'+0d000000h ;Change 'Linux',0ah to 'DOS',0dh,0ah
mov bx, 0ah ;
mov [OrNot-ComOff],eax ;
mov [OrNot+4-ComOff],bx ;Messy offsets !
mov si,msg-ComOff
call dword OutputIt ;Print message
mov ax,4c00h ;Exit
int 21h
int 20h ;Just in case
;----------------------------------------------------------
align 16 ;Keep Linux happy :)
phdr: ;Elf program header
dd 1 ;Load segment
dd 0 ;offset
dd $$ ;Virtual address
dd $$ ;Physical address
dd filesize ;File image size
dd filesize ;Memory image size
dd 5 ;Executable / readable x_r
dd 1000h ;Alignment
phdrsize equ $-phdr
;----------------------------------------------------------
use32
start:
mov eax,4 ;Write msg to console
mov ebx,1 ;
mov ecx,msg ;
mov edx,msg_size ;
int 80h ;
mov eax,1 ;Exit
xor ebx,ebx ;
int 80h ;
;----------------------------------------------------------
msg db 'This program has been run from some sort of '
OrNot:
db 'Linux',0ah
msg_size equ $-msg
;----------------------------------------------------------
use16
OutputIt: ;Print text string from si
mov ah,0eh ;could use int 21h and ah=2 instead with dl
mov bx,05h ;probably don't need this (graphics mode color)
@1: lodsb
or al,al
jz @2 ;End of string
int 10h
jmp @1
@2: ret
filesize equ $-$$
