怪事,汇编成这样的结果:undefined symbol:start????

[wuoaimaomi]

我编了个文件aaaaaaa.asm:
title aaaaaaa
;*******************************
data segment ;define data segment
source_buffer db 40 dup ('a')
data ends
;*******************************
extra segment ;define extra segment
dest_buffer db 40 dup (?)
extra ends
;*******************************
code segment ;define code segment
main proc far ;main part of program
assume cs:code,ds:data,es:extra
start: ;start execution address
;set up stack for return
push ds ;save old data segment
sub ax,ax ;put zero in AX
push ax ;save it on stack
;set DS register to current data segment
mov ax,data ;data segment addr
mov ds,ax ;into DS register
;set ES register to current extra segment
mov ax,extra ;extra segment addr
mov es,ax ;into ES register
;MAIN PART OF PROGRAM GOES HERE
lea si,source_buffer ;put offset addr of source
;buffer in SI
lea di,dest_buffer ;put offset addr of dest
;buffer in DI
cld ;set DF flag to forward
mov cx,40 ;put count in CX
rep movsb ;move extire string
ret ;return to DOS

main endp ;end of main part of program
;********************************
code ends ;end of code segment
;********************************
end start ;end of assembly
我输入下面命令时(ml aaaaaaa.asm),系统提示undefined symbol:start
我百思不得其解,请高手指教.....

[hkbyest]

main proc far？

[evileagle]

title aaaaaaa
;*******************************
data segment ;define data segment
source_buffer db 40 dup ( 'a ')
data ends
;*******************************
extra segment ;define extra segment
dest_buffer db 40 dup (?)
extra ends
;*******************************
code segment ;define code segment
;main part of program
assume cs:code,ds:data,es:extra
start: ;start execution address
;set up stack for return
push ds ;save old data segment
sub ax,ax ;put zero in AX
push ax ;save it on stack
;set DS register to current data segment
mov ax,data ;data segment addr
mov ds,ax ;into DS register
;set ES register to current extra segment
mov ax,extra ;extra segment addr
mov es,ax ;into ES register
;MAIN PART OF PROGRAM GOES HERE
lea si,source_buffer ;put offset addr of source
;buffer in SI
lea di,dest_buffer ;put offset addr of dest
;buffer in DI
cld ;set DF flag to forward
mov cx,40 ;put count in CX
rep movsb ;move extire string
ret ;return to DOS

;********************************
code ends ;end of code segment
;********************************
end start ;end of assembly