Hi Maarten,
       Forget what I wrote about porting.I've asked my lab's 8051 developers.Eventually I realized that what the question is converting the assembly code to binary code,for the assembly process can't pass through.Since it is so,the compile seems trivial.
      In addition, you mentioned above is just what I want to express.The modified 8051 just what you understand.It is just added a extern flash, some new instructions to control the flash and a few registers in SFR to deposit instructions, no stack changed,just so simple.
       I guess I must modify the source of sdas8051 and build a new assmbler to support the new 8051.The new question comes, what should I do if I must do something about modifying the source.
      Hoping the language barrier has been eliminated.
ps:I am a newbie about modifying assembler.