I once wrote an assembler but that was for a mini for which the 'provided' (and only available) assembler was buggy like .... and the supplier did not fix it, while at the same time insisting that the source code was proprietary.

I have coded various minis and micros with the knowledge that instead of xxx yyy,zzz you had to insert the hex code because that particular combination made the assembler burp.

Now, the '51 is a "commodity architecture" and thus bug free assemblers are aboundant.

I can say with certainty, that if I were to write an assembler for the '51 it would not be bug free (initially) and there is no worse debugging than the one where you do not know if it is your code or the tool.

I know of 3 assemblers for the '51 with "no known bugs": Keil, Ad2500 and MetaLink. The last one is free. So, since there is no advantage whatsoever gained, I would suggest you use a commercial product.

I 'smell' that you may want to write your own assembler becaude your downloader can not handle non-sequential .hex files. If that is the case, there are tools that make .hex files sequential and I do believe the MetaLink pop sequential .hes.

erik