Look around here and on web for reference to the Metalink assembler.

You can also get free eval copies of software from the major 8051 tools vendors. Keil being one example. Try clicking on some of the links to the left to visit some of the tool vendors web sites to see what they offer. I know Keil even offers their C51 compiler on an evaluation for small projects (that have small code size).

Also consider that if you are at a university it is not uncommon for the university to have made an agreement with a tools vendor to make some of their
tools available to students through the university web site. You should check with your instructor or graduate advisor regarding this.

Michael Karas