On the Internet you can find a lot of examples, assemblers, simulators etc. Below you will find a 8051-simulator that is working VERY well. Working with the 515/535 you will be missing Port 4, Port 5 and Port 6 in this simulator, but you can emulate ports on Port 3.

8051-Simulator for DOS

Below you will find a 80C535-simulator. If you want to test the 515/535-mainboard you will have to switch External code ram on from 8000h to FFFFh.

80C535-simulator