There are several options, probably none is seamless.

You appear to be using asm and perhaps the simulator from that IDE, is that correct? Any other features, particular device support, integrated FLASH downloader or similar?

While every tool has some form of assembler, they tend to come in various flavours and you may not like what is integrated into the commercial IDEs. Perhaps the best way is to try the downloadable trials, if available. From the high $, Keil, IAR, Tasking and Raisonance have offerings. Low$/free you have to find yourself, as they mostly wore out in the last years - http://www.moravia-microsystems.com/mcu-8051-ide/ appears to be up and kicking, but I have no personal experience.

Simulators is another issue, they are not that common. http://home.arcor.de/jensaltmann/jsdl/jsim-e.htm is one of I know of, but that's been quite some time too.

Giving up requirement for simulator, it's often easier to go for a decent programmer's editor plus command-line assembler, rather than an IDE. Such setup usually lasts longer than a dedicated IDE/toolchain, and can be used for various MCU families. But of course it's about your needs and your decision.


JW