You are asking about editor, but you should use assembler to assemble source written in asm. If are writing in C, you should use compiler first to compile c source to assembler. And then assemble it. There are a lot of IDE - Integrated Development Environments, whitch are editor, compiler, and asembler in a one place. Of course, it can be only editor and assembler, or even editor, assembler, compiler, simulator and etc.

If you work with IDE everything is in one place - you press button, and everyting is done - compiled, assembled and linked ;)

You can use some evaluation IDE - Keil, Raisonance, Pinnacle, or some free assemblers, or compilers. Check for them in link section ;)

Have fun