by using KEIL;
open ur asm file, and there is an option for generation of hex file. check that...
and specify the crystal frequency.
when u build ur programe, its hex file will be automatically generated in that folder
FREE:
you might use asem51.
and if you like and IDE version , I recomend you to use MIDE .. its IDE for Asem51 and SDCC.
you can download it for free here:
If you like the commercial one, you can choose between IAR Workbench and KEIL uVision (two the best compiler) for me.
but, my self use KEIL.. it's all that I need on 8051 compiler.
first rebuild your project, if error are not found then you are ready to create the hex file.for hex file, go to option for targets>>output>where u will find check box for creating hex file..........remember to select the correct device and oscillator