Just to be clear on the terminology here:

uVision is not a Compiler; it's not an Assembler; it's just an interface which (hopefully) makes it easier to bring together Compiler, Assembler, Editor, Linker, Debugger, Project Manager, etc, etc.

C51 is Keil's C Compiler;
A51 is Keil's Assembler.

The assembler, linker, compiler, etc can be used independently of uVision.

For examples, read the Keil manuals & look on their support pages at www.keil.com