As has already been explained to you, the errors & warnings you see are because you have included the C51 startup code, startup.a51 in your project.

startup.a51 is for C51 projects - you don't need it for an assembler project.
See: http://www.keil.com/support/m...tartup.htm

Specifically:

*** WARNING L5: CODE SPACE MEMORY OVERLAP 
FROM: 0000H 
TO: 0002H

Both your code and the startup.a51 place code place something at CODE address 0000H - the Reset Vector address.
Solution: remove startup.a51 from your project.

*** WARNING L1: UNRESOLVED EXTERNAL SYMBOL 
SYMBOL: ?C_START 
MODULE: STARTUP.obj (?C_STARTUP)

The last thing that startup.a51 is to jump to a label called "?C_START".
This label would be provided by the C51 compiler; but you're not using C51, so it's missing - or "Unresolved"
Solution: remove startup.a51 from your project.

*** WARNING L2: REFERENCE MADE TO UNRESOLVED EXTERNAL 
SYMBOL: ?C_START 
MODULE: STARTUP.obj (?C_STARTUP) 
ADDRESS: 080AH

This is really just a restatement of the previous message.
Solution: remove startup.a51 from your project.