mahmood Elnasser wrote
"I removed the A51 startup file from project and fortunately, everything works fine."

is this the same mahmood Elnasser who previously wrote:
"any old crap software works especially with a c compiler like Keil."

;-)

If you don't explicitly include it in your project, Keil will automatically use its default startup code - just like if you don't provide your own putchar it automatically uses the default.