technically it starts at 0000h, but it usually contains a jump instruction to a different block, considered the true start of the code. the reason to to skip past reserved entry points for interrupt routines. even if no interrupts are used, and they must be enabled by code to be used, most programmers still maintain the interrupt vector block by jumping around it.

why?

basically we distrust the micros we program. :)

while we may think and know we've not enabled interrupts, the microcontroller doesn't always agree. voodoo and murphy's law has more to do with programming than logic and software theory.

duh