"To be honest, I have no idea what's happening there.. I'm not quite sure how to force the chip to jump to 0800h (for all I know, that segment at 0000h may do that)..."

It is a specific limitation of the free Keil evaluation software that it always locates code starting at 0x0800 (or whatever) - this will be documented somewhere in the "readme" files, or see the Keil site, http://www.keil.com/demo/limits.htm, or try searching this forum.

Address zero is the "Reset Vector" for the 8051; ie, immediately after a reset, the 8051 starts executing code at adress zero - therefore, the free Keil evaluation software places a jump here to address 0x0800 (or whatever)