Default values for registers

Hi,
I'm pretty new to ASM. There is a sample source code for a boot
loader:

**********************************
org 0x7C00
start:

cli
;define stack
mov ax, 0x9000
mov ss, ax
mov sp, 0
sti

mov [bootdriv], dl
.....
***********************************

whereas bootdriv is a defined as a variable byte.

Is DX (resp. all registers) initiated with 0 for sure? Is there a
default value defined for the registers?

THX, Langi

p.s.: I use TASM
.

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