Nmap Development: At what point NMAP decides if host is UP ?

[micro dev <microdev1 () yahoo com>]

Hi,
I use TCP SYN scan to scan remote hosts and also use OS fingerprinting.
I use command something like that - 
 
nmap -sS -O -p <port list> <ip address>
I also depend upon nmap response to find if host is UP or DOWN.
 
So I am just qurious to know how NMAP will decide if host is UP if command listed above is used. 
Does it use ICMP at all in this case ?
 
If NMAP uses SYN packets to find if host is UP, then it uses any default port or uses list of ports specified in the 
command. 
 
Thanks
 

                
---------------------------------
Do you Yahoo!?
Read only the mail you want - Yahoo! Mail SpamGuard.