Decoded format

[radone]

I have YUV 420i video format

+------------------+
| |
| Y |
| |
+------------------+
| U |
+------------------+
| V |
+------------------+

and after
./xvid_encraw .... | ./xvid_decraw -f pnm -d
and the output is probably YUV (encapsulated in PNM):

+------------------+
| |
| Y |
| |
+------------------+
| U | V |
+------------------+
x x nothing x x
+- - - - - - - - - +


where 'nothing' is difference against original YUV420 data format and is not present.

Please, could anyone point me :
- what is the name of this format (or how to interpret this data)?
- exist any tool that could help me to display it?