top shows that multithreaded program uses 19623.14% CPU

[Yuri]

I ran then program with 1500 threads and in top it looked the this:

  PID USERNAME    THR PRI NICE   SIZE    RES STATE   C   TIME    CPU COMMAND
20382 yuri       1500  99    0   641M   462M umtxn   0   0:00 19623.14% 
quicksort
<skipped further>

Where can I read how CPU column is calculated?

Some time ago I saw another weird fenomenon when CPU column: 5% CPU load 
for the pocess that just cycles in CPU.

Yuri

7.2-STABLE, i7-920 (4 CPUs)