ReportingServicesService.exe only use 8 Cores

When I click on the ReportingServicesService.exe on the task manager, it has
only 8 CPUs among 24 CPUs as affinity.

The server environment is :

Windows 2008 Ent. 64 bit
SQL Server 2008 Std. 64 bit

When I choose all CPUs the process, it worked faster but the way is not everlasting.
After restarting, the affinity goes back to the default.

Is there any limitation in the stardard version?

Thanks in advance.