STM32 (ARM Cortex-M3) NVIC

Does anyone have any information or resources regarding the NVIC (Nested Vectored Interrupt Controller) in the STM32? I've gone through the Keil USART IRQ example, but it simply configures it on one line and doesn't elaborate:

NVIC->Enable[1] |= (1 << (USART1_IRQChannel & 0x1F)); // enable interrupt

Keil STM32 USART (interrupt mode) Example:
keil.com/.../359.asp

There are only 3 pages in the STM32 Reference Manual covering the NVIC (chapter 6), none of which really explain how it works:
STM32 Reference Manual (PDF, 8MB)
www.st.com/.../13902.pdf

Thanks...

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