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Help with GPIO on STM32F4 discovery

Question asked by super.mario1 on Apr 24, 2013
Latest reply on Apr 25, 2013 by super.mario1
i'm studying about GPIOs on STM32F4 Discovery but in the User Manual i don't understand very well how GPIOs work.
Can someone helps me to understand how it works?

GPIOs should be 9 ports of 16 bits that SoC uses to communicate with external Connectors and in general with peripheral on board, like LEDs.
Is it correct? and Has the SoC STM32F407VG of the STM32F4 Discovery 100 pinout?

A doubt about the PIN PA9, which can turn on the green LED but it can be used with Alternate Function as USART.
When i use this PIN for USART what happened to green LED?

Every GPIO has two weak resistance (pull-up/pull-down) and we can use a pin with open-drain or push-pull configuration. How do these two resistance work?

Thanks and sorry for my english.