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Differences between Privilege Modes and Non-Privilege Mode ?

Rui

Hi everyone ,

I'm currently using a Cortex-A9 processor (NXP Freescale i.MX6S).

My project is to develop a simple OS, but I met a problem:   

When I am trying to control some peripherals (such as UART and GPIO) directly under ARM USER MODE, the program stopped and jumped into some other exceptional handlers.

However, if I control these peripherals under ARM SYSTEM MODE, everything is fine.

I know that there are 7 ARM modes, USER mode is the non-privilege mode and the others are privilege modes. USER mode is redistricted in accessing some resources, but I didn't find out any document explain these restrictions explicitly.

Could you give me some advises? Very Appreciate it !!!

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