As a core component of embedded systems, the presence of the ARM processor ensures the stability and performance of the system. To better ensure the normal operation of embedded systems, the ARM processor provides seven operating modes, each with its specific application scenarios and privilege levels.
1. User Mode (USR)
Normal program execution mode.
Cannot directly switch to other modes.
Lowest privilege, cannot access protected system resources.
2. System Mode (SYS)
Privileged tasks running the operating system.
Similar to user mode, but has the privilege to switch directly to other modes.
Commonly used for operating system kernel tasks.
3. Fast Interrupt Mode (FIQ)
Supports high-speed data transfer and channel processing.
Enters this mode when responding to FIQ exceptions.
Higher priority than interrupt mode (IRQ).
4. Interrupt Mode (IRQ)
Used for general interrupt handling.
Enters this mode when responding to IRQ exceptions.
Lower priority than fast interrupt mode (FIQ).
5. Supervisor Mode (SVC)
Operating system protection mode.
Enters this mode during system reset and software interrupt responses.
Triggered by executing the software interrupt SWI command via system calls.
6. Abort Mode (ABT)
Supports virtual memory and/or memory protection.
Not very useful in ARM7TDMI.
Related to memory access exceptions.
7. Undefined Mode (UND)
Supports software emulation of hardware coprocessors.
Enters this mode when responding to undefined instruction exceptions.
Used for handling unknown or unsupported instructions.
Privilege Levels
System mode: has the highest privileges and can perform all operations.
Exception modes (FIQ, IRQ, SVC, ABT, UND): have higher privileges than user mode but lower than system mode.
User mode: lowest privileges, cannot directly access protected system resources.
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