ARM CPU Registers Quiz and Flashcards: MSP and More

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Questions and Answers

What is the primary use of the Process Stack Pointer (PSP) in ARM documentation?

Used by the base-level application code (when not running an exception handler)

What is the purpose of the Link Register R14 (LR) in the Cortex-M3 processor?

To store the return address when a subroutine or function is called

What is the significance of the program counter (R15) in the Cortex-M3 processor?

Due to pipelining, the value read from the program counter is different from the executing instruction by 4

What is the requirement for the least significant bit (LSB) of the target address when branching in the Cortex-M3 processor?

The LSB of the target address should be set to 1 Signup and view all the answers

What are the two operation modes of the Cortex-M3 processor?

Thread mode and Handler mode Signup and view all the answers

What is the purpose of the two privilege levels in the Cortex-M3 processor?

To safeguard memory accesses to critical regions and provide a basic security model Signup and view all the answers

What is the typical number of interrupt inputs on a Cortex-M3 microcontroller?

16 or 32 Signup and view all the answers

What is the purpose of the Non Maskable Input (NMI) signal on a Cortex-M3 microcontroller?

To warn the processor of critical events such as low voltage levels Signup and view all the answers

What is the role of the NVIC in handling exceptions on a Cortex-M3 microcontroller?

To provide fault status registers to determine the cause of exceptions Signup and view all the answers

How does the Cortex-M3 microcontroller determine the starting address of an exception handler?

Using a vector table mechanism Signup and view all the answers

What is the characteristic of the vector table in a Cortex-M3 microcontroller?

It is relocatable Signup and view all the answers

When can the NMI signal be activated on a Cortex-M3 microcontroller?

Any time, even right after the core exits reset Signup and view all the answers

What is the impact on the processor mode when an exception occurs and the Control register bit 0 is zero?

The processor mode changes. Signup and view all the answers

What happens to the processor mode and access level when an exception occurs and the Control register bit 0 is one?

Both processor mode and access level change. Signup and view all the answers

What is the default stack arrangement after power-up in the Cortex-M3?

The MSP is used for both Thread mode and handler mode. Signup and view all the answers

What is the purpose of using the PSP in Thread mode when the Control register bit 1 is 1?

To prevent a stack error in a user application from damaging the stack used by the OS. Signup and view all the answers

What is the effect of using PSP in Thread mode on the automatic stacking and unstacking mechanism?

The automatic stacking and unstacking mechanism will use PSP. Signup and view all the answers

Is it possible to perform read/write operations directly to the MSP and PSP without confusion?

Yes, it is possible. Signup and view all the answers

What happens when the priority of the new interrupt is higher than the current level?

The interrupt handler of the new interrupt will override the current running task. Signup and view all the answers

What is the advantage of the Cortex-M3 processor's vectored interrupt support?

The starting address of the interrupt service routine (ISR) is located from a vector table in memory, eliminating the need for software to determine and branch to the starting address. Signup and view all the answers

Why can the priority levels of interrupts be changed by software during run time without risk?

Interrupts that are being serviced are blocked from further activation until the interrupt service routine is completed. Signup and view all the answers

What is the benefit of the Cortex-M3 processor's ability to dynamically change priority levels?

It allows the system to adapt to changing conditions and priorities, ensuring that high-priority interrupts are attended to promptly. Signup and view all the answers

What is the initial stack value that should be set for a stack memory range from 0x20007C00 to 0x20007FFF?

0x20008000 Signup and view all the answers

What is the broader goal of the Cortex-M3 processor's advanced features for interrupt handling?

To reduce interrupt latency, enabling the system to respond quickly to interrupts. Signup and view all the answers

Why do vector addresses in the vector table have their LSB set to 1 in the Cortex-M3?

To indicate that they are Thumb code. Signup and view all the answers

What is the reason for initializing the stack pointer after reset?

Because some exceptions (such as NMI) can happen right after reset, and the stack memory could be required for the handler of those exceptions. Signup and view all the answers

What is the address of the boot code in the example given?

0x100 Signup and view all the answers

What happens after the reset vector is fetched in the Cortex-M3?

The Cortex-M3 can start to execute the program from the reset vector address and begin normal operations. Signup and view all the answers

Study Notes

Interrupt Handling

When a new interrupt with a higher priority occurs, its interrupt handler will override the current running task.
Vectored interrupt support allows the Cortex-M3 processor to directly locate the starting address of the interrupt service routine (ISR) from a vector table in memory.
Dynamic priority changes are supported, allowing priority levels of interrupts to be changed by software during runtime.

Registers and Memory

Process Stack Pointer (PSP) is used by the base-level application code (when not running an exception handler).
Link Register R14 (LR) stores the return address when a subroutine or function is called.
Program Counter R15 (PC) is used to store the current instruction address; due to pipelining, the read value is different from the executing instruction by 4.
Writing to the program counter causes a branch, and the target address should have its LSB set to 1 to indicate Thumb state operations.

Operation Modes and Privilege Levels

The Cortex-M3 processor has two operation modes: Thread mode (normal program execution) and Handler mode (exception handler or system exception handler).
There are two privilege levels: privileged level and user level, which provide memory access control and a basic security model.

Interrupt Inputs and NMI

The number of interrupt inputs depends on the microcontroller design, typically 16 or 32.
Interrupts generated by peripherals (except System Tick Timer) are connected to interrupt input signals.
Non-Maskable Input (NMI) is a separate input signal that can be connected to a watchdog timer or voltage monitoring block.

Vector Tables and Exception Handling

When an exception occurs, the corresponding exception handler is executed, and its starting address is determined from a vector table.
The vector table is an array of word data, each representing the starting address of one exception type.
The vector table is relocatable, controlled by a relocation register in the NVIC.

Stack Model and Control Register

The Cortex-M3 has a two-stack model: Main Stack Pointer (MSP) and Process Stack Pointer (PSP).
The Control register bit 0 determines the processor mode change when an exception takes place.
The Control register bit 1 determines the stack used in Thread mode (PSP or MSP).
In the default setting, the MSP is used for both Thread mode and handler mode.
Direct read/write operations can be performed on the MSP and PSP.

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Description

Learn about the different registers in ARM processor, including PSP, MSP, Link Register, and Program Counter. Understand their roles and how they interact with the processor's pipelined nature.

ARM Processor Registers

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