Banked Registers in ARM: What They Are and How They Work

In ARM architecture, banked registers are special sets of registers that exist separately for different processor modes. They allow the CPU to switch modes quickly without saving and restoring all registers, improving performance during interrupts and exceptions.

How It Works

Banked registers in ARM are like having multiple copies of certain registers, each reserved for a specific processor mode. Imagine you have a desk with drawers labeled for different tasks. Instead of clearing your desk every time you switch tasks, you just open the drawer for that task and continue working. Similarly, ARM uses banked registers to keep separate register sets for modes like User, FIQ, IRQ, Supervisor, and others.

This mechanism helps the processor handle interrupts and exceptions efficiently. When the CPU switches to a mode like FIQ (Fast Interrupt Request), it automatically uses a different set of registers without overwriting the User mode registers. This avoids the overhead of saving and restoring registers, making context switches faster and smoother.

Example

This example shows how ARM uses banked registers for the R8-R14 registers in FIQ mode, which are separate from User mode registers.

c

/* Pseudocode to illustrate banked registers usage in ARM */
// Assume CPU is in User mode
R8 = 10; // User mode R8

// Switch to FIQ mode
switch_to_FIQ_mode();
R8 = 20; // FIQ mode R8 (banked, separate from User mode)

// Switch back to User mode
switch_to_User_mode();
print(R8); // Outputs 10, showing User mode R8 unchanged

// Switch again to FIQ mode
switch_to_FIQ_mode();
print(R8); // Outputs 20, showing FIQ mode R8 preserved

Output

10
20

When to Use

Banked registers are used internally by the ARM processor to speed up mode switches, especially during interrupts and exceptions. As a programmer, you benefit from them when writing low-level code like operating system kernels or interrupt handlers. They allow the CPU to quickly switch context without manually saving and restoring all registers.

For example, the FIQ mode has its own banked registers to handle fast interrupts with minimal delay. This is critical in real-time systems where quick response to hardware events is required. Similarly, Supervisor and IRQ modes have banked registers to isolate their context from User mode, improving system stability and security.

Key Points

• Banked registers are separate copies of some CPU registers for different ARM modes.
• They enable fast context switching by avoiding saving/restoring registers.
• Commonly banked registers include R8-R12 and R13-R14 in FIQ mode and R13-R14 in other modes.
• Used mainly in interrupt and exception handling for performance.
• Programmers interact with them mostly in low-level system programming.

Key Takeaways

Banked registers provide separate register sets for different ARM processor modes.

They speed up mode switches by avoiding manual saving and restoring of registers.

FIQ mode has the most banked registers for fast interrupt handling.

Banked registers improve performance and reliability in interrupt and exception handling.

They are mainly relevant in low-level programming like OS kernels and device drivers.