The Cortex-M processor is an extremely popular 32-bit ARM processor optimized for embedded applications. One of the defining characteristics of Cortex-M is that its first word must be configured as the initial stack pointer. This design choice provides some important benefits but also has tradeoffs. Understanding why the stack pointer is set up this way helps illuminate the design philosophy behind Cortex-M.
The Stack and Stack Pointer
In most computer architectures, including Cortex-M, the stack is a region of memory used to temporarily store data during function calls. The stack grows and shrinks dynamically at runtime as functions are called and return. The stack pointer is a register that points to the top of the stack. Pushing data onto the stack involves decrementing the stack pointer and writing data to the new memory address. Popping data off the stack is the reverse: reading data from the current stack pointer address and incrementing it. The stack implemention in Cortex-M is a descending stack that grows downwards in memory.
Why Set the Initial Stack Pointer in the First Word
There are several reasons why Cortex-M requires the first word of memory to be configured as the initial stack pointer on startup:
- It allows for consistent, standardized initialization across Cortex-M devices and toolchains.
- Setting up the stack pointer early ensures there is a valid stack before executing any other code.
- It enables efficient exception and interrupt handling by having a stack ready.
- The fixed address simplifies stack overflow protection and monitoring.
- It provides flexibility in locating the stack in memory later on.
Having a standardized procedure for initializing the processor helps with portability across different Cortex-M implementations. Developers can rely on the first word containing a valid stack pointer value regardless of the specific chip being used.
Setting up the stack pointer early on is important because many operations will require pushing data onto the stack. Without initializing it first, these operations would fail unpredictably. The initialization code itself may even utilize the stack in some way before the main program begins.
Handling Exceptions and Interrupts
A key benefit of the upfront stack pointer configuration is enabling immediate support for exceptions and interrupts. When an exception or interrupt occurs, the processor automatically pushes context data onto the current stack before executing the handler. With the stack ready to go, this will work seamlessly right from reset.
If the stack pointer was uninitialized, an exception could corrupt other data in memory before the handler starts. The fixed location of the initial stack avoids this issue and allows instant interrupt responsiveness.
Stack Overflow Protection
Knowing the exact stack location also makes stack overflow protection easier. The Cortex-M Memory Protection Unit (MPU) can be configured to generate an exception if the descending stack grows beyond a set address. This provides insurance against stack overflows corrupting other data.
Software can also check against a fixed stack overflow address to trigger error handling before corruption occurs. Without the standardized initialization address, overflow detection would need to be relative to the current stack pointer rather than a hardcoded value.
Flexibility for Positioning the Main Stack
Using the first word as the initial stack pointer gives flexibility when it comes to allocating the main program stack later on. The startup stack can be set up independently in a different memory region from the main application stack. This allows the application stack to be placed in the optimal memory area for performance or power reasons.
The initial stack can also act as an overflow guard area. The main stack can be based off a stack pointer value in the second word, underflowing into the initial stack area before hitting other memory.
Tradeoffs and Alternatives
While using the first word for the stack pointer has advantages, there are also some tradeoffs. It imposes an architectural constraint that the beginning of memory must be available and cannot contain anything else. The fixed location also means the initial stack cannot be placed in the fastest or most appropriate memory region.
Some embedded systems initialize the stack pointer elsewhere and relocate it later. However, this loses some of the benefits around consistency, interrupts, and overflow protection outlined earlier. It also increases complexity by requiring extra initialization steps.
Overall the requirement for an initial stack pointer makes sense given the design goals and usage scenarios Cortex-M targets. The rigidity ultimately provides more robustness and ensures the processor starts in a known, usable state right after reset.
Conclusion
Forcing the first word in Cortex-M to be the initial stack pointer provides a firm starting point for the processor before application code takes over. It enables standardized initialization, instant interrupt handling, and simpler stack overflow checks. While requiring dedicated memory, the advantages outweigh the tradeoffs for Cortex-M’s primary use cases. Understanding the reasons for this architectural choice provides helpful insight into the design philosophy behind this ubiquitous ARM processor.
