Hexagon_V65_Programmers_Reference_Manual (11)

8. Software stack

The Hexagon processor includes special registers and instructions to support calls

Subroutine execution stack.

The stack structure follows standard C conventions

8.1 stack structure

The stack is defined to grow from a high address to a low address.The stack pointer register SP points to the data element currently at the top of the stack.

Note that the Hexagon processor supports three dedicated stack instructions:

allocframe, deallocframe, and Deallocu_returns (Section 8.5).

For stack instructions, the SP address must always be 8-byte aligned to work properly.

8.2 stack frame

The stack is used to store stack frames, which are data structures that store state

Information about active subroutines in the program (ie those that are called but have

Not yet returned).Each stack frame corresponds to an active subroutine in the program.

The stack frame contains the following elements:

• Local variables and data used by subroutines

• Return address of subroutine call (pushed from link register LR)

• Address of the previous stack frame allocated on the stack (from the frame pointer register FP)

The frame pointer register FP always contains the address of the frame pointer held in the current stack frame.It makes it easy to determine call sequences, function arguments, etc. by allowing the debugger to inspect the stack in memory.

Note: For leaf functions, it is usually not necessary to save FP and LR.In this case, the FP contains the frame pointer of the calling function, not the current function.

8.3 Stack Protection

Hexagon V6x processors support the following features to protect the software stack:

• Stack bounds checking

• Stack Shatter Protection

8.3.1 Stack Bounds Check

Stack bounds checking prevents stack frames from being allocated beyond the bounds of the lower software stack.

FRAMELIMIT is a 32-bit control register that stores the lower limit of the memory area designated as reserved for the software stack.When the allocframe instruction allocates a new stack frame, it binds the new stack pointer value in SP to the stack bound value in FRAMELIMIT.If SP is less than FRAMELIMIT, the Hexagon processor raises exception 0x27 (Section 7.10).

Note: Stack bounds checking mode is performed when the processor is in user and guest state, but not in monitor mode.

8.3.2 Stack Bounds Check

Stack bounds checking prevents stack frames from being allocated beyond the bounds of the lower software stack.

FRAMELIMIT is a 32-bit control register that stores the lower limit of the memory area designated as reserved for the software stack.When the allocframe instruction allocates a new stack frame, it binds the new stack pointer value in SP to the stack bound value in FRAMELIMIT.If SP is less than FRAMELIMIT, the Hexagon processor raises exception 0x27 (Section 7.10).

Note: Stack bounds checking mode is performed when the processor is in user and guest state, but not in monitor mode.