NonStop S-Series Server Description Manual (G06.24+)

Native Execution Mode

HP NonStop S-Series Server Description Manual—520331-003

7-4

RISC Stack Frames

The primary stacks used by a native process (that is, the main stack and the privileged

stack) use RISC stack frame conventions as illustrated in Figure 7-2. These stacks

grow downward, toward smaller addresses. They both originate at the high-address

end of their containing segments. (In diagrams throughout this manual, addresses are

shown as increasing downward on the page.)

The sp (stack pointer) register points at the stack tip, which is the lowest-addressed

byte in the current stack frame. Typically, sp is static for the frame and, in most cases,

also serves as the frame pointer for the current frame. (If a procedure has

dynamically allocated variables, a separate frame pointer register is used as the static

reference point for the frame and sp is adjusted to the new tip.) The frame pointer is

used as a base reference to address local variables, saved registers, and formal

parameters.

The virtual frame pointer (vfp) defines the frame origin. The origin is the trailing

edge of the stack frame. It coincides with the tip of the frame of the caller. Thus, sp for

the caller becomes vfp for the called procedure. No register actually contains the vfp

address.

As shown in Figure 7-2, the formal parameters of a procedure call can be considered

part of either or both frames—the caller’s and the called. The caller’s procedure frame

is allocated with enough call-out space to hold the parameters for any procedure that it

might invoke. Actual parameters are stored into this space by offset references to sp

(not pushed, as in TNS frames). The called procedure addresses the formal

parameters by offsets from its own frame pointer.

Native stack frames can be increased only in multiples of eight bytes, and sp is

rounded up as necessary to accommodate this convention.

Subprocedures use exactly the same stack format as procedures, and sp is adjusted

each time any nested subprocedure is called. To accommodate the need for

subprocedures to access the outer procedure’s local variables, the outer procedure’s

vfp address is passed as a hidden parameter in the v0 register.