94.574 Elements of Computer Systems

SCE 574

101

Stack Frame Pointers

äValues and references on the stack can’t just be popped off because the SP must remain pointed at the return address

äSolution

äLoad the SP into the base pointer BP and access the parameters through the BP

äNote: BP is used by many subprograms, so it must be saved first! Where? Programmer saves it on the stack.

äDS, SS, BP are also saved/restored to original status (by the programmer) if changed by a subprogram (more calling conventions)

äBP is said to point to a stack frame

BP(L)

BP(H)

OFFSET(L)

OFFSET(H)

SEGMENT(L)

SEGMENT(H)

P2 ADDR(L)

P2 ADDR(H)

P1 ADDR(L)

P1 ADDR(H)

Original BP value placed on

stack by PUSH BP

Offset of return address pushed

by CALL

Segment of return address

pushed by call (FAR call)

Address of second parameter

Address of first parameter

BP+2

BP+4

BP+6

BP+8