What are the purposes of the stack pointer and frame pointer registers? Why does a subroutine often need both? Why do modern machines typically pass subroutine parameters in registers rather than on...

What are the purposes of the stack pointer and frame pointer registers? Why does a subroutine often need both? Why do modern machines typically pass subroutine parameters in registers rather than on the stack? Why do subroutine calling conventions often give the caller responsibility for saving half the registers and the callee responsibility for saving the other half?