Conceptually, the local variables cease to exist when the stack frame that implements them is popped.
$30 is reserved,
by software convention,
for use as a frame pointer.
In the extended assembler it has the mnemonic name
When a subroutine starts running,
the frame pointer and the stack pointer
contain the same address.
While the subroutine is active, the frame pointer,
points at the top of the stack.
(Remember, our stacks grow downward, so in the picture
$fp is correctly pointing at the last word that was pushed onto
the stack, the top of the stack.)
But the stack (and the stack pointer) may be involved in
arithmetic expression evaluation.
This often involves pushing and popping values onto
and off of the stack.
$sp keeps changing,
it would be hard to access a variable
at a fixed location on the stack.
To make things easy for compilers (and for human assembly language programmers) it is convenient to have a frame pointer that does not change its value while a subroutine is active. The variables will always be the same distance from the unchanging frame pointer.
In the subroutine prolog,
the caller's frame pointer is pushed onto the stack
along with the stack pointer and any
Now the subroutine makes room on the stack for
variables and points the frame pointer
to the top of the stack frame.