Push

By software convention, $sp always points to the top of the stack. Also by convention, the stack grows downward (in terms of memory addresses).

So, for our stack of 4-byte (full word) data, adding an item means subtracting four from $sp and storing the item in that address. This operation is called a push operation.

To push an item onto the stack, first subtract 4 from the stack pointer, then store the item at the address in the stack pointer.

Here is what that looks like in code. Say that the value to push on the stack is in register $t0:

                    # PUSH the item in $t0:
subu $sp,$sp,4      #   point to the place for the new item,
sw   $t0,($sp)      #   store the contents of $t0 as the new top.

The extended assembler allows you to write ($sp) for 0($sp). The machine instruction will have the displacement of zero filled in.