Stack Allocation .

. A procedure call sequence increments SP, saves the return address and transfers control to the called procedure

ADD #caller.recordsize, SP

MOVE #here+ 16, *SP

GOTO callee.code_area

Consider the situation when a function (caller) calls the another function(callee), then procedure call sequence increments SP by the caller record size, saves the return address and transfers control to the callee by jumping to its code area. In the MOV instruction here, we only need to add 16, as SP is a register, and so no space is needed to store *SP. The activations keep getting pushed on the stack, so #caller.recordsize needs to be added to SP, to update the value of SP to its new value. This works as #caller.recordsize is a constant for a function, regardless of the particular activation being referred to.