Stack Pointer
From Computer History Wiki
A Stack Pointer (often abbreviated as SP) is a special register inside a CPU, used by the CPU to do operations on a stack held in main memory.
In many machines (e.g. the PDP-11) any of the CPU's general purpose registers can be used as a stack pointer, but there is still usually one that is 'special' to the hardware (e.g. used during the processing of interrupts), and this is usually distinguished as 'the' SP.
