There are only 256 different values for the stack pointer. That’s low enough to fit in a 8-bit register. See it as a digital display with two hex digits and th

There are only 256 different values for the stack pointer. That’s low enough to fit in a 8-bit register. See it as a digital display with two hex digits and the value “0x01” painted in front of it: 0x0142 ^^ two hex digits: the value in the register ^^^^ painted on the front plate of the display The cpu, similarly, hard-wires those 8 bits (seven zeros and a single one) in front of the register value when accessing the stack.