Program counter and stack pointer || stack pointer

Program Counter: A program counter is a register in a computer processor that contains the address (location) of the instruction being executed at the current time. As each instruction gets fetched, the program counter increases its stored value by 1. After each instruction is fetched, the program counter points to the next instruction in the sequence. When the computer restarts or is reset, the program counter normally reverts to 0.

In computing, a program is a specific set of ordered operations for a computer to perform. An instruction is an order given to a computer processor by a program. Within a computer, an address is a specific location in memory or storage. A register is one of a small set of data holding places that the processor uses.

Some engineers refer to a program counter as an instruction address register or an address pointer. 

Stack Pointer: A stack pointer is a small register that stores the address of the last program request in a stack. A stack is a specialized buffer which stores data from the top down. As new requests come in, they "push down" the older ones. The most recently entered request always resides at the top of the stack and the program always takes requests from the top.

A stack (also called a pushdown stack) operates in a last-in/first-out sense. When a new data item is entered or "pushed" onto the top of a stack, the stack pointer increments to the next physical memory address, and the new item is copied to that address. When a data item is "pulled" or "popped" from the top. of a stack, the item is copied from the address of the stack pointer, and the stack pointer decrements to the next available item at the top of the stack.