ZiLOG Z80 User Manual page 21

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The program counter holds the 16-bit address of the current instruction
being fetched from memory. The PC is automatically incremented after its
contents have been transferred to the address lines. When a program jump
occurs, the new value is automatically placed in the PC, overriding the
incrementer.
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The stack pointer holds the 16-bit address of the current top of a stack
located anywhere in external system RAM memory. The external stack
memory is organized as a last-in first-out (LIFO) file. Data can be pushed
onto the stack from specific CPU registers or popped off of the stack to
specific CPU registers through the execution of PUSH and POP instructions.
The data popped from the stack is always the last data pushed onto it. The
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