Copies the operand into (SP), then increments SP by 2.
push af push bc push de push hl push ix push iy
Flags are unaffected.
Used for saving register values onto the stack. Also used when you want to load a 16-bit register into another 16-bit register (the LD instruction won't work for this).
rr represents a two byte register pair: BC, DE, HL, SP