That is a pretty useful command, especially if you need more registers, or you need to preserve some and you don't use interrupts. I personally rarely use interrupts, so super complicated math functions get to use b'c'd'e'h'l'
4 cycles, 1 byte, hex code: D9
Swaps registers with shadow registers
So as a really poor example, say I need to copy some data to memory somewhere, but I need to preserve all registers for some reason. Also, assume you don't need interrupts
And yay, your registers are preserved