Yeah, it's a register issue. IIRC the z80 proccessor has 14 general purpose registers while the ARM one in the nspire has 37(?) registers, if the documentation I was reading is correct.

By 'combining' registers it allows for storage of 16-bit data, but alone they can only hold 8-bits of data.  So AF, BC, DE, HL, IX, IY, and SP can hold up to 16-bits of data(I believe).  But IX, IY, and SP are reserved for the OS or something and shouldn't be modified.

I may not totally sure about IX, IY, and SP, I know that SP is the stack pointer, but I am not entirely sure what IX and IY are for. Maybe someone with more knowledge could enlighten us since I only know some basic stuff

Only IY is used by the OS, and SP is used pretty much everywhere (you need to preserve SP to use call, ret, push, and pop)
IY is actually the start of the OS flags, so you can use it by disabling interrupts then restoring it when done.
IX can be used by programs, and along with IY is an index register. They're called that because when using them indirectly, you can add a constant index (8-bit signed) to them, like "res 3,(ix+9)", which resets the third bit of the byte nine after IX.
The disadvantage to using them is that they are larger and slower. The instruction above is 4 bytes, while "res 3,(hl)", the equivalent version with hl instead of ix+9, is only 2.