16 8-bit general registers, V0 -> VF
1 16-bit register I, only 12 bits are used

Program counter (PC) should be 16-bit, and is used to store the currently executing address. 
The stack pointer (SP) can be 8-bit, it is used to point to the topmost level of the stack.
The stack is an array of 16 16-bit values, used to store the address that the interpreter shoud return to when finished with a subroutine.
Chip-8 allows for up to 16 levels of nested subroutines.