To further aid this type of access, the 6809 renamed the SP to U for "user", and added a second stack pointer, S, for "system". The idea was user programs would use U while the CPU itself would use S to store data during subroutine calls. This allowed system code to be easily called by changing S without affecting any other running program. For instance, a program calling a floating-point routine in ROM would place its data on the U stack and then call the routine, which could then perform the calculations using data on its own private stack pointed to by S, and then return, leaving the U stack untouched.