This graphic shows you the register line-up in Thumb state versus ARM state. In ARM state, you have all registers available, including r0 through r12, Program Counter (PC), the Stack Pointer (SP), a Link Register (LR), and the Current Program Status Register (CPSR). In Thumb state, all of these registers are available except for r8 through r12. As stated previously, this was one of the compromises that was made to make the Thumb mode a 16-bit wide instruction.