Depending on the mode of the operation (protected or long), there are 8 to 16 available general purpose registers in modern Intel processors. Each register is divided into subregisters, allowing access to data with a bit width lower than the width of the register.
The following table shows general purpose registers (further referred to as GPR):
Table 1: x86/x86_64 registers
All R* registers are only available in the long mode. Registers SIL, DIL, BPL, and SPL are only available in the long mode. Registers AH, BH, ...
