- Recall we only use 48 bits for addresses
- Below are the most important registers
- Sixteen 64 bit general purpose registers
- 64 bit flags register (RFLAGS)
- 64 bit instruction pointer (RIP)
- Eight 64 bit MMX registers
- Sixteen 128 bit XMM registers (versus 8 in 32 bit)
- General Purpose Registers
- Even in 64 bit mode there are only eight 32 bit registers by default
- Adding REX prefix to the instruction you now get access to sixteen 64 bit registers
- You cannot access a high-byte register at the same time as a new low-byte register
- The upper 32 bits of RFLAGS is not used, old flags remain
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