What does the R stand for in RAX, RBX, RCX, RDX, RSI, RDI, RBP, RSP?

assembly x86 x86-64 cpu-registers cpu-architecture
Wuschelbeutel Kartoffelhuhn picture Wuschelbeutel Kartoffelhuhn · Jun 12, 2012 · Viewed 27.2k times · Source

The x86 assembler language has had to change as the x86 processor architecture has changed from 8bit to 16bit to 32bit and now 64bit.

I know that in 32bit assembler register names (EAX, EBX, etc.), the E prefix for each of the names stands for Extended meaning the 32bit form of the register rather than the 16bit form (AX, BX, etc.).

What does the R prefix for these register names stand for in 64bit?

Answer

Paul R picture Paul R · Jun 12, 2012

I think it's just R for "register", since there are additional registers R8 - R15 on x86-64, and R is a common prefix on many CPU architectures where registers are numbered.

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