What do the E and R prefixes stand for in the names of Intel 32-bit and 64-bit registers?

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  • R just stands for “register”. The AMD64 ISA extension added 8 additional general-purpose registers, named R8 through R15. The 64-bit extended versions of the original 8 registers had an R prefix added to them for symmetry.

  • E stands for “extended” or “enhanced”. (Wikipedia says “extended”.) They are the “extended” versions of the 16-bit registers, in that they offer 16 additional bits for 32 bits total.

  • X is also for “extended”—or perhaps it implies 16 as in hexadecimal.* The X-suffixed registers are the 16-bit extended versions of the 8-bit registers. For 8-bit registers, the L suffix means “low”, and the H suffix means “high”.

Therefore, taking one particular register as an example, you have the 8-bit AL and AH registers, which are the low and high bytes of the 16-bit AX register, which is the low word of the 32-bit EAX register, which is the low double-word of the 64-bit RAX register.

| 63 - 32 | 31 - 16 | 15 - 8 | 7 - 0 |
======================================
.         .         | AH     | AL    |
.         .         | AX             |
.         | EAX                      | 
| RAX                                |
======================================
| 63 - 32 | 31 - 16 | 15 - 8 | 7 - 0 |

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* X was used in the mnemonics (such as LXI and DCX) on the 8080 for instructions that treated a pair of otherwise-separate 8-bit registers as a 16-bit integer, similar to how AX represents the AH:AL pair. Thus, another possible interpretation is that X means pair, and this usage was continued when naming the high:low pairs on subsequent processors, including the 8086, which was a full 16-bit extension of the 8080.

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