23 Dec register
Register as a verb can be used like “remember”.
A register (noun) can be a set of digital “locations” that, combined, are treated as a base unit of computing. A byte register is 8 bits in length, a word 2 bytes, a doubleword 4 bytes and a quadword 8 bytes or 64 bits (commonly in use as a personal computing standard at the time of this writing).
The Tortall YASM Manual tells us about Intel CPU architecture:
The 64-bit x86 register set consists of 16 general purpose registers, only 8 of which are available in 16-bit and 32-bit mode. The core eight 16-bit registers are
SP. The least significant 8 bits of the first four of these registers are accessible via the
DL in all execution modes. In 64-bit mode, the least significant 8 bits of the other four of these registers are also accessible; these are named
BPL. The most significant 8 bits of the first four 16-bit registers are also available, although there are some restrictions on when they can be used in 64-bit mode; these are named
The 80386 extended these registers to 32 bits while retaining all of the 16-bit and 8-bit names that were available in 16-bit mode. The new extended registers are denoted by adding a E prefix; thus the core eight 32-bit registers are named
ESP. The original 8-bit and 16-bit register names map into the least significant portion of the 32-bit registers.
64-bit long mode further extended these registers to 64 bits in size by adding a R prefix to the 16-bit name; thus the base eight 64-bit registers are named
RBX, etc. Long mode also added eight extra registers named numerically
r15. The least significant 32 bits of these registers are available via a d suffix (
r15d), the least significant 16 bits via a w suffix (
r15w), and the least significant 8 bits via a b suffix (
The figure below summarizes the full 64-bit x86 general purpose register set.