RAX register

x86
Registers
Designer	Intel, AMD
Bits	16-bit, 32-bit and 64-bit
Introduced	1978 (16-bit), 1985 (32-bit), 2003 (64-bit)
Design	CISC
Type	Register–memory
Encoding	Variable (1 to 15 bytes)
Branching	Condition code
Endianness	Little
Page size	8086–i286: None i386, i486: 4 KB pages P5 Pentium: added 4 MB pages (Legacy PAE: 4 KB→2 MB) x86-64: added 1 GB pages
Extensions	x87, IA-32, x86-64, MMX, 3DNow!, SSE, SSE2, SSE3, SSSE3, SSE4, SSE4.2, SSE5, AES-NI, CLMUL, RdRand, SHA, MPX, SGX, XOP, F16C, ADX, BMI, FMA, AVX, AVX2, AVX512, VT-x, AMD-V, TSX, ASF
Open	Partly. For some advanced features, x86 may require license from Intel; x86-64 may require an additional license from AMD. The 80486 processor has been on the market for more than 20 years and so cannot be subject to patent claims. The pre-586 subset of the x86 architecture is therefore fully open.
General purpose	16-bit: 6 semi-dedicated registers, BP and SP are not general-purpose 32-bit: 8 GPRs, including EBP and ESP 64-bit: 16 GPRs, including RBP and RSP
Floating point	16-bit: optional separate x87 FPU 32-bit: optional separate or integrated x87 FPU, integrated SSE2 units in later processors 64-bit: integrated x87 and SSE2 units

x86 is a family of backward-compatible instruction set architectures based on the Intel 8086 CPU and its Intel 8088 variant. The 8086 was introduced in 1978 as a fully 16-bit extension of Intel's 8-bit-based 8080 microprocessor, with memory segmentation as a solution for addressing more memory than can be covered by a plain 16-bit address. The term "x86" came into being because the names of several successors to Intel's 8086 processor end in "86", including the 80186, 80286, 80386 and 80486 processors.

Many additions and extensions have been added to the x86 instruction set over the years, almost consistently with full backward compatibility. The architecture has been implemented in processors from Intel, Cyrix, AMD, VIA and many other companies; there are also open implementations, such as the Zet SoC platform. Nevertheless, of those, only Intel, AMD, and VIA hold x86 architectural licenses, and are producing modern 64-bit designs.

The term is not synonymous with IBM PC compatibility, as this implies a multitude of other computer hardware; embedded systems, as well as general-purpose computers, used x86 chips before the PC-compatible market started, some of them before the IBM PC (1981) itself.

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Wikipedia