X86 & X86-64

x86 Designer     Intel, AMD
Bits                   16-bit, 32-bit, and/or 64-bit
Introduced        1978
Design              CISC
Type                 Register-memory
Encoding          Variable (1 to 15 bytes)
Branching         Status register
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, P6, MMX, SSE, SSE2, x86-64, SSE3, SSSE3, SSE4, SSE5, AVX

see Itanium.

x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other enhancements. The original specification was created by AMD, and has been implemented by AMD, Intel, VIA, and others. It is fully backwards compatible with Intel x86 16-bit and 32-bit code.[1](p13-14) Because the full x86 16-bit and 32-bit instruction sets remains implemented in hardware without any intervening emulation, existing x86 executables run with no compatibility or performance penalties,[2] although existing applications that are recoded to take advantage of new features of the processor design may see performance increases.

AMD's method of extending Intel's x86 32-bit instruction set to be a subset of its x86-64 instruction set is the same technique Intel employed to extend its 16-bit x86 instruction set to 32-bits.

Prior to launch, "x86-64" and "x86_64" were used to refer to the instruction set. Upon release, AMD named it AMD64[3] Intel initially used the names IA-32e and EM64T before finally settling on Intel 64 for their implementation. x86-64 is still used by many in the industry, while others, notably Sun Microsystems[4] (now Oracle Corporation) and Microsoft,[5] use x64 while the BSD family of OS's use AMD64.

64-bit Core microarchitecture based
The successor to Core is the mobile version of the Intel Core 2 line of processors using cores based upon the Intel Core microarchitecture,[6] released on July 27, 2006. The release of the mobile version of Intel Core 2 marks the reunification of Intel's desktop and mobile product lines as Core 2 processors were released for desktops and notebooks, unlike the first Intel Core CPUs that were targeted only for notebooks (although some small form factor and all-in-one desktops, like the iMac and the Mac Mini, also used Core processors).
Unlike the Intel Core, Intel Core 2 is a 64-bit processor, supporting Intel 64. Another difference between the original Core Duo and the new Core 2 Duo is an increase in the amount of Level 2 cache. The new Core 2 Duo has tripled the amount of on-board cache to 6 MB. Core 2 also introduced a quad-core performance variant to the single- and dual-core chips, branded Core 2 Quad, as well as an enthusiast variant, Core 2 Extreme. All three chips are manufactured at a 65 nm lithography, and in 2008, a 45 nm lithography and support Front Side Bus speeds ranging from 533 MHz to 1600 MHz. In addition, the 45 nm die shrink of the Core microarchitecture adds SSE4.1 support to all Core 2 microprocessors manufactured at a 45 nm lithography, therefore increasing the calculation rate of the processors.