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existing instructions extended to a 64 bit operand size (remaining instructions) Most instructions with a 64 bit operand size encode this using a REX.W prefix; in the absence of the REX.W prefix, the corresponding instruction with 32 bit operand size is encoded. This mechanism also applies to most other instructions with 32 bit operand size.
The REX prefix provides additional space for encoding 64-bit addressing modes and additional registers present in the x86-64 architecture. Bit-field W changes the operand size to 64 bits, R expands reg to 4 bits, B expands r/m (or opreg in the few opcodes that encode the register in the 3 lowest opcode bits, such as "POP reg"), and X and B expand index and base in the SIB byte.
IA-32 is the first incarnation of x86 that supports 32-bit computing; [4] as a result, the "IA-32" term may be used as a metonym to refer to all x86 versions that support 32-bit computing. [5] [6] Within various programming language directives, IA-32 is still sometimes referred to as the "i386" architecture.
Instructions that have been added to the x86 instruction set in order to assist efficient calculation of cryptographic primitives, such as e.g. AES encryption, SHA hash calculation and random number generation.
The EVEX prefix (enhanced vector extension) and corresponding coding scheme is an extension to the 32-bit x86 (IA-32) and 64-bit x86-64 (AMD64) instruction set architecture. EVEX is based on, but should not be confused with the MVEX prefix [ 1 ] used by the Knights Corner processor.
For XOP-encoded integer-register instructions (the TBM and LWP instruction set extensions, see below), W is used for operand size. (0=32-bit, 1=64-bit) vvvv is an extra source register argument, normally the first non-r/m source argument for instructions with ≥3 register arguments. L is a vector length specifier.
The x86 instruction set has several times been extended with SIMD (Single instruction, multiple data) instruction set extensions.These extensions, starting from the MMX instruction set extension introduced with Pentium MMX in 1997, typically define sets of wide registers and instructions that subdivide these registers into fixed-size lanes and perform a computation for each lane in parallel.
Bit manipulation instructions sets (BMI sets) are extensions to the x86 instruction set architecture for microprocessors from Intel and AMD. The purpose of these instruction sets is to improve the speed of bit manipulation. All the instructions in these sets are non-SIMD and operate only on general-purpose registers.