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19-661: The EVEX scheme is a 4-byte extension to the VEX scheme which supports the AVX-512 instruction set and allows addressing new 512-bit ZMM registers and new 64-bit operand mask registers. With Advanced Performance Extensions , the Extended EVEX prefix redefines the semantics of several payload bits. EVEX coding can address 8 operand mask registers, 16 general-purpose registers and 32 vector registers in 64-bit mode (otherwise, 8 general-purpose and 8 vector), and can support up to 4 operands. Like

38-501: A 4th register operand (IS4 above). At most one of the operands can be a memory operand; and at most one of the operands can be an immediate constant of 4 or 8 bits. The remaining operands are registers. The AVX instruction set is the first instruction set extension to use the VEX coding scheme. The AVX instruction set uses VEX prefix only for instructions using the SIMD XMM registers. However,

57-464: A VEX prefix added are equivalent to the same instructions without VEX prefix with the following differences: Instructions that use the whole 256-bit YMM register should not be mixed with non-VEX instructions that leave the upper half of the register unchanged, for reasons of efficiency. The VEX prefix is not supported in real mode and virtual-8086 mode (all instructions with the VEX prefix will cause #UD in these modes). ModR From Misplaced Pages,

76-507: A memory operand may use the ModR/M byte which specifies the addressing mode. This byte has three bit fields: The base-plus-index and scale-plus-index forms of 32-bit addressing (encoded with r/m = 100 and mod ≠ 11) require another addressing byte, the SIB byte. It has the following fields: The REX prefix provides additional space for encoding 64-bit addressing modes and additional registers present in

95-550: A register source operand; i.e., be of the form 11xxxxxx . Various bit-fields in the VEX prefix's second byte are inverted to ensure that the byte is always of this form. Similarly, the REX prefix's one-byte form has the four high-order bits set to four, which replaces sixteen opcodes numbered 0x40–0x4F. Previously, those opcodes were individual INC and DEC instructions for the eight standard processor registers; x86-64 code must use ModR/M INC and DEC instructions. Legacy SIMD instructions with

114-609: Is a 2-byte variant of the VEX3 prefix, that differs from the latter in the following points: Instructions that require any of these bit-fields need to be encoded with the VEX3 prefix. The REX2 prefix is a 2-byte variant of the REX prefix, introduced with Intel APX extensions which add 16 Extended GPR registers. Instructions coded with the VEX prefix can have up to four variable operands (in registers or memory) and one constant operand (immediate value). Instructions that need more than three variable operands use immediate operand bits to specify

133-444: Is longer or shorter than the legacy code. In 32-bit mode VEX encoded instructions can only access the first 8 YMM/XMM registers; the encodings for the other registers would be interpreted as the legacy LDS and LES instructions that are not supported in 64-bit mode. The VEX coding scheme uses a code prefix consisting of two or three bytes , which may be added to existing or new instruction codes. In x86 architecture, instructions with

152-511: The article wizard to submit a draft for review, or request a new article . Search for " ModR " in existing articles. Look for pages within Misplaced Pages that link to this title . Other reasons this message may be displayed: If a page was recently created here, it may not be visible yet because of a delay in updating the database; wait a few minutes or try the purge function . Titles on Misplaced Pages are case sensitive except for

171-460: The 16 general purpose registers): A few VEX-encoded AVX blending instructions have 4 operands. To accommodate this, VEX has IS4 addressing mode, which encodes 4th operand (a vector register) in bits Imm8[7:4] of the immediate constant. Similar EVEX-encoded blend instructions have their 4th operand in a mask register. No EVEX-encoded instruction uses IS4 addressing mode encoding. Intel Advanced Performance Extensions introduce several new variants of

190-449: The 3-byte payload in the EVEX prefix, which are used to encode Extended GPR registers R16-R31 and new conditional instructions. EVEX extension of EVEX instructions: EVEX extension of VEX instructions: EVEX extension for legacy instructions: EVEX prefix for conditional CMP and TEST: When the new EGPR registers and operand destinations can be encoded by both extended EVEX and REX2 prefixes,

209-438: The 32-bit BOUND instruction, which is not supported in 64-bit mode. The ModR/M byte specifies one operand (always a register) with reg field, and the second operand is encoded with mod and r/m fields, specifying either a register or a location in memory. Base-plus-index and scale-plus-index addressing require the SIB byte, which encodes 2-bit scale factor as well as 3-bit index and 3-bit base registers. Depending on

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228-445: The VEX coding scheme has been used for other instruction types as well in subsequent expansions of the instruction set. For example: The VEX prefix's initial-byte values, 0xC4 and 0xC5, are the same as the opcodes of the LDS and LES instructions. Not supported in 64-bit mode, the ambiguity is resolved in 32-bit mode by exploiting the fact that a legal LDS or LES's ModR/M byte cannot specify

247-402: The VEX coding scheme, the EVEX prefix unifies existing opcode prefixes and escape codes, memory addressing and operand length modifiers of the x86 instruction set. The following features are carried over from the VEX scheme: EVEX also extends VEX with additional capabilities: For example, the EVEX encoding scheme allows conditional vector addition in the form of where {k1} modifier next to

266-410: The addressing mode, Disp8/Disp16/Disp32 field may follow with displacement that needs to be added to the address. The EVEX prefix retains fields introduced in the VEX prefix : New functions of the existing fields: There are several new bit fields: The encoding of the EVEX prefix is as follows: The following table lists possible register addressing combinations (bit 4 is always zero when encoding

285-400: The destination operand encodes the use of opmask register k1 for conditional processing and updates to destination, and {z} modifier (encoded by EVEX.z) provides the two types of masking (merging and zeroing), with merging as default when no modifier is attached. The EVEX coding scheme uses a code prefix consisting of 4 bytes ; the first byte is always 62h and derives from an unused opcode of

304-456: The following purposes: The VEX prefix replaces the most commonly used instruction prefix bytes and escape bytes. In many cases, the number of prefix bytes and escape bytes that are replaced is the same as the number of bytes in the VEX prefix, so that the total length of the VEX-encoded instruction is the same as the length of the legacy instruction code. In other cases, the VEX-encoded version

323-885: The 💕 Look for ModR on one of Misplaced Pages's sister projects : [REDACTED] Wiktionary (dictionary) [REDACTED] Wikibooks (textbooks) [REDACTED] Wikiquote (quotations) [REDACTED] Wikisource (library) [REDACTED] Wikiversity (learning resources) [REDACTED] Commons (media) [REDACTED] Wikivoyage (travel guide) [REDACTED] Wikinews (news source) [REDACTED] Wikidata (linked database) [REDACTED] Wikispecies (species directory) Misplaced Pages does not have an article with this exact name. Please search for ModR in Misplaced Pages to check for alternative titles or spellings. You need to log in or create an account and be autoconfirmed to create new articles. Alternatively, you can use

342-448: The latter is preferred. VEX prefix The VEX prefix (from "vector extensions") and VEX coding scheme are an extension to the IA-32 and x86-64 instruction set architecture for microprocessors from Intel , AMD and others. The VEX coding scheme allows the definition of new instructions and the extension or modification of previously existing instruction codes . This serves

361-477: 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. The VEX3 prefix contains all bit-fields from the REX prefix as well as various other prefixes, expanding addressing mode, register enumeration, operand size and width: The VEX2 prefix

#357642