Intel Opcode Table Code 80 Followed
THIS REFERENCE IS NOT PERFECT.For example, opcode 80 followed by a ModR/M byte with a reg of 4 is an AND Eb, Ib instruction, while that same opcode followed by a ModR/M byte with a reg of 7 is a CMP Eb, Ib instruction. Derived from the May 2019 version of the Intel 64 and IA-32 Architectures Software Developer’s Manual.Last updated. Reference documents for x86-64 instruction’s encodingx86 and amd64 instruction reference. Use the high-order four bits of the opcode as an index to a row of the opcode table use the low-order four bits as an index to a column of the table. According to Intels XED, as of this writing, there are 1503 define Appendix A - Opcode Map The opcode tables that follow aid in interpreting 80386 object code.
Brief introduction to x86-64 instruction encoding Trainer kits composed of a. Opcodes of Intel in Alphabetical Order. Tools and tips for finding out an x86-64 instruction’s encodingLoad interrupt descriptor table: LLDT: Load local descriptor table: LMSW: Load machine status word: LOADALL: Load all CPU registers, including internal ones such as GDT: Undocumented, 8026 only LSL: Load segment limit: LTR: Load task register: SGDT: Store global descriptor table: SIDT: Store interrupt descriptor table: SLDT: Store local descriptor table: SMSWOpcode Sheet for Microprocessor With Description. Opcode, Instruction, size, flags, function. I constructed this table specifically for examining raw code and to aid in writing a disassembler.
For the representation of operands with the SIB or displacement, the formats are different. Note that instructions with more than one source operand, such as the enter instruction, do not have reversed order. Intel syntax uses "dest, source" while the AT&T syntax uses "source, dest". With GNU tool chains on Linux, the default syntax used is usually the AT&T one.The most significant different between these 2 syntaxes is that AT&T and Intel syntax use the opposite order for source and destination operands. In Intel documents, it is usually in Intel syntax.
a displacement and an immediate data field (if required)An example: manually encode an x86-64 instructionLet’s take a look at the encoding of an instruction add r8,QWORD PTR (in Intel syntax) in the previous part. a register and/or address mode specifier consisting of the ModR/M byte and sometimes the scale-index-base (SIB) byte (if required) Each instruction’s encoding consists of: For example, to find out the encoding of the instruction addq 10(%rdi), %r8, you can do it as follows.First, create a file add.s containing one line addq 10(%rdi), %r8Second, assemble the add.s to and object file by $ as add.s -o add.oLast, deassemble the object file by objdump -d by $ objdump -d add.oIt will print out add.o: file format elf64-x86-64Here 4c 03 47 0a is the 4-byte encoding of the addq instruction.If you want to check the instructions in Intel syntax, you may doLast, deassemble the object file by objdump -d by $ objdump -d -disassembler-options=intel-mnemonic add.oYou will get add.o: file format elf64-x86-640: 4c 03 47 0a add r8,QWORD PTR Brief introduction to x86-64 instruction encodingThe x86-64 instructions are encoded one by one as a variable number of bytes for each. x86-64 Instruction Encoding is another very good page from OSDev as a quick reference.Tools and tips for finding out an x86-64 instruction’s encodingTo quickly find out the encoding of an instruction, you can use the GNU assembler as and the objdump tool together. the Intel 64 and IA-32 Architectures Software Developer’s Manuals‘ "APPENDIX B INSTRUCTION FORMATS AND ENCODINGS" is a good reference.
There are 3 parts in the ModRM byte: ‘mod’, ‘reg’ and ‘r/m’.There is a table "Table 2-2. Memory Addressing Without an SIB Byte REX.X Not Used" from Volume 2 shows the encoding for this case.The REX.R and REX.B bits and the ModeRM byte will be decided accordingly. The first part will be ModRM:reg(r,w) and the second part will be ModRM:r/m(r).
Hence, we only use it to find out bits for ‘Mod’ only for the addq instruction we are encoding it. Bits needed toDefine fields in the 64-bit context are provided by the addition of REX prefixes" and hence the same value can be used) in Volume 2 which shows mapping of the operands combinations to the bits values of ‘mod’.Although the table applies to 64-bit modes too, it does not show the additional registers like r8. But from 2.2.1.1, "In 64-bit mode, these formats do not change.
You may use the same method to encode all other instruction by checking the reference documents for details of every instruction/operand combinations’ cases. Hence, in bits, we get reg = 000By putting the ‘WRXB’ bits ( ) together, we get the REX prefix for this instruction is 0100 1100Together with the 03 in REX.W+03/r from the reference for the ADD instruction, the opcode part, in hexadecimal, is 4c 03By putting the mod, reg and r/m together, we get the ModRM byte (in bits) 01 000 111Following the ModRM byte is the displacement is 0xa( 10‘s hexadecimal representation) in one byte ( disp8).Putting all these together, we finally get the encoding of add r8,: 4c 03 47 0aIn this example, to show the process, I have shown how to manually do an instruction’s encoding which is usually done by the assembler. From the row of +disp8 (actually, all disp8 ones share the same ‘Mod’ bits), Mod = 01 (in bits)For the encoding of the registers, I compiled a table for the general purpose 64-bit registers for your reference: _.Reg RegisterThe ‘ ‘ in the ‘.Reg’ are usually a bit in the REX prefix, such as REX.B and REX.R, depending on specific instructions and operand combinations.For the addq instruction in this case, r8 is 1.000 and rdi is 0.111.
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