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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Section 17.2</TITLE></HEAD><BODY><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B> <A HREF="s17_01.htm">17.1 Operand Size and Address-Size Attributes</A><BR><B>next:</B> <A HREF="AAA.htm"> AAA ASCII Adjust after Addition</A> <P><HR><P><H1>17.2 Instruction Format</H1>All instruction encodings are subsets of the general instruction formatshown in <A HREF="#fig17-1">Figure 17-1</A> . Instructions consist of optional instructionprefixes, one or two primary opcode bytes, possibly an address specifierconsisting of the ModR/M byte and the SIB (Scale Index Base) byte, adisplacement, if required, and an immediate data field, if required.<P>Smaller encoding fields can be defined within the primary opcode oropcodes. These fields define the direction of the operation, the size of thedisplacements, the register encoding, or sign extension; encoding fieldsvary depending on the class of operation.<P>Most instructions that can refer to an operand in memory have an addressingform byte following the primary opcode byte(s). This byte, called the ModR/Mbyte, specifies the address form to be used. Certain encodings of the ModR/Mbyte indicate a second addressing byte, the SIB (Scale Index Base) byte,which follows the ModR/M byte and is required to fully specify theaddressing form.<P>Addressing forms can include a displacement immediately following eitherthe ModR/M or SIB byte. If a displacement is present, it can be 8-, 16- or32-bits.<P>If the instruction specifies an immediate operand, the immediate operandalways follows any displacement bytes. The immediate operand, if specified,is always the last field of the instruction.<P>The following are the allowable instruction prefix codes:<PRE> F3H <A HREF="REP.htm">REP</A> prefix (used only with string instructions) F3H <A HREF="REP.htm">REPE/REPZ</A> prefix (used only with string instructions F2H <A HREF="REP.htm">REPNE/REPNZ</A> prefix (used only with string instructions) F0H <A HREF="LOCK.htm">LOCK</A> prefix</PRE>The following are the segment override prefixes:<PRE> 2EH CS segment override prefix 36H SS segment override prefix 3EH DS segment override prefix 26H ES segment override prefix 64H FS segment override prefix 65H GS segment override prefix 66H Operand-size override 67H Address-size override</PRE><A NAME="fig17-1"><IMG align=center SRC="fig17-1.gif" border=0><H2>17.2.1 ModR/M and SIB Bytes</H2>The ModR/M and SIB bytes follow the opcode byte(s) in many of the 80386instructions. They contain the following information:<UL> <LI> The indexing type or register number to be used in the instruction <LI> The register to be used, or more information to select the instruction <LI> The base, index, and scale information</UL>The ModR/M byte contains three fields of information:<UL> <LI> The mod field, which occupies the two most significant bits of the byte, combines with the r/m field to form 32 possible values: eight registers and 24 indexing modes <LI> The reg field, which occupies the next three bits following the mod field, specifies either a register number or three more bits of opcode information. The meaning of the reg field is determined by the first (opcode) byte of the instruction. <LI> The r/m field, which occupies the three least significant bits of the byte, can specify a register as the location of an operand, or can form part of the addressing-mode encoding in combination with the field as described above</UL>The based indexed and scaled indexed forms of 32-bit addressing require theSIB byte. The presence of the SIB byte is indicated by certain encodings ofthe ModR/M byte. The SIB byte then includes the following fields:<UL> <LI> The ss field, which occupies the two most significant bits of the byte, specifies the scale factor <LI> The index field, which occupies the next three bits following the ss field and specifies the register number of the index register <LI> The base field, which occupies the three least significant bits of the byte, specifies the register number of the base register</UL><A HREF="#fig17-2">Figure 17-2</A> shows the formats of the ModR/M and SIB bytes.The values and the corresponding addressing forms of the ModR/M and SIBbytes are shown in Tables 17-2, 17-3, and 17-4. The 16-bit addressingforms specified by the ModR/M byte are in Table 17-2. The 32-bit addressingforms specified by ModR/M are in Table 17-3. Table 17-4 shows the 32-bitaddressing forms specified by the SIB byte<P><A NAME="fig17-2"><IMG align=center SRC="fig17-2.gif" border=0><PRE>Table 17-2. 16-Bit Addressing Forms with the ModR/M Byter8(/r) AL CL DL BL AH CH DH BHr16(/r) AX CX DX BX SP BP SI DIr32(/r) EAX ECX EDX EBX ESP EBP ESI EDI/digit (Opcode) 0 1 2 3 4 5 6 7REG = 000 001 010 011 100 101 110 111 Effective +---Address--+ +Mod R/M+ +--------ModR/M Values in Hexadecimal--------+[BX + SI] 000 00 08 10 18 20 28 30 38[BX + DI] 001 01 09 11 19 21 29 31 39[BP + SI] 010 02 0A 12 1A 22 2A 32 3A[BP + DI] 011 03 0B 13 1B 23 2B 33 3B[SI] 00 100 04 0C 14 1C 24 2C 34 3C[DI] 101 05 0D 15 1D 25 2D 35 3Ddisp16 110 06 0E 16 1E 26 2E 36 3E[BX] 111 07 0F 17 1F 27 2F 37 3F[BX+SI]+disp8 000 40 48 50 58 60 68 70 78[BX+DI]+disp8 001 41 49 51 59 61 69 71 79[BP+SI]+disp8 010 42 4A 52 5A 62 6A 72 7A[BP+DI]+disp8 011 43 4B 53 5B 63 6B 73 7B[SI]+disp8 01 100 44 4C 54 5C 64 6C 74 7C[DI]+disp8 101 45 4D 55 5D 65 6D 75 7D[BP]+disp8 110 46 4E 56 5E 66 6E 76 7E[BX]+disp8 111 47 4F 57 5F 67 6F 77 7F[BX+SI]+disp16 000 80 88 90 98 A0 A8 B0 B8[BX+DI]+disp16 001 81 89 91 99 A1 A9 B1 B9[BX+SI]+disp16 010 82 8A 92 9A A2 AA B2 BA[BX+DI]+disp16 011 83 8B 93 9B A3 AB B3 BB[SI]+disp16 10 100 84 8C 94 9C A4 AC B4 BC[DI]+disp16 101 85 8D 95 9D A5 AD B5 BD[BP]+disp16 110 86 8E 96 9E A6 AE B6 BE[BX]+disp16 111 87 8F 97 9F A7 AF B7 BFEAX/AX/AL 000 C0 C8 D0 D8 E0 E8 F0 F8ECX/CX/CL 001 C1 C9 D1 D9 E1 E9 F1 F9EDX/DX/DL 010 C2 CA D2 DA E2 EA F2 FAEBX/BX/BL 011 C3 CB D3 DB E3 EB F3 FBESP/SP/AH 11 100 C4 CC D4 DC E4 EC F4 FCEBP/BP/CH 101 C5 CD D5 DD E5 ED F5 FDESI/SI/DH 110 C6 CE D6 DE E6 EE F6 FEEDI/DI/BH 111 C7 CF D7 DF E7 EF F7 FF</PRE><EM><H3>Notes</H3> disp8 denotes an 8-bit displacement following the ModR/M byte, to be sign-extended and added to the index. disp16 denotes a 16-bit displacement following the ModR/M byte, to be added to the index. Default segment register is SS for the effective addresses containing a BP index, DS for other effective addresses.</EM><PRE>Table 17-3. 32-Bit Addressing Forms with the ModR/M Byter8(/r) AL CL DL BL AH CH DH BHr16(/r) AX CX DX BX SP BP SI DIr32(/r) EAX ECX EDX EBX ESP EBP ESI EDI/digit (Opcode) 0 1 2 3 4 5 6 7REG = 000 001 010 011 100 101 110 111 Effective+---Address--+ +Mod R/M+ +---------ModR/M Values in Hexadecimal-------+[EAX] 000 00 08 10 18 20 28 30 38[ECX] 001 01 09 11 19 21 29 31 39[EDX] 010 02 0A 12 1A 22 2A 32 3A[EBX] 011 03 0B 13 1B 23 2B 33 3B[--] [--] 00 100 04 0C 14 1C 24 2C 34 3Cdisp32 101 05 0D 15 1D 25 2D 35 3D[ESI] 110 06 0E 16 1E 26 2E 36 3E[EDI] 111 07 0F 17 1F 27 2F 37 3Fdisp8[EAX] 000 40 48 50 58 60 68 70 78disp8[ECX] 001 41 49 51 59 61 69 71 79disp8[EDX] 010 42 4A 52 5A 62 6A 72 7Adisp8[EPX]; 011 43 4B 53 5B 63 6B 73 7Bdisp8[--] [--] 01 100 44 4C 54 5C 64 6C 74 7Cdisp8[ebp] 101 45 4D 55 5D 65 6D 75 7Ddisp8[ESI] 110 46 4E 56 5E 66 6E 76 7Edisp8[EDI] 111 47 4F 57 5F 67 6F 77 7Fdisp32[EAX] 000 80 88 90 98 A0 A8 B0 B8disp32[ECX] 001 81 89 91 99 A1 A9 B1 B9disp32[EDX] 010 82 8A 92 9A A2 AA B2 BAdisp32[EBX] 011 83 8B 93 9B A3 AB B3 BBdisp32[--] [--] 10 100 84 8C 94 9C A4 AC B4 BCdisp32[EBP] 101 85 8D 95 9D A5 AD B5 BDdisp32[ESI] 110 86 8E 96 9E A6 AE B6 BEdisp32[EDI] 111 87 8F 97 9F A7 AF B7 BFEAX/AX/AL 000 C0 C8 D0 D8 E0 E8 F0 F8ECX/CX/CL 001 C1 C9 D1 D9 E1 E9 F1 F9EDX/DX/DL 010 C2 CA D2 DA E2 EA F2 FAEBX/BX/BL 011 C3 CB D3 DB E3 EB F3 FBESP/SP/AH 11 100 C4 CC D4 DC E4 EC F4 FCEBP/BP/CH 101 C5 CD D5 DD E5 ED F5 FDESI/SI/DH 110 C6 CE D6 DE E6 EE F6 FEEDI/DI/BH 111 C7 CF D7 DF E7 EF F7 FF</PRE><EM><H3>Notes</H3> [--] [--] means a SIB follows the ModR/M byte. disp8 denotes an 8-bit displacement following the SIB byte, to be sign-extended and added to the index. disp32 denotes a 32-bit displacement following the ModR/M byte, to be added to the index.</EM><PRE>Table 17-4. 32-Bit Addressing Forms with the SIB Byte r32 EAX ECX EDX EBX ESP [*] Base = 0 1 2 3 4 5 6 7 Base = 000 001 010 011 100 101 110 111+Scaled Index+ +SS Index+ +--------ModR/M Values in Hexadecimal--------+[EAX] 000 00 01 02 03 04 05 06 07[ECX] 001 08 09 0A 0B 0C 0D 0E 0F[EDX] 010 10 11 12 13 14 15 16 17[EBX] 011 18 19 1A 1B 1C 1D 1E 1Fnone 00 100 20 21 22 23 24 25 26 27[EBP] 101 28 29 2A 2B 2C 2D 2E 2F[ESI] 110 30 31 32 33 34 35 36 37[EDI] 111 38 39 3A 3B 3C 3D 3E 3F[EAX*2] 000 40 41 42 43 44 45 46 47[ECX*2] 001 48 49 4A 4B 4C 4D 4E 4F[ECX*2] 010 50 51 52 53 54 55 56 57[EBX*2] 011 58 59 5A 5B 5C 5D 5E 5Fnone 01 100 60 61 62 63 64 65 66 67[EBP*2] 101 68 69 6A 6B 6C 6D 6E 6F[ESI*2] 110 70 71 72 73 74 75 76 77[EDI*2] 111 78 79 7A 7B 7C 7D 7E 7F[EAX*4] 000 80 81 82 83 84 85 86 87[ECX*4] 001 88 89 8A 8B 8C 8D 8E 8F[EDX*4] 010 90 91 92 93 94 95 96 97[EBX*4] 011 98 89 9A 9B 9C 9D 9E 9Fnone 10 100 A0 A1 A2 A3 A4 A5 A6 A7[EBP*4] 101 A8 A9 AA AB AC AD AE AF[ESI*4] 110 B0 B1 B2 B3 B4 B5 B6 B7[EDI*4] 111 B8 B9 BA BB BC BD BE BF[EAX*8] 000 C0 C1 C2 C3 C4 C5 C6 C7[ECX*8] 001 C8 C9 CA CB CC CD CE CF[EDX*8] 010 D0 D1 D2 D3 D4 D5 D6 D7[EBX*8] 011 D8 D9 DA DB DC DD DE DFnone 11 100 E0 E1 E2 E3 E4 E5 E6 E7[EBP*8] 101 E8 E9 EA EB EC ED EE EF[ESI*8] 110 F0 F1 F2 F3 F4 F5 F6 F7[EDI*8] 111 F8 F9 FA FB FC FD FE FF</PRE><EM>⌨️ 快捷键说明
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