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Lect 4: Instruction Encoding and Instruction Set

Instruction Encoding T T T T mod TTT r/m ss index base d32| 16 | 8 | nonedata32 |16 |8 | none opcode (one or two bytes) (T represents an opcode bit.) “mode r/m” byte “s-i-b” byte register and address mode specifier address displacement immediate data General Instruction Format

Encoding of Operand Length Field w FieldOperation SizeOperation SizeDuring 16-bit Data Operations 0 8 Bits 8 Bits 1 16 Bits 16 Bits Encoding of reg Field When w Field is not present in Instruction

Encoding of 16-bit Address Mode with “mod r/m” Byte

Encoding of 32-bit Address Mode with “mod r/m” Byte (no ‘s-i-b’)

Encoding of 32-bit Address Mode( “mod r/m” byte and “s-i-b” byte present): * Important Note: When index field is 100, indicating no index register, then ss field MUST equal 00. If index is 100 and ss does not equal 00, the effective address is undefined.

Encoding the instruction in Machine code Example: –MOV BL, AL ; MOV opcode byte: dw d = 0 REG field is source; w =1 for word operation; mod r/m = ; REG = 000 First byte : (88H); Second byte: (C3H) –ADD [BX][DI]+1234H, AX; ADD opcode byte : dw d=0 REG field is source; w =1 for word operation; mod r/m = ; REG = 000 First byte : (01H); Second byte: (81H) Resulting Machine code: H

Encoding the instruction in Machine code –MOV ECX,2 ; MOV opcode byte: 1011w reg (immediate to reg short form) MOV AX,[EBX+2*ECX] MOV opcode byte: w (reg/mem to reg) MOV ECX,2 ; (B9H) MOV AX, [EBX+2*ECX] First byte : Second Byte: Third byte: mod REG r/m ss index base The Resulting Machine code: 8B044B Address size prefix : (67) ; Operand size prefix: (66) The Result: 66 | B | 8B 04 4B

Instruction Set base instruction set encoding table: See Fig 4.5 Instruction Types –General Data Transfer –Arithmetic/Logic –Shift/Rotate –Bit test and bit scan –Flag Control –Control Transfer –String handling –System Control

Data Transfer Instructions –MOV : 8 instructions ; operation: (dest)  (src), Flags Affected: none MOV AX,[BP] mod reg r/m displacement There are several machine codes to represent one MOV instruction. The assembler must choose an efficient one. –MOVSX: sign-extend MOVSX EBX, AX (reg from reg/mem) –MOVZX: zero-extend MOVZX CX, DATA (reg from reg/mem) General Data Transfer

General Data Transfer –XCHG operation (dest)  (src) ; flags affected : none reg/mem with reg, reg with accumulator (short form) XCHG AX,DX –XLAT : Translate String ; operation (AL)  ((AL)+(BX)) ; flags affected: none –Load Effective Address and Load Pointer Instructions LEA : Load effective address ; LEA SI, [DI+BX+5H] LDS, LSS, LES, LFS, LGS : Load full pointer (offset: SBA) into reg and seg reg –PUSH operation : ((SP))  (src) (SP)  (SP)-2 flags affected: none –POP –PUSHA, POPA (all general registers onto/from stack) –PUSHF, POPF

Arithmetic Instructions ADD, ADC, INC, AAA, DAA SUB, SBB, DEC, NEG, AAS, DAS MUL, IMUL, AAM, DIV, IDIV, AAD, CBW, CWD –AAA : ASCII adjust for addition; flags affected: AF, CF, OF, SF, ZF, PF undefined operation: ALcarry  (AL) >0F9H; if ((AL)&0FH) >9 or (AF) = 1 then (AL)  (AL)+6)&0FH; (AH)  (AH)+1+ALcarry; (AF)  1; (CF)  1; else AF  0; CF  0; (AL)  (AL)&0FH ;

Arithmetic Instructions Example : AL : 32H ASCII code for number 2; BL: 34H ASCII code for 4 ADD AL,BL ---- AL :66H AAA AL : 6H –DAA (Decimal adjust for addition: two packed decimal) operation: if ((AL)&0FH)>9 or (AF)=1 then (AL)  (AL)+6, (AF)  1 if ((AL)&0F0H) >90H or (CF) =1 then (AL)  (AL)+60H, (CF)  1

Logic Instructions –AND, OR, XOR, NOT Shift Instructions –SAL/SHL, SHR/SAR, SHLD, SHRD: –SHL AX, 1; SAR AX, CL Rotate Instructions –ROL, ROR, RCL, RCR –ROR AX,1; ROL AX, CL

Bit Test Instructions Bit Test and Bit Scan Instructions –BT(Bit Test), BTR(Bit test and reset), BTS(Bit test and set), BTC(Bit test and complement), BSF(Bit scan forward), BSR(Bit scan reverse) –BT D, S ; CF  BIT[D,S]; saves the value of the bit indicated by the first operand and the bit offset into the CF flag –BSF r16,r/m16 or BSF r32, r/m32 ; scans the bits in the second word or double word operand starting with bit 0. The ZF flag is set if all the bits are 0; otherwise, the ZF flag is cleared and the destination register is loaded with the bit index of the first set bit.

Flag Control Set Flag-Control Instructions –LAHF(Load AH from flags), SAHF(Store AH into flags), CLC, STC, CMC, CLI, STI –AH : |SF|ZF| - |AF| - |PF| - |CF| Compare and Set Instructions –CMP –SETcc r/m8: if condition then r/m8  1(not FF) else r/m8  0; SETA, SETAE,.....

Control Transfer Jump Instructions –Unconditional(JMP) and Conditional Jumps(JA, JAE,.... ) –JMP : Intrasegment jump, Intersegment jump Intrasegment jump : short(8-bit displacement: -126 to +129), direct within segment (16-bit or 32-bit relative), r/m indirect with segment Intersegment jump : direct intersegment (full offset and selector), indirect intersegment –Jcc : conditional jump; 8-bit or full displacement JA, JAE,....

Control Transfer Subroutines and Subroutine-Handling Instructions –CALL and Return Instructions –CALL : intrasegment and intersegment call; Intrasegment : CALL rel16, or rel32, CALL indirect within segment Intersegment : CALL direct intersegment, CALL indirect intersegment –RET Intrasegment : RET within segment, RET within segment Adding Immediate to SP Intersegment : RET, RET adding immediate to SP

Control Transfer –Stack Frame Instructions: ENTER and LEAVE to allocate and deallocate a data area called a stack frame ENTER : make a stack frame –ENTER imm16,0; Make procedure stack frame –ENTER imm16,1; Make stack frame for procedure parameter –ENTER imm16,imm8: Make stack frame for procedure parameter –first operand : the number of bytes to be allocated on the stack for local data storage second operand: lexical nesting level of the routine

Control Transfer Procedure AProcedure B Enter 32, 1Enter 12, 2 Leave Ret Procedure C Enter 16, 3 Leave Ret Leave Ret Data for Proc. C (16 bytes) BP for Proc. C BP for Proc. B BP for Proc. A BP for Proc. B Ret addr for proc. B Data for Proc. B (12 bytes) BP for Proc. B BP for Proc. A Ret addr for proc. A Data for Proc. A (32 bytes) BP for Proc. A Old BP Stack frame for A Stack frame for C Stack frame for B BP when executing Procedure C BP when executing Procedure B BP when executing Procedure A

String Handling Loop and Loop-handling Instructions –LOOP, LOOPE/Z, LOOPNE/NZ: CX must be preloaded with a count String and String-Handling Instructions –MOVSB/W/D, compare string, scan string, load string, store string –the contents of both SI and DI are automatically incremented or decremented. –REP : repeat string; Check Array Index Against Bounds –BOUND r16, m16&16 : check if r16 is within bounds –BOUND r32, m32&m32: check if r32 is within bounds operation: if (LeftSRC [RightSRC+OperandSize/8]) then Interrupt 5;