Selected Pentium Instructions. Chapter 12 S. Dandamudi

Transcription

1 Selected Pentium Instructions Chapter 12 S. Dandamudi

2 Outline Status flags Zero flag Carry flag Overflow flag Sign flag Auxiliary flag Parity flag Arithmetic instructions Multiplication instructions Division instructions Application examples PutInt8 GetInt8 Conditional execution Indirect jumps Conditional jumps Single flags Unsigned comparisons Signed comparisons Implementing high-level language decision structures Selection structures Iteration structures S. Dandamudi Chapter 12: Page 2 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

3 Outline (cont d) Logical expressions in high-level languages Representation of Boolean data Logical expressions Logical expression evaluation Full evaluation Partial evaluation Bit instructions Bit test and modify Bit scan Illustrative examples String representation String instructions Repetition prefixes Direction flag String move instructions String compare instructions String scan instructions Illustrative examples str_len str_mov Indirect procedure call S. Dandamudi Chapter 12: Page 3 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

4 Status Flags S. Dandamudi Chapter 12: Page 4 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

5 Status Flags (cont d) Status flags are updated to indicate certain properties of the result Example: If the result is zero, zero flag is set Once a flag is set, it remains in that state until another instruction that affects the flags is executed Not all instructions affect all status flags add and sub affect all six flags inc and dec affect all but the carry flag mov, push, and pop do not affect any flags S. Dandamudi Chapter 12: Page 5 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

6 Status Flags (cont d) Example ; initially, assume ZF = 0 mov AL,55H ; ZF is still zero sub AL,55H ; result is 0 ; ZF is set (ZF = 1) push BX ; ZF remains 1 mov BX,AX ; ZF remains 1 pop DX ; ZF remains 1 mov CX,0 ; ZF remains 1 inc CX ; result is 1 ; ZF is cleared (ZF = 0) S. Dandamudi Chapter 12: Page 6 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

7 Status Flags (cont d) Zero Flag Indicates zero result If the result is zero, ZF = 1 Otherwise, ZF = 0 Zero can result in several ways (e.g. overflow) mov AL,0FH mov AX,0FFFFH mov AX,1 add AL,0F1H inc AX dec AX» All three examples result in zero result and set ZF Related instructions jz jump if zero (jump if ZF = 1) jnz jump if not zero (jump if ZF = 0) S. Dandamudi Chapter 12: Page 7 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

8 Uses of zero flag Status Flags (cont d) Two main uses of zero flag» Testing equality Often used with cmp instruction cmp char, $ ; ZF = 1 if char is $ cmp AX,BX» Counting to a preset value Initialize a register with the count value Decrement it using dec instruction Use jz/jnz to transfer control S. Dandamudi Chapter 12: Page 8 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

9 Status Flags (cont d) Consider the following code sum := 0 for (i = 1 to M) for (j = 1 to N) sum := sum + 1 end for end for Assembly code sub AX,AX ; AX := 0 mov DX,M outer_loop: mov CX,N inner_loop: inc AX loop inner_loop dec DX jnz outer_loop exit_loops: mov sum,ax S. Dandamudi Chapter 12: Page 9 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

10 Status Flags (cont d) Two observations loop instruction is equivalent to dec DX jnz outer_loop» This two instruction sequence is more efficient than the loop instruction (takes less time to execute)» loop instruction does not affect any flags! This two instruction sequence is better than initializing DX = 1 and executing inc DX cmp DX,M jle inner_loop S. Dandamudi Chapter 12: Page 10 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

11 Status Flags (cont d) Carry Flag Records the fact that the result of an arithmetic operation on unsigned numbers is out of range The carry flag is set in the following examples mov AL,0FH mov AX,12AEH add AL,0F1H sub AX,12AFH Range of 8-, 16-, and 32-bit unsigned numbers size range 8 bits 0 to 255 (2 8 1) 16 bits 0 to 65,535 (2 16 1) 32 bits 0 to 4,294,967,295 (2 32 1) S. Dandamudi Chapter 12: Page 11 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

12 Status Flags (cont d) Carry flag is not set by inc and dec instructions» The carry flag is not set in the following examples mov AL,0FFH mov AX,0 inc AL dec AX Related instructions jc jump if carry (jump if CF = 1) jnc jump if no carry (jump if CF = 0) Carry flag can be manipulated directly using stc set carry flag (set CF to 1) clc clear carry flag (clears CF to 0) cmc complement carry flag (inverts CF value) S. Dandamudi Chapter 12: Page 12 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

13 Status Flags (cont d) Uses of carry flag To propagate carry/borrow in multiword addition/subtraction 1 carry from lower 32 bits x = A AE7H y = 489B A321 FE H 7FAB C9CA 10B7 DCFAH To detect overflow/underflow condition» In the last example, carry out of leftmost bit indicates overflow To test a bit using the shift/rotate instructions» Bit shifted/rotated out is captured in the carry flag» We can use jc/jnc to test whether this bit is 1 or 0 S. Dandamudi Chapter 12: Page 13 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

14 Status Flags (cont d) Overflow flag Indicates out-of-range result on signed numbers Signed number counterpart of the carry flag The following code sets the overflow flag but not the carry flag mov AL,72H ; 72H = 114D add AL,0EH ; 0EH = 14D Range of 8-, 16-, and 32-bit signed numbers size range 8 bits 128 to to (2 7 1) 16 bits 32,768 to +32, to (2 15 1) 32 bits 2,147,483,648 to +2,147,483, to (2 31 1) S. Dandamudi Chapter 12: Page 14 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

15 Status Flags (cont d) Signed or unsigned: How does the system know? The processor does not know the interpretation It sets carry and overflow under each interpretation Unsigned interpretation mov AL,72H add AL,0EH jc overflow no_overflow: (no overflow code here).... overflow: (overflow code here).... Signed interpretation mov AL,72H add AL,0EH jo overflow no_overflow: (no overflow code here).... overflow: (overflow code here).... S. Dandamudi Chapter 12: Page 15 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

16 Status Flags (cont d) Related instructions jo jump if overflow (jump if OF = 1) jno jump if no overflow (jump if OF = 0) There is a special software interrupt instruction into interrupt on overflow Details on this instruction in Chapter 20 Uses of overflow flag Main use» To detect out-of-range result on signed numbers S. Dandamudi Chapter 12: Page 16 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

17 Status Flags (cont d) Sign flag Indicates the sign of the result Useful only when dealing with signed numbers Simply a copy of the most significant bit of the result Examples mov AL,15 add AL,97 clears the sign flag as the result is 112 (or in binary) Related instructions js jump if sign (jump if SF = 1) jns jump if no sign (jump if SF = 0) mov AL,15 sub AL,97 sets the sign flag as the result is 82 (or in binary) S. Dandamudi Chapter 12: Page 17 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

18 Usage of sign flag Status Flags (cont d) To test the sign of the result Also useful to efficiently implement countdown loops Consider the count down loop: for (i = M downto 0) <loop body> end for If we don t use the jns, we need cmp as shown below: cmp CX,0 jl for_loop The count down loop can be implemented as mov CX,M for_loop: <loop body> dec CX jns for_loop S. Dandamudi Chapter 12: Page 18 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

19 Auxiliary flag Status Flags (cont d) Indicates whether an operation produced a carry or borrow in the low-order 4 bits (nibble) of 8-, 16-, or 32- bit operands (i.e. operand size doesn t matter) Example mov AL,43 add AL,94 1 carry from lower 4 bits 43D = B 94D = B 137D = B» As there is a carry from the lower nibble, auxiliary flag is set S. Dandamudi Chapter 12: Page 19 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

20 Status Flags (cont d) Related instructions» No conditional jump instructions with this flag» Arithmetic operations on BCD numbers use this flag aaa ASCII adjust for addition aas ASCII adjust for subtraction aam ASCII adjust for multiplication aad ASCII adjust for division daa Decimal adjust for addition das Decimal adjust for subtraction Appendices I has more details on these instructions Usage» Main use is in performing arithmetic operations on BCD numbers S. Dandamudi Chapter 12: Page 20 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

21 Status Flags (cont d) Parity flag Indicates even parity of the low 8 bits of the result PF is set if the lower 8 bits contain even number 1 bits For 16- and 32-bit values, only the least significant 8 bits are considered for computing parity value Example mov AL,53 53D = B add AL,89 89D = B 142D = B» As the result has even number of 1 bits, parity flag is set Related instructions jp jump on even parity (jump if PF = 1) jnp jump on odd parity (jump if PF = 0) S. Dandamudi Chapter 12: Page 21 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

22 Status Flags (cont d) Usage of parity flag» Useful in writing data encoding programs» Example: Encodes the byte in AL (MSB is the parity bit) parity_encode PROC shl AL jp parity_zero stc ; CF = 1 jmp move_parity_bit parity_zero: clc ; CF = 0 move_parity_bit: rcr AL parity_encode ENDP S. Dandamudi Chapter 12: Page 22 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

23 Arithmetic Instructions Pentium provides several arithmetic instructions that operate on 8-, 16- and 32-bit operands» Addition: add, adc, inc» Subtraction: sub, sbb, dec, neg, cmp» Multiplication: mul, imul Discussed in Chapter 9» Division: div, idiv» Related instructions: cbw, cwd, cdq, cwde, movsx, movzx There are few other instructions such as aaa, aas, etc. that operate on decimal numbers» See Appendix I for details S. Dandamudi Chapter 12: Page 23 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

24 Arithmetic Instructions (cont d) Multiplication More complicated than add/sub» Produces double-length results E.g. Multiplying two 8 bit numbers produces a 16-bit result» Cannot use a single multiply instruction for signed and unsigned numbers add and sub instructions work both on signed and unsigned numbers For multiplication, we need separate instructions mul for unsigned numbers imul for signed numbers S. Dandamudi Chapter 12: Page 24 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

25 Arithmetic Instructions (cont d) Unsigned multiplication mul source» Depending on the source operand size, the location of the other source operand and destination are selected S. Dandamudi Chapter 12: Page 25 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

26 Arithmetic Instructions (cont d) Example mov AL,10 mov DL,25 mul DL produces 250D in AX register (result fits in AL) Theimul instruction can use the same syntax» Also supports other formats Example mov DL,0FFH ; DL = -1 mov AL,0BEH ; AL = -66 imul DL produces 66D in AX register (again, result fits in AL) S. Dandamudi Chapter 12: Page 26 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

27 Arithmetic Instructions (cont d) Division instruction Even more complicated than multiplication» Produces two results Quotient Remainder» In multiplication, using a double-length register, there will not be any overflow In division, divide overflow is possible Pentium provides a special software interrupt when a divide overflow occurs Two instructions as in multiplication div source for unsigned numbers idiv source for signed numbers S. Dandamudi Chapter 12: Page 27 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

28 Arithmetic Instructions (cont d) Dividend is twice the size of the divisor Dividend is assumed to be in AX (8-bit divisor) DX:AX (16-bit divisor) EDX:EAX (32-bit divisor) S. Dandamudi Chapter 12: Page 28 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

29 Arithmetic Instructions (cont d) S. Dandamudi Chapter 12: Page 29 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

30 Arithmetic Instructions (cont d) Example mov AX,251 mov CL,12 div CL produces 20D in AL and 11D as remainder in AH Example sub DX,DX ; clear DX mov AX,141BH ; AX = 5147D mov CX,012CH ; CX = 300D div CX produces 17D in AX and 47D as remainder in DX S. Dandamudi Chapter 12: Page 30 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

31 Arithmetic Instructions (cont d) Signed division requires some help» We extended an unsigned 16 bit number to 32 bits by placing zeros in the upper 16 bits» This will not work for signed numbers To extend signed numbers, you have to copy the sign bit into those upper bit positions Pentium provides three instructions in aiding sign extension» All three take no operands cbw converts byte to word (extends AL into AH) cwd converts word to doubleword (extends AX into DX) cdq converts doubleword to quadword (extends EAX into EDX) S. Dandamudi Chapter 12: Page 31 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

32 Arithmetic Instructions (cont d) Some additional related instructions» Sign extension cwde converts word to doubleword (extends AX into EAX)» Two move instructions movsx dest,src (move sign-extended src to dest) movzx dest,src (move zero-extended src to dest)» For both move instructions, dest has to be a register» The src operand can be in a register or memory If src is 8-bits, dest must be either a 16- or 32-bit register Ifsrc is 16-bits, dest must be a 32-bit register S. Dandamudi Chapter 12: Page 32 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

33 Arithmetic Instructions (cont d) Example mov AL,-95 cbw ; AH = FFH mov CL,12 idiv CL produces 7D in AL and 11D as remainder in AH Example mov AX,-5147 cwd ; DX := FFFFH mov CX,300 idiv CX produces 17D in AX and 47D as remainder in DX S. Dandamudi Chapter 12: Page 33 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

34 Arithmetic Instructions (cont d) Use of Shifts for Multiplication and Division Shifts are more efficient Example: Multiply AX by 32 mov CX,32 imul CX takes 12 clock cycles Using sal AX,5 takes just one clock cycle S. Dandamudi Chapter 12: Page 34 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

35 PutInt8 procedure Application Examples To display a number, repeatedly divide it by 10 and display the remainders obtained quotient remainder 108/ / / To display digits, they must be converted to their character form» This means simply adding the ASCII code for zero line 24: add AH, 0 S. Dandamudi Chapter 12: Page 35 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

36 Application Examples (cont d) GetInt8 procedure To read a number, read each digit character» Convert to its numeric equivalent» Multiply the running total by 10 and add this digit Input digit Numeric Number := Number*10 + N value (N) Initial value * = * = * = 158 S. Dandamudi Chapter 12: Page 36 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

37 Direct jump Indirect Jumps Target address is encoded in the instruction itself Indirect jump Introduces a level of indirection» Address is specified either through memory of a generalpurpose register Example jmp CX jumps to the address in CX Address is absolute» Not relative as in direct jumps S. Dandamudi Chapter 12: Page 37 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

38 Indirect Jumps (cont d) Switch (ch) { Case 0 : count[0]++; break; Case 1 : count[1]++; break; Case 2 : count[2]++; break; Case 3 : count[3]++; break; Default: count[3]++; } S. Dandamudi Chapter 12: Page 38 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

39 Indirect Jumps (cont d) Turbo C assembly code for the switch statement _main PROC NEAR... mov AL,ch cbw sub AX,48 ; 48 = ASCII for 0 mov BX,AX cmp BX,3 Indirect ja default jump shl BX,1 ; BX = BX * 2 jmp WORD PTR CS:jump_table[BX] S. Dandamudi Chapter 12: Page 39 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

40 Indirect Jumps (cont d) case_0: inc WORD PTR [BP-10] jmp SHORT end_switch case_1: inc WORD PTR [BP-8] jmp SHORT end_switch case_2: inc WORD PTR [BP-6] jmp SHORT end_switch case_3: inc WORD PTR [BP-4] jmp SHORT end_switch default: inc WORD PTR [BP-2] end_switch:... _main ENDP S. Dandamudi Chapter 12: Page 40 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

41 Indirect Jumps (cont d) jump_table LABEL WORD DW case_0 DW case_1 DW case_2 DW case_3... Jump table for the indirect jump Indirect jump uses this table to jump to the appropriate case routine The indirect jump instruction uses segment override prefix to refer to the jump_table in the CODE segment S. Dandamudi Chapter 12: Page 41 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

42 Conditional Jumps Three types of conditional jumps Jumps based on the value of a single flag» Arithmetic flags such as zero, carry can be tested using these instructions Jumps based on unsigned comparisons» Operands of cmp instruction are treated as unsigned numbers Jumps based on signed comparisons» Operands of cmp instruction are treated as signed numbers S. Dandamudi Chapter 12: Page 42 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

43 Jumps Based on Single Flags Testing for zero jz jump if zero jumps if ZF = 1 je jump if equal jumps if ZF = 1 jnz jump if not zero jumps if ZF = 0 jne jump if not equal jumps if ZF = 0 jcxz jump if CX = 0 jumps if CX = 0 (Flags are not tested) S. Dandamudi Chapter 12: Page 43 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

44 Jumps Based on Single Flags (cont d) Testing for carry jc jump if carry jumps if CF = 1 jnc jump if no carry jumps if CF = 0 Testing for overflow jo jump if overflow jumps if OF = 1 jno jump if no overflow jumps if OF = 0 Testing for sign js jump if negative jumps if SF = 1 jns jump if not negative jumps if SF = 0 S. Dandamudi Chapter 12: Page 44 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

45 Jumps Based on Single Flags (cont d) Testing for parity jp jump if parity jumps if PF = 1 jpe jump if parity jumps if PF = 1 is even jnp jump if not parity jumps if PF = 0 jpo jump if parity jumps if PF = 0 is odd S. Dandamudi Chapter 12: Page 45 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

46 Jumps Based on Unsigned Comparisons Mnemonic Meaning Condition je jump if equal ZF = 1 jz jump if zero ZF = 1 jne jump if not equal ZF = 0 jnz jump if not zero ZF = 0 ja jump if above CF = ZF = 0 jnbe jump if not below CF = ZF = 0 or equal S. Dandamudi Chapter 12: Page 46 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

47 Jumps Based on Unsigned Comparisons Mnemonic Meaning Condition jae jump if above CF = 0 or equal jnb jump if not below CF = 0 jb jump if below CF = 1 jnae jump if not above CF = 1 or equal jbe jump if below CF=1 or ZF=1 or equal jna jump if not above CF=1 or ZF=1 S. Dandamudi Chapter 12: Page 47 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

48 Jumps Based on Signed Comparisons Mnemonic Meaning Condition je jump if equal ZF = 1 jz jump if zero ZF = 1 jne jump if not equal ZF = 0 jnz jump if not zero ZF = 0 jg jump if greater ZF=0 & SF=OF jnle jump if not less ZF=0 & SF=OF or equal S. Dandamudi Chapter 12: Page 48 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

49 Jumps Based on Signed Comparisons (cont d) Mnemonic Meaning Condition jge jump if greater SF = OF or equal jnl jump if not less SF = OF jl jump if less SF OF jnge jump if not greater SF OF or equal jle jump if less ZF=1 or SF OF or equal jng jump if not greater ZF=1 or SF OF S. Dandamudi Chapter 12: Page 49 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

50 Implementing HLL Decision Structures High-level language decision structures can be implemented in a straightforward way See Section 12.4 for examples that implement if-then-else if-then-else with a relational operator if-then-else with logical operator AND if-then-else with logical operator OR while loop repeat-until loop for loops S. Dandamudi Chapter 12: Page 50 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

51 Logical Expressions in HLLs Representation of Boolean data Only a single bit is needed to represent Boolean data Usually a single byte is used» For example, in C All zero bits represents false A non-zero value represents true Logical expressions Logical instructions AND, OR, etc. are used Bit manipulation Logical, shift, and rotate instructions are used S. Dandamudi Chapter 12: Page 51 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

52 Evaluation of Logical Expressions Two basic ways Full evaluation» Entire expression is evaluated before assigning a value» PASCAL uses full evaluation Partial evaluation» Assigns as soon as the final outcome is known without blindly evaluating the entire logical expression»two rules help: cond1 AND cond2 If cond1 is false, no need to evaluate cond2 cond1 OR cond2 If cond1 is true, no need to evaluate cond2 S. Dandamudi Chapter 12: Page 52 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

53 Evaluation of Logical Expressions (cont d) Partial evaluation Used by C Useful in certain cases to avoid run-time errors Example if ((X > 0) AND (Y/X > 100)) If x is 0, full evaluation results in divide error Partial evaluation will not evaluate (Y/X > 100) if X = 0 Partial evaluation is used to test if a pointer value is NULL before accessing the data it points to S. Dandamudi Chapter 12: Page 53 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

54 Bit Instructions Bit Test and Modify Instructions Four bit test instructions Each takes the position of the bit to be tested Instruction Effect on the selected bit bt (Bit Test) No effect bts (Bit Test and Set) selected bit 1 btr (Bit Test and Reset) selected bit 0 btc selected bit NOT(selected bit) (Bit Test and Complement) S. Dandamudi Chapter 12: Page 54 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

55 Bit Instructions (cont d) All four instructions have the same format We use bt to illustrate the format bt operand,bit_pos operand is word or doubleword» Can be in a register or memory bit_pos indicates the position of the bit to be tested» Can be an immediate value or in a 16/32-bit register Instructions in this group affect only the carry flag» Other five flags are undefined S. Dandamudi Chapter 12: Page 55 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

56 Bit Scan Instructions These instructions scan the operand for a 1 bit return the bit position in a register Two instructions bsf dest_reg,operand ;bit scan forward bsr dest_reg,operand ;bit scan reverse» operand can be a word or doubleword in a register or memory» dest_reg receives the bit position Must be a 16- or 32-bit register Only ZF is updated (other five flags undefined) ZF = 1 if all bits of operand are 0 ZF = 0 otherwise (position of first 1 bit in dest_reg) S. Dandamudi Chapter 12: Page 56 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

57 Example 1 Illustrative Examples Linear search of an integer array Example 2 Selection sort on an integer array Example 3 Multiplication using shift and add operations» Multiplies two unsigned 8-bit numbers Uses a loop that iterates 8 times Example 4 Multiplication using bit instructions S. Dandamudi Chapter 12: Page 57 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

58 Two types Fixed-length Variable-length Fixed length strings String Representation Each string uses the same length» Shorter strings are padded (e.g. by blank characters)» Longer strings are truncated Selection of string length is critical» Too large ==> inefficient» Too small ==> truncation of larger strings S. Dandamudi Chapter 12: Page 58 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

59 String Representation (cont d) Variable-length strings Avoids the pitfalls associated with fixed-length strings Two ways of representation Explicitly storing string length (used in PASCAL) string DB Error message str_len DW $-string $ represents the current value of the location counter $ points to the byte after the last character of string Using a sentinel character (used in C)» Uses NULL character Such NULL-terminated strings are called ASCIIZ strings S. Dandamudi Chapter 12: Page 59 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

60 Five string instructions String Instructions LODS LOaD String source STOS STOre String destination MOVS MOVe String source & destination CMPS CoMPare String source & destination SCAS SCAn String destination Specifying operands 32-bit segments: DS:ESI = source operand ES:EDI = destination operand 16-bit segments: DS:SI = source operand ES:DI = destination operand S. Dandamudi Chapter 12: Page 60 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

61 String Instructions (cont d) Each string instruction Can operate on 8-, 16-, or 32-bit operands Updates index register(s) automatically» Byte operands: increment/decrement by 1» Word operands: increment/decrement by 2» Doubleword operands: increment/decrement by 4 Direction flag DF = 0: Forward direction (increments index registers) DF = 1: Backward direction (decrements index registers) Two instructions to manipulate DF std set direction flag (DF = 1) cld clear direction flag (DF = 0) S. Dandamudi Chapter 12: Page 61 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

62 Repetition Prefixes String instructions can be repeated by using a repetition prefix Two types Unconditional repetition rep REPeat Conditional repetition repe/repz REPeat while Equal REPeat while Zero repne/repnz REPeat while Not Equal REPeat while Not Zero S. Dandamudi Chapter 12: Page 62 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

63 Repetition Prefixes (cont d) rep while (CX 0) execute the string instruction CX := CX 1 end while CX register is first checked If zero, string instruction is not executed at all More like the JCXZ instruction S. Dandamudi Chapter 12: Page 63 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

64 repe/repz Repetition Prefixes (cont d) while (CX 0) execute the string instruction CX := CX 1 if (ZF = 0) then exit loop end if end while Useful with cmps and scas string instructions S. Dandamudi Chapter 12: Page 64 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

65 repne/repnz Repetition Prefixes (cont d) while (CX 0) execute the string instruction CX := CX 1 if (ZF = 1) then exit loop end if end while S. Dandamudi Chapter 12: Page 65 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

66 String Move Instructions Three basic instructions movs, lods, and stos Move a string (movs) Format movs dest_string,source_string movsb ; operands are bytes movsw ; operands are words movsd ; operands are doublewords First form is not used frequently Source and destination pointed by DS:(E)SI and ES:(E)DI, respectively S. Dandamudi Chapter 12: Page 66 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

67 String Move Instructions (cont d) movsb --- move a byte string ES:DI := (DS:SI) ; copy a byte if (DF=0) ; forward direction then SI := SI+1 DI := DI+1 else SI := SI 1 DI := DI 1 end if Flags affected: none ; backward direction S. Dandamudi Chapter 12: Page 67 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

68 String Move Instructions (cont d) Example.DATA string1 DB 'The original string',0 strlen EQU $ - string1 string2 DB 80 DUP (?).CODE.STARTUP mov AX,DS ; set up ES mov ES,AX ; to the data segment mov CX,strLen ; strlen includes NULL mov SI,OFFSET string1 mov DI,OFFSET string2 cld ; forward direction rep movsb S. Dandamudi Chapter 12: Page 68 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

69 String Move Instructions (cont d) Load a String (LODS) Copies the value from the source string at DS:(E)SI to AL (lodsb) AX (lodsw) EAX (lodsd) Repetition prefix does not make sense It leaves only the last value in AL, AX, or EAX register S. Dandamudi Chapter 12: Page 69 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

70 String Move Instructions (cont d) lodsb --- load a byte string AL := (DS:SI) ; copy a byte if (DF=0) ; forward direction then SI := SI+1 else SI := SI 1 end if Flags affected: none ; backward direction S. Dandamudi Chapter 12: Page 70 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

71 String Move Instructions (cont d) Store a String (STOS) Performs the complementary operation Copies the value in»al (lodsb)»ax (lodsw)» EAX (lodsd) to the destination string at ES:(E)DI Repetition prefix can be used to initialize a block of memory S. Dandamudi Chapter 12: Page 71 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

72 String Move Instructions (cont d) stosb --- store a byte string ES:DI := AL ; copy a byte if (DF=0) ; forward direction then DI := DI+1 else ; backward direction DI := DI 1 end if Flags affected: none S. Dandamudi Chapter 12: Page 72 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

73 String Move Instructions (cont d) Example: Initializes array1 with -1.DATA array1 DW 100 DUP (?).CODE.STARTUP mov AX,DS ; set up ES mov ES,AX ; to the data segment mov CX,100 mov DI,OFFSET array1 mov AX,-1 cld ; forward direction rep stosw S. Dandamudi Chapter 12: Page 73 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

74 String Move Instructions (cont d) In general, repeat prefixes are not useful with lods and stos Used in a loop to do conversions while copying mov CX,strLen mov SI,OFFSET string1 mov DI,OFFSET string2 cld ; forward direction loop1: lodsb or AL,20H stosb loop loop1 done: S. Dandamudi Chapter 12: Page 74 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

75 String Compare Instruction cmpsb --- compare two byte strings Compare two bytes at DS:SI and ES:DI and set flags if (DF=0) then SI := SI+1 DI := DI+1 else SI := SI 1 DI := DI 1 end if ; forward direction ; backward direction Flags affected: As per cmp instruction (DS:SI) (ES:DI) S. Dandamudi Chapter 12: Page 75 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

76 String Compare Instruction (cont d).data string1 DB 'abcdfghi',0 strlen EQU $ - string1 string2 DB 'abcdefgh',0.code.startup mov AX,DS ; set up ES mov ES,AX ; to the data segment mov CX,strLen mov SI,OFFSET string1 mov DI,OFFSET string2 cld ; forward direction repe cmpsb dec SI dec DI ; leaves SI & DI pointing to the last character that differs S. Dandamudi Chapter 12: Page 76 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

77 String Compare Instruction (cont d).data string1 DB 'abcdfghi',0 strlen EQU $ - string1-1 string2 DB 'abcdefgh',0.code.startup mov AX,DS ; set up ES mov ES,AX ; to the data segment mov CX,strLen mov SI,OFFSET string1 + strlen - 1 mov DI,OFFSET string2 + strlen - 1 std ; backward direction repne cmpsb inc SI ; Leaves SI & DI pointing to the first character that matches inc DI ; in the backward direction S. Dandamudi Chapter 12: Page 77 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

78 String Scan Instruction scasb --- Scan a byte string Compare AL to the byte at ES:DI and set flags if (DF=0) ; forward direction then DI := DI+1 else ; backward direction DI := DI 1 end if Flags affected: As per cmp instruction (DS:SI)-(ES:DI) scasw uses AX and scasd uses EAX registers instead of AL S. Dandamudi Chapter 12: Page 78 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

79 String Scan Instruction (cont d).data string1 DB 'abcdefgh',0 Example 1 strlen EQU $ - string1.code.startup mov AX,DS ; set up ES mov ES,AX ; to the data segment mov CX,strLen mov DI,OFFSET string1 mov AL,'e' ; character to be searched cld ; forward direction repne scasb dec DI ; leaves DI pointing to e in string1 S. Dandamudi Chapter 12: Page 79 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

80 String Scan Instruction (cont d).data string1 DB ' abc',0 strlen EQU $ - string1.code.startup mov AX,DS ; set up ES Example 2 mov ES,AX ; to the data segment mov CX,strLen mov DI,OFFSET string1 mov AL,' ' ; character to be searched cld ; forward direction repe scasb dec DI ; leaves DI pointing to the first non-blank character a S. Dandamudi Chapter 12: Page 80 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

81 Illustrative Examples LDS and LES instructions String pointer can be loaded into DS/SI or ES/DI register pair by using lds or les instructions Syntax lds register,source les register,source register should be a 16-bit register source is a pointer to a 32-bit memory operand register is typically SI in lds and DI in les S. Dandamudi Chapter 12: Page 81 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

82 Illustrative Examples (cont d) Actions of lds and les lds les register := (source) DS := (source+2) register := (source) ES := (source+2) Pentium also supports lfs, lgs, and lss to load the other segment registers S. Dandamudi Chapter 12: Page 82 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

83 Illustrative Examples (cont d) Seven popular string processing routines are given as examples in string.asm str_len str_mov str-cpy str_cat str_cmp str_chr str_cnv Given in the text S. Dandamudi Chapter 12: Page 83 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.

84 Indirect Procedure Call Direct procedure calls specify the offset of the first instruction of the called procedure In indirect procedure call, the offset is specified through memory or a register If BX contains pointer to the procedure, we can use call BX If the word in memory at target_proc_ptr contains the offset of the called procedure, we can use call target_proc_ptr These are similar to direct and indirect jumps Last slide S. Dandamudi Chapter 12: Page 84 To be used with S. Dandamudi, Fundamentals of Computer Organization and Design, Springer,.