1. Selected Pentium Instructions Chapter 12 S. Dandamudi

2. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

3. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

4. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 4 Status Flags

5. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

6. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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)

7. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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)

8. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 8 Status Flags (cont’d) Uses of zero flag  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

9. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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 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

10. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

11. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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 bits0 to 255 (2 8  1) 16 bits0 to 65,535 (2 16  1) 32 bits0 to 4,294,967,295 (2 32  1)

12. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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)

13. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 13 Status Flags (cont’d) Uses of carry flag  To propagate carry/borrow in multiword addition/subtraction 1  carry from lower 32 bits x = 3710 26A8 1257 9AE7H y = 489B A321 FE60 4213H 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

14. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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 +127 2 7 to (2 7  1) 16 bits  32,768 to +32,767 2 15 to (2 15  1) 32 bits  2,147,483,648 to +2,147,483,647 2 31 to (2 31  1)

15. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 15 Status Flags (cont’d) 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).... Signed or unsigned: How does the system know?  The processor does not know the interpretation  It sets carry and overflow under each interpretation

16. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

17. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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  Examplesmov AL,15 add AL,97sub AL,97 clears the sign flag as sets the sign flag as the result is 112 the result is  82 (or 0111000 in binary) (or 10101110 in binary)  Related instructions js jump if sign (jump if SF = 1) jns jump if no sign (jump if SF = 0)

18. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 18 Status Flags (cont’d) Consider the count down loop: for (i = M downto 0) 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: dec CX jns for_loop Usage of sign flag  To test the sign of the result  Also useful to efficiently implement countdown loops

19. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 19 Status Flags (cont’d) Auxiliary flag  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 1  carry from lower 4 bits mov AL,43 43D = 0010 1011B add AL,94 94D = 0101 1110B 137D = 1000 1001B »As there is a carry from the lower nibble, auxiliary flag is set

20. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

21. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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 = 0011 0101B add AL,89 89D = 0101 1001B 142D = 1000 1110B »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)

22. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

23. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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 »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 Discussed in Chapter 9

24. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

25. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

26. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 26 Arithmetic Instructions (cont’d)  Example mov AL,10 mov DL,25 mul DL produces 250D in AX register (result fits in AL) The imul 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)

27. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

28. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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)

29. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 29 Arithmetic Instructions (cont’d)

30. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

31. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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)

32. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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 –If src is 16-bits, dest must be a 32-bit register

33. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

34. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

35. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 35 Application Examples PutInt8 procedure  To display a number, repeatedly divide it by 10 and display the remainders obtained quotient remainder 108/10 10 8 10/10 10 1/10 01  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’

36. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

37. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 37 Indirect Jumps Direct jump  Target address is encoded in the instruction itself Indirect jump  Introduces a level of indirection »Address is specified either through memory of a general- purpose register  Example jmp CX jumps to the address in CX  Address is absolute »Not relative as in direct jumps

38. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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]++; }

39. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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 ja default shl BX,1 ; BX = BX * 2 jmp WORD PTR CS:jump_table[BX] Indirect jump

40. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

41. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 41 Indirect Jumps (cont’d) jump_table LABEL WORD DW case_0 DW case_1 DW case_2 DW case_3... 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 Jump table for the indirect jump

42. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

43. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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)

44. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

45. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

46. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 46 Jumps Based on Unsigned Comparisons MnemonicMeaningCondition je jump if equalZF = 1 jz jump if zeroZF = 1 jne jump if not equalZF = 0 jnz jump if not zeroZF = 0 ja jump if aboveCF = ZF = 0 jnbe jump if not below CF = ZF = 0 or equal

47. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 47 Jumps Based on Unsigned Comparisons MnemonicMeaningCondition jae jump if aboveCF = 0 or equal jnb jump if not belowCF = 0 jb jump if belowCF = 1 jnae jump if not aboveCF = 1 or equal jbe jump if belowCF=1 or ZF=1 or equal jna jump if not above CF=1 or ZF=1

48. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 48 Jumps Based on Signed Comparisons MnemonicMeaningCondition je jump if equalZF = 1 jz jump if zeroZF = 1 jne jump if not equalZF = 0 jnz jump if not zeroZF = 0 jg jump if greaterZF=0 & SF=OF jnle jump if not less ZF=0 & SF=OF or equal

49. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 49 Jumps Based on Signed Comparisons (cont’d) Mnemonic MeaningCondition jge jump if greater SF = OF or equal jnl jump if not less SF = OF jl jump if lessSF  OF jnge jump if not greaterSF  OF or equal jle jump if lessZF=1 or SF  OF or equal jng jump if not greater ZF=1 or SF  OF

50. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

51. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

52. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

53. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

54. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 54 Bit Instructions Bit Test and Modify Instructions  Four bit test instructions  Each takes the position of the bit to be tested InstructionEffect 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)

55. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

56. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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 )

57. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 57 Illustrative Examples Example 1  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

58. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 58 String Representation Two types  Fixed-length  Variable-length Fixed length strings  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

59. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

60. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 60 String Instructions Five string instructions LODSLOaD Stringsource STOSSTOre Stringdestination MOVSMOVe Stringsource & destination CMPSCoMPare Stringsource & destination SCASSCAn Stringdestination Specifying operands  32-bit segments: DS:ESI = source operand ES:EDI = destination operand  16-bit segments: DS:SI = source operand ES:DI = destination operand

61. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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)

62. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

63. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

64. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 64 Repetition Prefixes (cont’d) repe/repz 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

65. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 65 Repetition Prefixes (cont’d) repne/repnz while (CX  0) execute the string instruction CX := CX  1 if (ZF = 1) then exit loop end if end while

66. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

67. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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 ; backward direction SI := SI  1 DI := DI  1 end if Flags affected: none

68. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

69. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

70. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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 ; backward direction SI := SI  1 end if Flags affected: none

71. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

72. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

73. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

74. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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:

75. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 75 String Compare Instruction cmpsb --- compare two byte strings Compare two bytes at DS:SI and ES:DI and set flags if (DF=0); forward direction then SI := SI+1 DI := DI+1 else ; backward direction SI := SI  1 DI := DI  1 end if Flags affected: As per cmp instruction (DS:SI)  (ES:DI)

76. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

77. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

78. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

79. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 79 String Scan Instruction (cont’d).DATA string1 DB 'abcdefgh',0 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 Example 1

80. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 80 String Scan Instruction (cont’d).DATA string1 DB ' abc',0 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,' ' ; character to be searched cld ; forward direction repe scasb dec DI ; leaves DI pointing to the first non-blank character a Example 2

81. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

82. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 82 Illustrative Examples (cont’d) Actions of lds and les lds register := (source) DS := (source+2) les register := (source) ES := (source+2) Pentium also supports lfs, lgs, and lss to load the other segment registers

83. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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

84. 2003 To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.  S. Dandamudi Chapter 12: Page 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