Al2ed chapter8

Selection and Iteration Chapter 8 S. Dandamudi

Outline Unconditional jump Compare instruction Conditional jumps Single flags Unsigned comparisons Signed comparisons Loop instructions Implementing high-level language decision structures Selection structures Iteration structures Illustrative examples Indirect jumps Multiway conditional statements

Unconditional Jump Unconditional jump transfers control to the instruction at the target address Format jmp target Specification of target Direct Target address is specified as a part of the instruction Indirect Target address is specified indirectly either through memory or a general-purpose register

Unconditional Jump (cont’d) Example Two jump instructions Forward jump jmp ECX_init_done Backward jump jmp repeat1 Programmer specifies target by a label Assembler computes the offset using the symbol table . . . mov ECX,10 jmp ECX_init_done init_CX_20: mov ECX,20 CX_init_done: mov EAX,ECX repeat1: dec ECX . . . jmp repeat1 . . .

Unconditional Jump (cont’d) Address specified in the jump instruction is not the absolute address Uses relative address Specifies relative byte displacement between the target instruction and the instruction following the jump instruction Displacement is w.r.t the instruction following jmp Reason: EIP is already pointing to this instruction Execution of jmp involves adding the displacement value to current EIP Displacement is a signed number Negative value for backward jumps Positive value for forward jumps

Target Location Inter-segment jump Target is in another segment CS = target-segment (2 bytes) EIP = target-offset (4 bytes) Called far jumps (needs five bytes to encode jmp ) Intra-segment jumps Target is in the same segment EIP = EIP + relative-displacement Uses 1-byte displacement if target is within  128 to +127 Called short jumps (needs two bytes to encode jmp ) If target is outside this range, uses 2/4-byte displacement Called near jumps (needs 3 or 5 bytes to encode jmp )

Target Location (cont’d) In most cases, the assembler can figure out the type of jump For backward jumps, assembler can decide whether to use the short jump form or not For forward jumps, it needs a hint from the programmer Use SHORT prefix to the target label If such a hint is not given Assembler reserves three bytes for jmp instruction If short jump can be used, leaves one byte of rogue data See the next example for details

Example . . . 167 0…09 EB 14 jmp SHORT ECX_init_done 0…1F – 0…0B = 0…014 168 0…0B B9 78563412 mov ECX,12345678H 169 0…10 E9 0A000000 jmp ECX_init_done 0…1F – 0…15 = 0…00A 170 init_ECX: 171 0…15 B9 12EFCDAB mov ECX,0ABCDEF12H 172 0…1A E9 52060000 jmp near_jump 0…671 – 0…01F = 0…652 173 ECX_init_done: 174 0…1F 89 C8 mov EAX,ECX

Example (cont’d) 175 repeat1: 176 0…21 49 dec ECX 177 0…22 EB FD jmp repeat1 0…21 – 0…24 = -3 = FD . . . 557 0…662 EB 05000000 jmp short_jump 0…66C – 0…667 = 5 558 0…667 B9 FFFF00FF mov ECX, 0FF00FFFFH 559 short_jump: 560 0…66C BA 32547698 mov EDX, 98765432H 561 near_jump: 562 0…671 E9 9FF9FFFF jmp init_ECX 0…015 – 0…676 = FFFFF99F

Compare Instruction Compare instruction can be used to test the conditions Format cmp destination, source Updates the arithmetic flags by performing destination - source The flags can be tested by a subsequent conditional jump instruction

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 The operands of cmp instruction are treated as unsigned numbers Jumps based on signed comparisons The operands of cmp instruction are treated as signed numbers

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)

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

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

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

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

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

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

A Note on Conditional Jumps target: . . . cmp AX,BX je target mov CX,10 . . . traget is out of range for a short jump Use this code to get around target: . . . cmp AX,BX jne skip1 jmp target skip1: mov CX,10 . . . All conditional jumps are encoded using 2 bytes Treated as short jumps What if the target is outside this range?

Loop Instructions Loop instructions use CX/ECX to maintain the count value target should be within the range of a short jump as in conditional jump instructions Three loop instructions loop target Action: ECX = ECX  1 Jump to target if ECX  0

Loop Instructions (cont’d) The following two loop instructions also test the zero flag status loope/loopz target Action: ECX = ECX  1 Jump to target if (ECX  0 and ZF = 1) loopne/loopnz target Action: ECX = ECX  1 Jump to target if (ECX  0 and ZF = 0)

Instruction Execution Times Functionally, loop instruction can be replaced by dec ECX jnz target loop instruction is slower than dec/jnz version loop requires 5/6 clocks whereas dec/jnz takes only 2 clocks jcxz also takes 5/6 clocks Equivalent code (shown below) takes only 2 clocks cmp ECX,0 jz target

Implementing HLL Decision Structures High-level language decision structures can be implemented in a straightforward way See Section 8.5 for examples that implement if-then-else if-then-else with a relational operator if-then-else with logical operators AND and OR while loop repeat-until loop for loop

Illustrative Examples Two example programs Linear search LIN_SRCH.ASM Searches an array of non-negative numbers for a given input number Selection sort SEL_SORT.ASM Uses selection sort algorithm to sort an integer array in ascending order

Indirect Jumps Jump target address is not specified directly as a part of the jump instruction With indirect jump, we can specify target via a general-purpose register or memory Example: Assuming ECX has the offset value jmp [ECX] Note: The offset value in indirect jump is the absolute value (not relative value as in the direct jumps) Program example IJUMP.ASM Uses a jump table to direct the jump

Indirect Jumps (cont’d) Another example Implementing multiway jumps We use switch statement of C We can use a table with appropriate target pointers for the indirect jump Segment override is needed jump_table is in the code segment (not in the data segment) switch (ch) { case '0': count[0]++; break; case '1': count[1]++; break; case '2': count[2]++; break; case '3': count[3]++; break; default: count[4]++; }

Indirect Jumps (cont’d) _main PROC NEAR . . . mov AL,ch cbw sub AX,48 ; 48 =‘0’ mov BX,AX cmp BX,3 ja default shl BX,1 ; BX= BX*2 jmp WORD PTR CS:jump_table[BX] 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 ;end of main jump_table LABEL WORD dw case_0 dw case_1 dw case_2 dw case_3 . . . Last slide

Al2ed chapter8

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Al2ed chapter8