IKI10230 Pengantar Organisasi Komputer Kuliah no. 06: Control Structures

1. IKI10230Pengantar Organisasi KomputerKuliah no. 06: Control Structures Sumber:1. Paul Carter, PC Assembly Language2. Hamacher. Computer Organization, ed-53. Materi kuliah CS61C/2000 & CS152/1997, UCB 24 Maret 2004 L. Yohanes Stefanus (yohanes@cs.ui.ac.id)Bobby Nazief (nazief@cs.ui.ac.id) bahan kuliah: http://www.cs.ui.ac.id/kuliah/POK/

2. Control Transfer Instructions • The processor provides both: • conditional transfers • taken only for specified states of thestatus flags in the EFLAGS register • unconditional transfers • always executed

3. Control Transfer Instructions: Summary • JMP Jump • JE/JZ Jump if equal/Jump if zero • JNE/JNZ Jump if not equal/Jump if not zero • JA/JNBE Jump if above/Jump if not below or equal • JAE/JNB Jump if above or equal/Jump if not below • JB/JNAE Jump if below/Jump if not above or equal • JBE/JNA Jump if below or equal/Jump if not above • JG/JNLE Jump if greater/Jump if not less or equal • JGE/JNL Jump if greater or equal/Jump if not less • JL/JNGE Jump if less/Jump if not greater or equal • JLE/JNG Jump if less or equal/Jump if not greater • JC Jump if carry • JNC Jump if not carry • JO Jump if overflow • JNO Jump if not overflow • JS Jump if sign (negative) • JNS Jump if not sign (non-negative) • JPO/JNP Jump if parity odd/Jump if not parity • JPE/JP Jump if parity even/Jump if parity • JCXZ/JECXZ Jump register CX zero/Jump register ECX zero • LOOP Loop with ECX counter • LOOPZ/LOOPE Loop with ECX and zero/Loop with ECX and equal • LOOPNZ/LOOPNE Loop with ECX and not zero/Loop with ECX and not equal

4. JMP • The JMP (jump) instruction unconditionally transfers program control to a destination instruction. • A destination operand specifiesthe address (the instruction pointer) of the destination instruction. • The address can be arelative address or an absolute address. • A relative address is a displacement (offset) with respect to the address in the EIP register. • Thedestination address (a near pointer) is formed by adding the displacement to the address in theEIP register. • The displacement is specified with a signed integer, allowing jumps either forwardor backward in the instruction stream. • An absolute address is a offset from address 0 of a segment. It can be specified in either of thefollowing ways: • An address in a general-purpose register • An address specified using the standard addressing modes of the processor.

5. Format Instruksi JMP • JMP rel8Jump short, relative, displacement relative to next instruction • JMP rel16Jump near, relative, displacement relative to next instruction • JMP rel32Jump near, relative, displacement relative to next instruction • JMP r/m16Jump near, absolute indirect, address given in r/m16 • JMP r/m32Jump near, absolute indirect, address given in r/m32 • JMP ptr16:16Jump far, absolute, address given in operand • JMP ptr16:32Jump far, absolute, address given in operand • JMP m16:16Jump far, absolute indirect, address given in m16:16 • JMP m16:32Jump far, absolute indirect, address given in m16:32

6. Type of Target Addresses • Short jump—A near jump where the jump range is limited to –128 to +127 from the currentEIP value. • To specifya short jump, use the SHORT keyword immediately before the label inthe JMP instruction • Near jump—A jump to an instruction within the current code segment (the segmentcurrently pointed to by the CS register), sometimes referred to as an intrasegment jump. • The two byte type can be specified by putting the WORD keywordbefore the label in the JMP instruction • Far jump—A jump to an instruction located in a different segment than the current codesegment but at the same privilege level, sometimes referred to as an intersegment jump. • Task switch—A jump to an instruction located in a different task.

7. Conditional Jump: unsigned • The Jcc(conditional) jump instructions transfer program control to a destination instruction if the conditions specified with the condition code (cc) associated with the instruction are satisfied.

8. Conditional Jump: signed • Assume comparison was conducted to signed numbers

9. Format Instruksi: JCC • JA rel8Jump short if above (CF=0 and ZF=0) • JAE rel8Jump short if above or equal (CF=0) • JB rel8Jump short if below (CF=1) • JBE rel8Jump short if below or equal (CF=1 or ZF=1) • JC rel8Jump short if carry (CF=1) • JNC rel8Jump short if not carry (CF=0) • JE rel8Jump short if equal (ZF=1) • JNE rel8Jump short if not equal (ZF=0) • JZ rel8Jump short if zero (ZF = 1) • JNZ rel8Jump short if not zero (ZF=0) • JA rel16/32Jump near if above (CF=0 and ZF=0) • JAE rel16/32Jump near if above or equal (CF=0) • JB rel16/32Jump near if below (CF=1) • JBE rel16/32Jump near if below or equal (CF=1 or ZF=1) • JC rel16/32Jump near if carry (CF=1) • JNC rel16/32Jump near if not carry (CF=0) • JE rel16/32Jump near if equal (ZF=1) • JNE rel16/32Jump near if not equal (ZF=0) • JZ rel16/32Jump near if 0 (ZF=1) • JNZ rel16/32Jump near if not zero (ZF=0)

10. Simple Condition • the following pseudo-code: if ( EAX == 0 ) EBX = 1; else EBX = 2; • could be written in assembly as: • cmp eax, 0; set flags (ZF set if eax - 0 = 0) • jz thenblock ; if ZF is set branch to thenblock • mov ebx, 2 ; ELSE part of IF • jmp next ; jump over THEN part of IF • thenblock: • mov ebx, 1 ; THEN part of IF • next:

11. Complex Condition (v1) • consider the following pseudo-code: if ( EAX >= 5 ) EBX = 1; else EBX = 2; • here is assembly code that tests for these conditions(assuming that EAX is signed): • cmp eax, 5 • js signon ; goto signon if SF = 1 • jo elseblock ; goto elseblock if OF = 1 and SF = 0 • jmp thenblock ; goto thenblock if SF = 0 and OF = 0 • signon: • jo thenblock ; goto thenblock if SF = 1 and OF = 1 • elseblock: • mov ebx, 2 • jmp next • thenblock: • mov ebx, 1 • next:

12. 0 1 1 0 0 1 1 1 7 1 1 0 0 –4 3 – 5 + 0 0 1 1 + 1 0 1 1 1 0 1 0 – 6 0 1 1 1 7 Review: Overflow Detection • Overflow: the result is too large (or too small) to represent properly • Example: - 8 < = 4-bit binary number <= 7 • When adding operands with different signs, overflow cannot occur! • Overflow occurs when adding: • 2 positive numbers and the sum is negative • 2 negative numbers and the sum is positive • Overflowcan be detected by evaluating: • Carry into MSB  Carry out of MSB

13. Complex Condition (v2) • consider the following pseudo-code: if ( EAX >= 5 ) EBX = 1; else EBX = 2; • here is assembly code that tests for these conditions(assuming that EAX is signed): • cmp eax, 5 • jge thenblock • mov ebx, 2 • jmp next • thenblock: • mov ebx, 1 • next:

14. Instruksi Loop • LOOP: • decrements the contents of the ECX register (or the CX register, if theaddress-size attribute is 16), • then tests the register for the loop-termination condition • if thecount in the ECX register is non-zero, program control is transferred to the instruction addressspecified by the destination operand • when the count in the ECX registerreaches zero, program control is transferred to the instruction immediately following theLOOP instruction, which terminates the loop • LOOPE (loop while equal) & LOOPZ (loop while zero): • same as LOOP, • theyalso test the ZF flag. If the count in the ECX register is not zero and the ZF flag is set, programcontrol is transferred to the destination operand. • When the count reaches zero or the ZF flag isclear, the loop is terminated by transferring program control to the instruction immediatelyfollowing the LOOPE/LOOPZ instruction. • LOOPNE (loop while notequal) &LOOPNZ (loop while not zero)instructions operate thesame as the LOOPE/LOOPPZ instructions, except that they terminate the loop if the ZF flagis set.

15. JCXZ & JECXZ • The JECXZ (jump if ECX zero) instruction jumps to the location specified in the destinationoperand if the ECX register contains the value zero. • JCXZ (jump if CX is zero) instruction operates the same as the JECXZ instruction using CX register instead.

16. Format Instruksi: JCXZ, JECXZ, LOOP, LOOPCC • JCXZ rel8Jump short if CX register is 0 • JECXZ rel8Jump short if ECX register is 0 • LOOP rel8Decrement count; jump short if count ≠ 0 • LOOPE rel8Decrement count; jump short if count ≠ 0 and ZF=1 • LOOPZ rel8Decrement count; jump short if count ≠ 0 and ZF=1 • LOOPNE rel8Decrement count; jump short if count ≠ 0 and ZF=0 • LOOPNZ rel8Decrement count; jump short if count ≠ 0 and ZF=0

17. Loop • thefollowing pseudo-code: sum = 0; for ( i=10; i >0; i-- ) sum += i; • could be translated into assembly as: • mov eax, 0; eax is sum • mov ecx, 10; ecx is i • loop_start: • add eax, ecx • loop loop_start

18. IF Statement • The following pseudo-code: if ( condition ) then_block ; else else_block ; • could be implemented as: • ; code to set FLAGS • jxx else_block ; select xx so that branches if ; condition false • ; code for then_block • jmp endif • else_block: • ; code for else_block • endif: • if there is no ‘else’: • ; code to set FLAGS • jxxendif ; select xx so that branches if ; condition false • ; code for then_block • endif:

19. WHILE Loop • The while loop is a top tested loop: while( condition ) { body of loop; } • This could be translated into: • while: • ; code to set FLAGS based on condition • jxx endwhile ; select xx so that branches if false • ; body of loop • jmp while • endwhile:

20. DO WHILE Loop • The do while loop is a bottom tested loop: do { body of loop; } while( condition ); • This could be translated into: • do: • ; body of loop • ; code to set FLAGS based on condition • jxx do ; select xx so that branches if true

21. ~prime.c #include <stdio.h> int main() { unsigned guess; /* current guess for prime */ unsigned factor; /* possible factor of guess */ unsigned limit; /* find primes up to this value */ printf("Find primes up to: "); scanf("%u", &limit); printf("2\n"); /* treat first two primes as special case */ printf("3\n"); guess = 5; /* initial guess */ while ( guess <= limit ) { /* look for a factor of guess */ factor = 3; while ( factor*factor < guess && guess % factor != 0 ) factor += 2; if ( guess % factor != 0 ) printf("%d\n", guess); guess += 2; /* only look at odd numbers */ } return 0; }

22. Contoh: prime.asm (1/3) • %include "asm_io.inc" • segment .data • Message db "Find primes up to: ", 0 • segment .bss • Limit resd 1 ; find primes up to this limit • Guess resd 1 ; the current guess for prime • segment .text • global _asm_main • _asm_main: • enter 0,0 ; setup routine • pusha • mov eax, Message • call print_string • call read_int ; scanf("%u", & limit ); • mov [Limit], eax • mov eax, 2 ; printf("2\n"); • call print_int • call print_nl

23. Contoh: prime.asm (2/3) • mov eax, 3 ; printf("3\n"); • call print_int • call print_nl • mov dword [Guess], 5 ; Guess = 5; • while_limit: ; while ( Guess <= Limit ) • mov eax,[Guess] • cmp eax, [Limit] • jnbe end_while_limit; use jnbe since numbersare ; unsigned • mov ebx, 3 ; ebx is factor = 3; • while_factor: • mov eax,ebx • mul eax ; edx:eax = eax*eax • jo end_while_factor ; if answer won't fit in eax alone • cmp eax, [Guess] • jnb end_while_factor ; if !(factor*factor < guess) • mov eax,[Guess] • mov edx,0 • div ebx ; edx = edx:eax % ebx • cmp edx, 0 • je end_while_factor ; if !(guess % factor != 0) • add ebx,2 ; factor += 2; • jmp while_factor

24. Contoh: prime.asm (3/3) • end_while_factor: • je end_if ; if !(guess % factor != 0) • mov eax,[Guess] ; printf("%u\n") • call print_int • call print_nl • end_if: • mov eax,[Guess] • add eax, 2 • mov [Guess], eax ; guess += 2 • jmp while_limit • end_while_limit: • popa • mov eax, 0 ; return back to C • leave • ret

25. prime.exe