Program Logic and Control

1. Program Logic and Control Mart 2006

2. Introduction • Instructions do not follow straight line • Transfer control forward or backward • Change IP content by special instructions • Compare • CMP, TEST • Transfer • CALL, JMP, Jnnn, LOOP, RET • Logical • AND, NOT, OR, XOR • Shift and Rotate • SAR/SHR, SAL/SHL, RCR/ROR, RCL/ROL

3. Short, Near, Far • Three type of address that are disguished by their distance from the current address. Distance is between • Short: -128 and +128 (one byte) • Near: -32K and +32K (one or two word) • Far: distance is greater than +-32K (means pass another segment , use CALL)

4. Transfer Control and Limits • JMP => short, near, far • Jnnn => short, near(+80386) • Loop => short • Call => (short), near, far Simple example JMP L10 …. L10: INC CX

5. JMP • Commanly used • Uniconditional • Flushes prefetched intruction queue • Format: • [label:] JMP short/near/far address • short JMP => EB (1byte) + address (1byte) adds the address to IP, means it is an offset • near JMP => E9(1byte) + 2 byte(80286) or 4 Byte (+80386) • Far JMP=> we will pass now

6. JMP • Forward JMP JMP L10; …. L10: … • Backward JMP L20: …. ….. JMP L20

7. Sample Program .MODE SMALL .CODE ORG 100H A10MAIN PROC NEAR MOV AX, 00 MOV, BX, 00 MOV CX, 01 A20: ADD AX, 01 ; 3 byte ADD BX, AX ; 2 byte SHL CX,1 ; 2 byte JMP A20 ; 2 byte A10MAIN ENDP END A10MAIN AX = 1,2,3,4….. BX = 1,3,6,10…. CX = 1,2,4,8…. No end Backward jmp with – 9 offset Hex representation of -9 is FFF7 Jmp addres = IP + jmp operand = 0112h + FFF7h = 109h

8. LOOP • Specified number of iteration • Initial value in CX • Automatic decrement in CX • If CX is non zero then go loop, else go next instruction • means short jmp • Format • [label:] LOOP short_address

9. Same Sample with Loop .MODE SMALL .CODE ORG 100H A10MAIN PROC NEAR MOV AX, 00 MOV BX, 00 MOV DX, 01 MOV CX, 08 A20: INC AX ; 3 byte ADD BX, AX SHL DX,1 LOOP A20 MOV AX, 4C00H INT 21H A10MAIN ENDP END A10MAIN Two variant of LOOP LOOPE /LOOPZ (while CX is zero) LOOPNE/ LOOPNZ (while CX is not zero)

10. The Flag Register C : unsigned arithmetic, shift-rotate, carry out of high order bit P : Odd parity after arithmetic operation A : skip until next chapter Z : result is zero or not S : sign of following arithmetic operation T : is single step mode or not I : Interrupt occured or not D : Direction of string processing (left to right or otherwise) O: signed arithmatic, carry out of high order sign bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Z C O D S A I T P

11. CMP • Compares two numeric data field • Format • [label:] CMP r/m, r/m/i • Cmp of string bytes (use CMPS) • Result of CMP effects A, C,O,P, Z • Example CMP DX, 00 ; Behaves like SUB JE L10 …. L10:…..

12. Conditional Jmp Instructions • JMP with condition • Format • [label:] Jnnn short-address • Example (equivalent of LOOP) DEC CX; JNZ A20 ; uses zero flag

13. Signed / Unsigned Data • Signed for numeric values, unsigned for data values • CX = 11000110 and DX = 00010110 • if they are signed then CX is smaller • İf they are unsigned CX is greater • Use suitable Jnnn’s for signed and unsigned data CMP DX, CX CMP DX,CX JB A20; for unsigned JL A20; signed data

14. Jump Based on UnSigned Data • JE/JZ = ZF • JNE/JNZ = ZF • JA/JNBE = CF,ZF • JAE/JNB = CF • JB/JNAE = CF • JBE/JNA = AF,CF

15. Jump Based on Signed Data • JE/ JZ = ZF • JNE/JNZ = ZF • JG/JNLE = OF,SF,ZF • JGE/JNL = OF, SF • JL/JNGE = OF,SF • JLE / JNG = OF,SF,ZF

16. Special Arithmetic Tests • JCXZ = none, cx is zero • JC = cf, carry on (same jb) • JNC = cf, carry off • JO = of, overflow on • JNO = of, overflow off • JP/JPE = pf, parity off or even parity • JNP/JPO= pf, parity off or odd parity • JS = sf, negative • JNS = sf, positive