Chapter 5The proessor status and the FLAGS registers
- 1. A solution manual to Assembly language Programming and the organization of the IBM PC Chapter 5 The processor status and the FLAGS register BY:- Ytha Yu & Charles Marut prepared by : Warda Aziz [email protected]
- 2. Question1: For each of the following instructions, give the new destination contents and the new settings of CF,SF,ZF,PF and OF. Suppose that the flags are initially 0 in each part of this question. A) ADD AX,BX ;AX=7FFFh and BX=0001h B) SUB AL, BL ;AL=01h and BL=FFh C) DEC AL ;AL=00h D) NEG AL ;AL=7Fh E) XCHG AX, BX ;AX=1ABCh, BX= 712Ah F) ADD AL, BL ;AL=80h, BL=FFh G) SUB AX, BX ; AX=0000h, BX= 8000h H) NEG AX AX=0001h ADD AX,BX ;AX=7FFFh and BX=0001h 7=0111 , F=1111 0111 1111 1111 1111 +0000 0000 0000 0001 1000 0000 0000 0000 AL=8000h CF=0 (No carry out) SF=1 ZF=0 PF=1 OF=1 (Carry in but no carry out) SUB AL, BL ;AL=01h and BL=FFh 0000 0001 -1111 1111 0000 0010 AL=02h CF=1 (borrow into MSB) SF=0 ZF=0 PF=0 OF=0 (Borrow in and borrow out) DEC AL ;AL=00h 0000 0000 -1 1111 1111 AL=FFh CF=no effect(increment/decrement do not effect CF) SF=1 ZF=0
- 3. PF=1 OF=0 (Borrow in and borrow out) NEG AL ;AL=7Fh 7=0111, F=1111 1’s compliment= 1000 0000 1000 0000 +1 1000 0001 AL=81h CF=1(result is not zero) SF=1 ZF=0 PF=1 OF=0 XCHG AX, BX ;AX=1ABCh, BX= 712Ah AX=712Ah BX=1ABCh No effect at any flag ADD AL, BL ;AL=80h, BL=FFh 1000 0000 +1111 1111 10111 1111 AL=17Fh CF=1 SF=0 ZF=0 PF=0 OF=1 SUB AX, BX ; AX=0000h, BX= 8000h 0000 0000 0000 0000 -1000 0000 0000 0000 1000 0000 0000 0000 AX=8000h CF=1 SF=1 ZF=0 PF=1 OF=1(borrow in but not borrow out) NEG AX AX=0001h 0000 0000 0000 0001 1’s compliment=1111 1111 1111 1110 1111 1111 1111 1110 +1 1111 1111 1111 1111
- 4. AX=FFFFh CF=1(result is not 0) SF=1 ZF=0 PF=1 OF=0 Question 2: a) Suppose that AX and BX both contains +ive numbers, and ADD AX, BX is executed , show that there is carry into the MSB but no carry out of the MSB if and only if, signed overflow occurs. Solution: As we know signed overflow occurs if and if: There is a carry into the MSB but no carry out of the MSB.(for addition) There is a carry out of the MSB but no carry into the MSB.(for addition) There is a borrow into the MSB but no borrow into the MSB.(for subtraction) There is a borrow out of the MSB but no borrow into the MSB.(for subtraction) So; For the proof you can see question 3 part a and d. Here signed flow is occurred because of a carry into the MSB but no carry out of the MSB. Also in part e where there is no carry out and no carry in; no signed overflow occurred. b) Suppose that AX and BX both contains -ive numbers, and ADD AX, BX is executed , show that there is carry out of the MSB but no carry into the MSB if and only if, signed overflow occurs. Solution: Let’s take an example 8Fh+C9 ; 1000 1111 +1100 1001 1 0101 1000 We can see there is a carry out of the MSB but no carry into the MSB and a signed overflow occurred. Remember: Whenever there is ‘but’ in carry of MSB or borrow of MSB there is a signed overflow. And Whenever there is an ‘and’ in carry of MSB or borrow of MSB there is no signed overflow. Question 3: Suppose ADD AX,BX is executed. In each of the following parts, the first number being added is the content of AX, and the second number is the contents of BX. Give the resulting value of AX and tell whether signed or unsigned overflow occurred. a) 512Ch + 4185h b) FE12h + 1ACBh c) E1E4h + DAB3h
- 5. d) 7132h + 7000h e) 6389h + 1176h 512Ch + 4185h 0101 0001 0010 1100 + 0100 0001 1000 0101 1001 0010 1011 0001 AX=92B1 Flow Set/clear reason Signed 1 Carry in but no carry out unsigned 0 CF=0 FE12h + 1ACBh FE12= 1111 1110 0001 0010 +1ACB= 0001 1010 1100 1011 118DD= 1 0001 1000 1101 1101 AX=118DDh Flow Set/clear reason Signed 0 carry in and carry out unsigned 1 CF=1 E1E4h + DAB3h E1E4= 1110 0001 1110 0100 + DAB3=1101 1010 1011 0011 1BC97 11011 1100 1001 0111 AX=BC97h ;1 will be in CF Flow Set/clear reason Signed 0 carry in and borrow out unsigned 1 CF=1 7132h + 7000h 7132h= 0111 0001 0011 0010 +7000h= 0111 0000 0000 0000 E132= 1110 0001 0011 0010 Flow Set/clear reason Signed 1 carry in but no carry out unsigned 0 CF=0 6389h + 1176h 6389h= 0110 0011 1000 1001 +1176h= 0001 0001 0111 0110 74FF= 0111 0100 1111 1111 Flow Set/clear reason Signed 0 No carry in, no carry out
- 6. unsigned 0 CF=0 Question 4: Suppose SUB AX,BX is executed. In each of the following parts, the first number being added is the content of AX, and the second number is the contents of BX. Give the resulting value of AX and tell whether signed or unsigned overflow occurred. a) 2143h-1986h b) 81FEh-1986h c) 19BCh-81FEh d) 0002h-FE0Fh e) 8BCDh-71ABh 2143h-1986h 2143= 0010 0001 0100 0011 1986= 0001 1001 1000 1101 0010 0001 0100 0011 - 0001 1001 1000 1101 0000 0111 1011 1101 AX=7BDh Flow Set/clear reason Signed 0 No borrow in , no borrow out unsigned 0 CF=0 81FEh-1986h 1000 0001 1111 1110 0001 1001 1000 0110 0110 1000 0111 1000 AX=6878h Flow Set/clear reason Signed 1 Borrow out but no borrow in unsigned 0 CF=0 19BCh-81FEh 0001 1001 1011 1100 1000 0001 1111 1110 1001 0111 1011 1110 AX=97BEh Flow Set/clear reason Signed 1 Borrow in but no borrow out
- 7. unsigned 1 CF=1 0002h-FE0Fh 0000 0000 0000 0010 1111 1110 0000 1111 0000 0001 1111 0011 AX=01F3h Flow Set/clear reason Signed 0 Borrow in and borrow out unsigned 1 CF=1 8BCDh-71ABh 1000 1011 1100 1101 0111 0001 1010 1011 0001 1010 0010 0010 AX=1A22h Flow Set/clear reason Signed 1 Borrow out but no borrow in unsigned 0 CF=0