logical instructions

Question 1

What are the three categories of 8086 bit manipulation instructions?

Answer 1

“1. Logical Instructions: AND, OR, XOR, NOT, NEG, TEST. 2. Shift Instructions: SHL, SHR, SAL, SAR. 3. Rotate Instructions: ROL, RCL, ROR, RCR.”

Question 2

How does AND modify bits?

Answer 2

“AND destination, source: Clears specific bits using a mask (0s clear, 1s preserve). Example: AND BL, 0FH clears upper nibble of BL. Flags: CF=0, OF=0; ZF/SF reflect result.”

Question 3

What is the purpose of OR?

Answer 3

“OR destination, source: Sets specific bits using a mask (1s set, 0s preserve). Example: OR AL, 80H sets the MSB of AL. Flags: CF=0, OF=0; ZF=1 if result is zero.”

Question 4

How does XOR work?

Answer 4

“XOR destination, source: Toggles bits where the mask is 1. Example: XOR AL, 04H toggles bit 2 of AL. Also used to clear registers: XOR AX, AX zeros AX. Flags: CF=0, OF=0.”

Question 5

What is the effect of NOT?

Answer 5

“NOT destination: Inverts all bits (1’s complement). Example: NOT AL changes 55H to AAH. Does not affect flags.”

Question 6

How does NEG work?

Answer 6

“NEG destination: Computes 2’s complement (subtracts operand from 0). Example: NEG AX turns 5 into -5. Flags: CF=1 if result ≠ 0, OF=1 for overflow (e.g., NEG 8000H → OF=1).”

Question 7

What does TEST do?

Answer 7

“TEST destination, source: Performs AND without altering operands. Sets flags to test bits. Example: TEST AL, 1 → ZF=1 if AL is even. Flags: CF=0, OF=0; ZF/SF reflect result.”

Question 8

What is the difference between SHL and SAL?

Answer 8

“SHL/SAL: Both shift left. SHL for unsigned, SAL for signed (identical operation). Example: SHL AL, 1 doubles AL. Flags: CF=last bit shifted out; OF=1 if sign changes.”

Question 9

How does SHR differ from SAR?

Answer 9

“SHR: Fills MSB with 0 (unsigned). SAR: Fills MSB with sign bit (signed). Example: SAR AL, 1 halves -15 (F1H) → -8 (F8H). Flags: CF=last bit shifted out.”

Question 10

What happens in ROL vs. RCL?

Answer 10

“ROL: Rotates left; MSB → LSB and CF. RCL: Rotates left through CF; MSB → CF, CF → LSB. Example: ROL AH, 1 shifts AH left with wrap-around.”

Question 11

What is the result of ROR BH, 2 if BH=88H?

Answer 11

“BH=22H. ROR: Rotates right; LSB → MSB and CF. Example: ROR 88H (10001000b) twice → 00100010b (22H). CF=1 after first shift.”

Question 12

How to reverse bits in AL using shifts?

Answer 12

“Use SHL AL,1 and RCR BL,1 in a loop:
1. MOV CX,8
2. Shift AL left → CF
3. Rotate CF into BL right
4. Loop until CX=0. Result in BL.”

Question 13

What flags are affected by logical instructions?

Answer 13

“CF=0, OF=0. SF/ZF/PF reflect result. Example: AND BL, 0FH → ZF=1 if lower nibble is 0.”

Question 14

How to convert lowercase to uppercase with AND?

Answer 14

“Clear bit 5: AND DL, 0DFH. Example: ‘a’ (61H) → ‘A’ (41H). Mask: 11011111b.”

Question 15

What happens in NEG AX if AX=8000H?

Answer 15

“Result: AX=8000H (no change). Flags: OF=1 (overflow), CF=1, SF=1. 8000H is its own 2’s complement in 16-bit.”

Question 16

How to test if AL is even?

Answer 16

“Use TEST AL, 1. If ZF=1, AL is even. Example: TEST AL, 1 → ZF=1 if bit 0 is 0.”

Question 17

What is the result of SAR AL, 3 if AL=9AH?

Answer 17

“AL=EDH (11101101b). SAR preserves sign: 9AH (-102) → shifted right 3 times → -13 (EDH).”

Question 18

How does RCR differ from ROR?

Answer 18

“RCR: Rotates right through CF. LSB → CF, CF → MSB. Example: RCR AH,1 → CF becomes new MSB. Used for multi-byte rotations.”

Question 19

What is the effect of SHL AL, CL if CL=3 and AL=8AH?

Answer 19

“AL=50H (01010000b). CF=1 (last shifted bit). Example: SHL 8AH (10001010b) 3 times → 01010000b.”

Question 20

How to count 1s in BX using ROL?

Answer 20

1. XOR AL,AL (counter).
2. ROL BX,1 → CF=1 if bit was 1.
3. JNC NEXT; else INC AL.
4. Loop 16 times. Result in AL.