Assembly Programming - Practice MCQs for CCAT

Correct Answer: A - Copies source to destination

MOV copies data from source to destination (source unchanged).

Correct Answer: B - Adds source to destination

ADD: destination = destination + source.

Correct Answer: B - Subtracts source from destination

SUB: destination = destination - source.

Correct Answer: D - Compares by subtracting, sets flags

CMP subtracts without storing result, only sets flags.

Correct Answer: B - Unconditional jump

JMP: unconditional jump to specified address.

Correct Answer: B - Zero flag is set

JZ (Jump if Zero): jumps when ZF=1.

Correct Answer: B - Pushes return address, jumps to subroutine

CALL saves return address on stack and jumps to subroutine.

Correct Answer: B - Returns from subroutine

RET pops return address from stack and returns.

Correct Answer: A - Decrements CX, loops if CX!= 0

LOOP decrements CX; if CX!= 0, jumps to label.

Correct Answer: C - Loads effective address

LEA (Load Effective Address) loads address, not data.

Correct Answer: C - Copies data from source to destination

MOV copies data from source operand to destination operand. Source remains unchanged.

Correct Answer: D - MOV copies data, XCHG exchanges data between operands

MOV copies from source to destination. XCHG exchanges contents between two operands.

Correct Answer: B - Writes data to stack and decrements SP

PUSH decrements the stack pointer and then copies the operand to the top of stack.

Correct Answer: C - Reads data from stack and increments SP

POP copies data from top of stack to operand and then increments the stack pointer.

Correct Answer: B - Pushes return address and jumps to subroutine

CALL pushes the return address (next instruction) onto stack and then jumps to the subroutine.

Correct Answer: D - Pops return address from stack and jumps to it

RET pops the return address from the stack into IP/PC, returning control to the caller.

Correct Answer: A - CALL saves return address, JMP does not

JMP simply transfers control without saving return address. CALL saves return address on stack for later RET.

Correct Answer: B - Instruction for the assembler, not converted to machine code

Assembler directives (like ORG, EQU, DB) instruct the assembler but are not converted to machine instructions.

Correct Answer: D - Sets the starting address for code/data

ORG (Origin) directive sets the memory address where the following code or data will be placed.

Correct Answer: A - Subtracts operands and sets flags without storing result

CMP subtracts second operand from first, setting flags accordingly, but does not store the result.

Correct Answer: C - Jump if Zero flag is set

JZ (Jump if Zero) transfers control if the Zero flag is set (previous operation result was zero).

Correct Answer: D - They are the same instruction

JE (Jump if Equal) and JZ (Jump if Zero) are identical - both check if Zero flag is set.

Correct Answer: C - Decrements CX and jumps if CX is not zero

LOOP decrements CX (counter) and jumps to target label if CX is not zero, used for counted loops.

Correct Answer: C - Loads effective address into register

LEA (Load Effective Address) calculates and loads the memory address of operand into register, not the value.

Correct Answer: D - No operation - does nothing but consume time

NOP (No Operation) performs no operation. Used for timing delays or reserving space for later patching.

Correct Answer: A - MOV

MOV Rd, Rs copies the content of source register (Rs) to the destination register (Rd). For example, MOV A, B copies the content of B into A.

Correct Answer: C - Immediate addressing

MVI A, 32H uses immediate addressing mode because the data (32H) is specified directly in the instruction as the second byte.

Correct Answer: A - LDA address

LDA (Load Accumulator Direct) loads the accumulator with the contents of the memory location specified by a 16-bit address. It is a 3-byte instruction.

Correct Answer: B - Stores BC pair on stack (B first, then C)

PUSH B pushes the BC register pair onto the stack. First, SP is decremented and B is stored, then SP is decremented again and C is stored.

Correct Answer: C - Decimal Adjust Accumulator for BCD arithmetic

DAA (Decimal Adjust Accumulator) adjusts the result in the accumulator after a BCD addition to ensure the result is valid BCD. It adds 06H or 60H as needed.

Correct Answer: B - CMA

CMA (Complement Accumulator) performs a bitwise complement (1's complement) of the accumulator. Each bit is inverted: 0 becomes 1 and 1 becomes 0.

Correct Answer: A - HL and DE register pairs

XCHG exchanges the contents of the HL register pair with the DE register pair. H↔D and L↔E. It is a 1-byte instruction requiring 4 T-states.

Correct Answer: D - Register indirect addressing

MOV A, M uses register indirect addressing. 'M' refers to the memory location whose address is in the HL register pair. The content at that address is moved to A.

Correct Answer: C - RAL

RAL (Rotate Accumulator Left through carry) rotates the accumulator bits left through the carry flag. The carry flag goes to bit 0, and bit 7 goes to carry.

Correct Answer: D - Indexed addressing with displacement

LEA BX, [SI+10] uses indexed addressing with displacement. The effective address is computed by adding the index register SI value with the displacement 10.

Correct Answer: D - Stops program execution until reset or interrupt

HLT stops the program execution. The processor enters a halt state and waits until an interrupt occurs or the processor is reset using RESET IN pin.

Correct Answer: C - MOVSB/MOVSW

MOVSB (Move String Byte) and MOVSW (Move String Word) are used with the REP prefix to transfer blocks of data from DS:SI to ES:DI automatically.

Correct Answer: C - HL pair from two consecutive memory locations

LHLD (Load HL Direct) loads the HL register pair from two consecutive memory locations. L is loaded from the specified address and H from the next address.

Correct Answer: A - Compares accumulator with immediate data

CPI (Compare Immediate) compares the accumulator with 8-bit immediate data by subtracting the data from A (without storing result). Flags are set based on the result.

Correct Answer: D - Software interrupt

INT 21H is a software interrupt instruction. It invokes interrupt service routine number 21H (DOS function calls). Software interrupts are triggered by the INT instruction.

Correct Answer: B - CALL and RET

CALL pushes the return address onto the stack and jumps to the subroutine address. RET pops the return address from the stack back into the PC to resume execution.

Correct Answer: A - 3 bytes

LDA is a 3-byte instruction: 1 byte for the opcode (3AH) and 2 bytes for the 16-bit memory address (lower byte first, then higher byte).

Correct Answer: D - INX H

INX H increments the 16-bit content of the HL register pair by 1. Unlike INR which works on 8-bit registers, INX works on 16-bit register pairs.

Correct Answer: A - Replaces AL with a byte from a lookup table at BX+AL

XLAT (Translate) loads AL with the byte from a table in memory. The table address is DS:BX, and AL serves as the index. Effective address = BX + AL.

Correct Answer: A - Sets the carry flag to 1

STC (Set Carry) sets the carry flag (CY) to 1. No other flags are affected by this instruction.

Correct Answer: C - Based indexed with displacement

MOV AX, [BX+SI+5] uses based indexed with displacement addressing. The effective address = BX + SI + 5, combining a base register, index register, and displacement.

Correct Answer: B - Rotate accumulator right without carry

RRC (Rotate Right Carry) rotates the accumulator right. Bit 0 goes to both the carry flag and bit 7. The carry flag is not included in the rotation path.

Correct Answer: A - Decrements the memory content pointed by HL

DCR M decrements the content of the memory location addressed by the HL register pair by 1. 'M' refers to the memory location pointed to by HL.

Correct Answer: A - Direction Flag

The Direction Flag (DF) controls the direction of string operations. DF=0 (CLD) causes auto-increment; DF=1 (STD) causes auto-decrement of SI and DI.

Correct Answer: D - Copies HL content to Program Counter

PCHL copies the contents of the HL register pair to the Program Counter. This effectively jumps to the address stored in HL, providing an indirect jump mechanism.