GATE 2020 EE Q.54: solution and answer

GATE 2020 · EEQ.54 · 2 marks · NAT

An 8085 microprocessor accesses two memory locations (2001H) and (2002H), that contain 8-bit numbers 98H and B1H, respectively. The following program is executed:

LXI H,2001H
MVI A, 21H
INX H
ADD M
INX H
MOV M, A
HLT

At the end of this program, the memory location 2003H contains the number in decimal (base 10) form ________.

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Answer and solution

ANSWER

Memory location 2003H ends up holding D2H, which is 210 in decimal.

Official answer: 210Source: Official answer key, GATE 2020 EE (IIT Delhi)

$LXI H,2001H$ loads the register pair with the address: $H L = 2001 H$ , which points to the location holding $98 H$ .
$MVI A, 21H$ loads the immediate byte into the accumulator: $A = 21 H$ .
$INX H$ increments the 16-bit pair: $H L = 2002 H$ . $M$ now refers to the location holding $B1H$ , not $98 H$ .
$ADD M$ computes $A \leftarrow A + M$ : $21 H + B1H = D2H$ In binary, $(00100001)_{2} + (10110001)_{2} = (11010010)_{2}$ . There is no carry out of bit 7, so the sum fits in 8 bits.
$INX H$ increments the pointer again: $H L = 2003 H$ .
$MOV M, A$ stores the accumulator at the address in $H L$ , so location $2003 H$ now holds $D2H$ . $HLT$ stops the program.
Convert to decimal: $(D2)_{16} = 13 \times 16 + 2 = 210$ . Check: $21 H = 33$ , $B1H = 177$ , and $33 + 177 = 210$ .

KEY IDEA

In the 8085, $M$ always means the memory byte at the address in $H L$ . Follow $H L$ through each $INX H$ : the first one runs before $ADD M$ , so the byte added is the one at $2002 H$ ( $B1H$ ), and $98 H$ at $2001 H$ is never read. The second $INX H$ moves $H L$ to $2003 H$ , and the sum is stored there.

WATCH OUT FOR

Adding $98 H$ as well, or adding it instead of $B1H$ , because the question names both locations. $98 H + B1H + 21 H$ overflows 8 bits and gives the wrong answer. The other common slip is writing the answer in hex ( $D2$ ), or converting $D2H$ as $12 \times 16 + 2 = 194$ by mistaking the value of the digit D, which is 13.

From the GATE 2020 EE paper. Every question of this paper · Download the full paper · All 9 Processors & memory questions, with notes

GATE 2020 EE Q.54: solution and answer

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