In the following 8051 assembly language program : MOV A, # 25H (A) A = 11H, B = 58H Choose the most appropriate answer from the options given below :
MOV B, # 78H
MUL AB
(B) A = 58H, B = 11H
(C) CY = 0 and OV = 1
(D) CY = 1 and OV = 1
(B) and (C) Only
Multiply the two bytes, then place the halves of the 16-bit product where MUL AB puts them.
Step 1 — the product.
\(25_{H}=37,\qquad 78_{H}=120\)
\(37\times120=4440=1158_{H}\)
Step 2 — where the halves go. The 8051 defines MUL AB to leave the low byte in A and the high byte in B:
\(A=58_{H},\qquad B=11_{H}\)
— statement (B). Statement (A) has them interchanged, which is the single most common error with this instruction; the mnemonic order "A, B" tempts one to expect the high byte in A.
Step 3 — the flags. Two rules apply, and they are independent:
| Flag | Rule for MUL AB | Here |
|---|---|---|
| CY | Always cleared — without exception | 0 |
| OV | Set if the product exceeds 255, i.e. if B ≠ 0 | 1 |
The carry is cleared because a multiplication cannot produce a carry in the sense the flag records — there is no ninth bit to carry into, since the whole 16-bit result is retained across the two registers. The overflow flag is used instead, and here it is set because \(B=11_{H}\neq0\), signalling that the result did not fit in a single byte. So statement (C) is correct and (D) is not.
Reading OV correctly matters in practice. A program that multiplies two bytes and expects a byte result should test OV afterwards: if it is clear, B is zero and A alone holds the answer; if it is set, both registers must be used. Testing CY instead would never detect anything, since it is always 0.
The companion instruction follows the opposite convention and is worth remembering alongside: DIV AB leaves the quotient in A and the remainder in B, clears CY, and sets OV only if B was zero — a division by zero.
Hence, the correct statements are (B) and (C).
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Important Questions from Different Processors - Teaching
Match List I with List II:
List I
(Communication Mode)
List II
(Features)
(A) Mode 0 (I) High speed; 8-bit shift register; one
baud rate of f/12
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baud rates of f/32 and f/64
Choose the correct answer from the options given below:
Which of the following is/are NOT processor control instructions?
A. STC
B. CMC
C. JNO
D. NOP
E. CWD
Choose the correct answer from the options given below:
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Read the following statements :
ST 1 : Queue is provided in 8086.
ST 2 : Slow memory does not degrade speed operation of 8086.Assertion (A) : Any program written for 8080 will run without any changes on 8085.
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