# Instructions of 8085 Microprocessor

 Question 1
The content of the registers are $R_{1} = 25H, R_{2} = 30H$ and $R_{3} = 40H$. The following machine instructions are executed.
$\text{PUSH } \{R_{1}\}$
$\text{PUSH } \{R_{2}\}$
$\text{PUSH } \{R_{3}\}$
$\text{POP } \{R_{1}\}$
$\text{POP } \{R_{2}\}$
$\text{POP } \{R_{3}\}$
After execution, the content of registers $R_{1},\:R_{2},\:R_{3}$ are
 A $R_{1}=40H,\:R_{2}=30H,\:R_{3}=25H$ B $R_{1}=25H,\:R_{2}=30H,\:R_{3}=40H$ C $R_{1}=30H,\:R_{2}=40H,\:R_{3}=25H$ D $R_{1}=40H,\:R_{2}=25H,\:R_{3}=30H$
GATE EC 2021   Microprocessors
Question 1 Explanation:
For PUSH SP is decremented and for POP SP is incremented. \begin{aligned} \therefore \qquad [\mathrm{R} 1]&=40 \\ [\mathrm{R} 2]&=30 \\ [\mathrm{R} 3]&=25 \end{aligned}
 Question 2
The clock frequency of an 8085 microprocessor is 5 MHz. If the time required to execute an instruction is 1.4 $\mu s$, then the number of T-states needed for executing the instruction is
 A 1 B 6 C 7 D 8
GATE EC 2017-SET-1   Microprocessors
Question 2 Explanation:
Given than,
$f_{\mathrm{CLK}}=5 \mathrm{MHz}$
Execution time $=1.4 \mu \mathrm{s}$
Execution time $=n(T-\text { state })$
$n=$ number of T-states required to execute the instruction
T- state (or) $T_{\mathrm{CLK}}=\frac{1}{f_{\mathrm{CLK}}}=0.2 \mu \mathrm{s}$
So, $\quad n=\frac{1.4 \mu \mathrm{s}}{T_{\mathrm{CLK}}}=\frac{1.4}{0.2}=7$

 Question 3
In an 8085 microprocessor, the contents of the accumulator and the carry flag are A7 (in hex) and 0, respectively. If the instruction RLC is executed, then the contents of the accumulator (in hex) and the carry flag, respectively, will be
 A 4E and 0 B 4E and 1 C 4F and 0 D 4F and 1
GATE EC 2016-SET-3   Microprocessors
Question 3 Explanation:
RLC: Rotate accumulator left by 1 bit without carry Before RLC operation:
\begin{aligned} A &=A 7 H=(10100111)_{2} \\ C Y &=0 \end{aligned}
After RLC operation:
\begin{aligned} A &=(01001111)_{2}=4 \mathrm{FH} \\ \mathrm{CY} &=1 \end{aligned}
 Question 4
In an 8085 system, a PUSH operation requires more clock cycles than a POP operation. Which one of the following options is the correct reason for this?
 A For POP, the data transceivers remain in the same direction as for instruction fetch (memory to processor), whereas for PUSH their direction B174has to be reversed. B Memory write operations are slower than memory read operations in an 8085 based system. C The stack pointer needs to be pre-decremented before writing registers in a PUSH, whereas a POP operation uses the address already in the stack pointer. D Order of registers has to be interchanged for a PUSH operation, whereas POP uses their natural order.
GATE EC 2016-SET-1   Microprocessors
Question 4 Explanation:
The stack pointer needs to be pre-decremented before wiriting data into stack. For reading data from stack, such pre-decrement or pre-increment operations are not needed, as already stack pointer indicates the address of stack top from where the read operation takes place. Hence, PUSH operation requires more clock cycles than POP operation.
 Question 5
In an 8085 microprocessor, which one of the following instructions changes the content of the accumulator?
 A MOV B,M B PCHL C RNZ D SBI BEH
GATE EC 2015-SET-2   Microprocessors
Question 5 Explanation:
The only instruction that changes the contents of accumulator is SBI BEH.

There are 5 questions to complete.