MICROPROCESSOR QUESTION AND ANSWER

Q.What is Microprocessor? Give the power supply & clock frequency of 8085 
A microprocessor is a multipurpose, programmable logic device that reads 
binary  instructions  from  a  storage  device  called  memory  accepts  binary  data  as 
input  and  processes  data  according  to  those  instructions  and  provides  result  as 
output. The power supply of 8085 is +5V and clock frequency in 3MHz. 

Q. List few applications of microprocessor-based system. 
It is used: 
i. For  measurements,  display  and  control  of  current,  voltage, temperature, pressure, etc. 
ii.  For traffic control and industrial tool control. 
iii.  For speed control of machines. 

Q. What are the functions of an accumulator? 
The  accumulator  is  the  register  associated  with  the  ALU  operations  and sometimes I/O operations. It is an integral part of ALU. It holds one of data to be processed by ALU. It also temporarily stores the result of the operation performed by the ALU. 

Q. List the 16 – bit registers of 8085 microprocessor. 
Stack pointer (SP) and Program counter (PC).

Q.  List the allowed register pairs of 8085. 
•  B-C register pair 
•  D-E register pair 
•  H-L register pair 

Q.  Mention the purpose of SID and SOD lines 
SID (Serial input data line): 
It is an input line through which the microprocessor accepts serial data. 
SOD (Serial output data line): 
It is an output line through which the microprocessor sends output serial data. 

Q.  What is an Opcode? 
The part of the instruction that specifies the operation to be performed is called the Operation code or opcode. 

Q.  What is the function of IO/M signal in the 8085? 
It is a status signal. It is used to differentiate between memory locations and  I/O  operations.  When  this  signal  is  low  (IO/M  =  0)  it  denotes the memory related operations. When this signal is high (IO/M = 1) it denotes an I/O operation. 

Q.  What is an Operand? 
The data on which the operation is to be performed is called as an Operand. 

Q.  How  many  operations  are  there  in  the  instruction  set  of 8085 microprocessor? 
There are 74 operations in the 8085 microprocessor. 

Q. List  out  the  five  categories  of  the  8085  instructions.  Give  examples  of  the instructions for each group. 
•  Data transfer group – MOV, MVI, LXI. 
•  Arithmetic group – ADD, SUB, INR. 
•  Logical group –ANA, XRA, CMP. 
•  Branch group – JMP, JNZ, CALL. 
•  Stack I/O and Machine control group – PUSH, POP, IN, HLT. 

Q. Explain the difference between a JMP instruction and CALL instruction. 
A  JMP  instruction  permanently  changes  the  program  counter.  A  CALL instruction  leaves  information  on  the  stack  so  that  the  original  program  execution sequence can be resumed. 

Q.  Explain the purpose of the I/O instructions IN and OUT. 
The  IN  instruction  is  used  to  move  data  from  an  I/O  port  into  the accumulator. 
The OUT instruction is used to move data from the accumulator to an I/O port. 
The IN & OUT instructions are used only on microprocessor, which use a separate address space for interfacing. 

Q.  What is the difference between the shift and rotate instructions? 
A rotate instruction is a closed loop instruction. That is, the data moved out at one end is put back in at the other end. The shift instruction loses the data that is moved out of the last bit locations. 

Q.  How many address lines in a 4096 x 8 EPROM CHIP? 
12 address lines. 

Q.  Control signals used for DMA operation are ____________ 
HOLD & HLDA. 

Q.  What is meant by Wait State? 
This state is used by slow peripheral devices. The peripheral devices can 
transfer the data to or from the microprocessor by using READYinput line. The 
microprocessor remains in wait state as long as READY line  is low. During the 
wait  state,  the  contents  of  the  address,  address/data  and  control  buses  are  held 
constant. 

Q.  List the four instructions which control the interrupt structure of the 8085 
microprocessor. 
•  DI ( Disable Interrupts ) 
•  EI ( Enable Interrupts ) 
•  RIM ( Read Interrupt Masks ) 
•  SIM ( Set Interrupt Masks ) 

Q.  What is meant by polling? 
Polling or device polling is a process which identifies the device that has interrupted the microprocessor. 

Q.  What is meant by interrupt? 
Interrupt is an external signal that causes a microprocessor to jump to a specific subroutine. 

Q.  Explain priority interrupts of 8085. 
The 8085 microprocessor has five interrupt inputs. They are TRAP, RST 7.5,  RST  6.5,  RST  5.5,  and  INTR.  These  interrupts  have  a  fixed  priority  of interrupt service. 
If two or more interrupts go high at the same time, the 8085 will service them on priority basis. The TRAP has the highest priority followed  bye RST 7.5, RST 6.5, RST 5.5. The priority of interrupts in 8085 is shown in the table. 
Interrupts  Priority 
TRAP 
RST 7.5 
RST 6.5 
RST 5.5 
INTR 

Q.  What is a microcomputer? 
 A computer that is designed using a microprocessor as its CPU is called microcomputer. 

Q.  What is the signal classification of 8085 
All the signals of 8085 can be classified into 6 groups 
•  Address bus 
•  Data bus 
•  Control and status signals 
•  Power supply and frequency signals 
•  Externally initiated signals 
•  Serial I/O ports 

Q.  What are operations performed on data in 8085 
The various operations performed are 
•  Store 8-bit data 
•  Perform arithmetic and logical operations 
•  Test for conditions 
•  Sequence the execution of instructions 
•  Store data temporarily during execution in the defined R/W memory locations called the stack 

Q.  Steps involved to fetch a byte in 8085 
i.  The PC places the 16-bit memory address on the address bus 
ii.  The  control  unit  sends  the  control  signal  RD  to  enable  the  memory chip 
iii.  The byte from the memory location is placed on the data bus 
iv.  The byte is placed in the instruction decoder of the microprocessor and 
the task is carried out according to the instruction 

Q.  How many interrupts does 8085 have, mention them 
The 8085 has 5 interrupt signals; they are INTR, RST7.5, RST6.5, RST5.5 
and TRAP 

Q.  Basic concepts in memory interfacing 
The  primary  function  of  memory  interfacing  is  that  the  microprocessor 
should be able to read from and write into a given register of a memory chip. To 
perform these operations the microprocessor should 
•  Be able to select the chip 
•  Identify the register 
•  Enable the appropriate buffer 

Q.  Define instruction cycle, machine cycle and T-state 
Instruction cycle is defined, as the time required completing theexecution of an instruction. Machine cycle is defined as the time required completing one operation  of  accessing  memory,  I/O  or  acknowledging  an  external  request.  Tcycle is defined as one subdivision of the operation performed in one clock period 

Q.  What is an instruction? 
An  instruction  is  a  binary  pattern  entered  through  an  input  device  to command the microprocessor to perform that specific function 

Q.  What is the use of ALE 
The ALE is used to latch the lower order address so that it can be available in T2 and T3 and used for identifying the memory address. During T1 the ALE goes  high,  the  latch  is  transparent  ie,  the  output  changes  according  to  the  input data, so the output of the latch is the lower order address. When ALE goes low the lower order address is latched until the next ALE. 

Q.  How many machine cycles does 8085 have, mention them 
The 8085 have seven machine cycles. They are 
•  Opcode fetch 
•  Memory read 
•  Memory write 
•  I/O read 
•  I/O write 
•  Interrupt acknowledge 
•  Bus idle 

Q.  Explain the signals HOLD, READY and SID 
HOLD  indicates  that  a  peripheral  such  as  DMA  controller  is  requesting 
the  use  of  address  bus,  data  bus  and  control  bus.  READY  is  used  to  delay  the 
microprocessor read or write cycles until a slow responding peripheral is ready to 
send or accept data. SID is used to accept serial data bit by bit 

Q.  Mention  the  categories  of  instruction  and  give  two  examples  for  each 
category 
The instructions of 8085 can be categorized into the following five 
•  Data transfer   MOV Rd,Rs STA 16-bit 
•  Arithmetic   ADD R  DCR M 
•  Logical   XRI 8-bit  RAR 
•  Branching   JNZ   CALL 16-bit 
•  Machine control  HLT   NOP 

Q.  Explain LDA, STA and DAA instructions 
LDA  copies  the  data  byte  into  accumulator  from  the  memory  location 
specified by the 16-bit address. STA copies the data byte from theaccumulator in 
the  memory  location  specified  by  16-bit  address.  DAA  changes  the  contents  of 
the accumulator from binary to 4-bit BCD digits. 

Q.  Explain the different instruction formats with examples 
The instruction set is grouped into the following formats 
•  One byte instruction   MOV C,A 
•  Two byte instruction   MVI A,39H 
•  Three byte instruction  JMP 2345H 

Q.  What is the use of addressing modes, mention the different types 
The various formats of specifying the operands are called addressing modes, it is used to 
access the operands or data. The different types are as follows 
•  Immediate addressing 
•  Register addressing 
•  Direct addressing 
•  Indirect addressing 
•  Implicit addressing 

Q. What is the use of bi-directional buffers? 
It is used to increase the driving capacity of the data bus. The data bus of a 
microcomputer system is bi-directional, so it requires a buffer that allows the data 
to flow in both directions. 

Q.  Define stack and explain stack related instructions 
The stack is a group of memory locations in the R/W memory thatis used 
for  the  temporary  storage  of  binary  information  during  the  execution  of  the 
program. The stack related instructions are PUSH & POP 

Q.  Why do we use XRA A instruction 
The XRA A instruction is used to clear the contents of the Accumulator and store the value 00H. 

Q.  What is Microcontroller and Microcomputer 
Microcontroller is a device that includes microprocessor; memory and I/O 
signal lines on a single chip, fabricated using VLSI technology. Microcomputer is 
a  computer  that  is  designed  using  microprocessor  as  its  CPU.  It includes 
microprocessor, memory and I/O. 

Q. Define Flags
The flags are used to reflect the data conditions in the accumulator. The 8085 
flags  are  S-Sign  flag,  Z-Zero  flag,  AC-Auxiliary  carry flag,  P-Parity  flag,  CYCarry flag 
D7 D6 D5 D4 D3 D2 D1 D0 
S  Z   AC   P   CY 

Q. How does the microprocessor differentiate between data and instruction 
When  the  first  m/c  code  of  an  instruction  is  fetched  and  decoded  in  the 
instruction register, the microprocessor recognizes the number of  bytes required 
to  fetch  the  entire  instruction.  For  example  MVI  A,  Data,  the  second  byte  is 
always considered as data. If the data byte is omitted by mistake whatever is in 
that memory location will be considered as data & the byte after the “data”will be 
treated as the next instruction. 


Q.  What is assembler 
The assembler translates the assembly language program text which is given as input 
to the assembler to their binary equivalents known as object code. The time required to 
translate the assembly code to object code is called access time. The assembler checks for 
syntax errors & displays them before giving the object code. 

Q.  What is loader 
The  loader  copies  the  program  into  the  computer’s  main  memory  at load time and begins the program execution at execution time. 

Q.  What is linker 
A linker is a program used to join together several object files into one large object 
file.  For  large  programs  it  is  more  efficient  to  divide  the large  program  modules  into 
smaller modules. Each module is individually written, tested &  debugged. When all the 
modules work they are linked together to form a large functioning program. 

Q.  Explain ALIGN & ASSUME 
The ALIGN directive forces the assembler to align the nextsegment at an address 
divisible by specified divisor. The format is ALIGN number where number can be 2, 4, 8 
or 16. Example ALIGN 8. 
The ASSUME directive assigns a logical segment to a physical segment at any given 
time.  It  tells  the  assembler  what  address  will  be  in  the  segment  registers  at  execution 
time. Example ASSUME CS: code, DS: data, SS: stack