COMPUTER ARCHITECTURE UNIT NOTES

COMPUTER ARCHITECTURE UNIT NOTES by

MICROPROCESSORS & MICROCONTROLLER 2 marks

COMPUTER SCIENCE AND ENGINEERING

MICROPROCESSORS & MICROCONTROLLER

1.  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.

2.  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.

3.  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.

4.  List the 16 – bit registers of 8085 microprocessor.

Stack pointer (SP) and Program counter (PC).

5.  List the allowed register pairs of 8085.

•    B-C register pair

•    D-E register pair

•    H-L register pair

6.  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.

7.  What is an Opcode?

The part of the instruction that specifies the operation to be performed is

called the operation code or opcode.

8.  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.

9.  What is an Operand?

The data on which the operation is to be performed is called as an

Operand.

10. How    many    operations    are    there    in    the    instruction    set    of    8085 microprocessor?

There are 74 operations in the 8085 microprocessor.

11. 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.

12. 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.

13. 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.

14. 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.

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

12 address lines.

16. Control signals used for DMA operation are                         

HOLD & HLDA.

17. 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 READY input 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.

18. 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 )

19. What is meant by polling?

Polling or device polling is a process which identifies the device that has

interrupted the microprocessor.

20. What is meant by interrupt?

Interrupt is an external signal that causes a microprocessor to jump to a

specific subroutine.

21. 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	1 2 3 4 5

22. What is a microcomputer?

A computer that is designed using a microprocessor as its CPU is called

microcomputer.

23. 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

24. 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

25. 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

26. 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

27. 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

28. Define instruction cycle, machine cycle and T-state

Instruction cycle is defined, as the time required completing the execution

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

29. What is an instruction?

An instruction  is  a  binary pattern  entered  through  an  input  device  to

command the microprocessor to perform that specific function

30. 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.

31. 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

32. 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

33. 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

34. 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 the accumulator in the memory location specified by 16-bit address. DAA changes the contents of the accumulator from binary to 4-bit BCD digits.

35. 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

36. 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

37. 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.

38. Give the register organization of 8085

W(8) Temp. Reg	Z(8) Temp. Reg
B(8) Register	C(8) Register
D(8) Register	E(8) Register
H(8) Register	L(8) Register
Stack Pointer(16)
Program Counter(16)

39. Define stack and explain stack related instructions

The stack is a group of memory locations in the R/W memory that is used

for  the  temporary  storage  of  binary  information  during  the  execution  of  the program. The stack related instructions are PUSH & POP

40. 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.

41. Compare CALL and PUSH instructions

CALL	PUSH
When CALL is executed the microprocessor automatically stores the 16-bit address of the instruction next to CALL on the stack	The programmer uses the instruction PUSH to save the contents of the register pair on the stack
When CALL is executed the stack pointer is decremented by two	When PUSH is executed the stack pointer register is decremented by two

42. 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.

43. 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, CY- Carry flag

D7     D6      D5      D4      D3      D2     D1      D0

S

Z

AC

P

CY

44. 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.

45. Compare RET and POP

RET	POP
RET transfers the contents of the top two locations of the stack to the PC	POP transfers the contents of the top two locations of the stack to the specified register pair
When RET is executed the SP is incremented by two	When POP is executed the SP is incremented by two
Has 8 conditional RETURN instructions	No conditional POP instructions

46. 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.

47. 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.

48. 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.

49. Explain ALIGN & ASSUME

The ALIGN directive forces the assembler to align the next segment 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

50. Explain PTR & GROUP

A program may contain several segments of the same type. The GROUP directive collects them under a single name so they can reside in a single segment, usually a data segment. The format is Name GROUP Seg-name,…..Seg-name

PTR is used to assign a specific type to a variable or a label. It is also used to override the declared type of a variable.

51. Explain about MODEL

This directive provides short cuts in defining segments. It initializes memory model before   defining  any  segment.  The  memory  model  can  be  SMALL,  MEDIUM, COMPACT or LARGE.

Model	Code segments	Data segments
Small	One	One
Medium	Multiple	One
Compact	One	Multiple
Large	Multiple	Multiple

52. Explain PROC & ENDP

PROC directive defines the procedures in the program. The procedure name must be unique. After PROC the term NEAR or FAR are used to specify the type of procedure. Example FACT PROC FAR. ENDP is used along with PROC and defines the end of the

procedure.

53. Explain SEGMENT & ENDS

An assembly program in .EXE format consists of one or more segments. The starts of

these segments are defined by SEGMENT and the end of the segment is indicated by

ENDS directive. Format Name SEGMENT

Name ENDS

54. Explain TITLE & TYPE

The TITLE directive helps to control the format of a listing of an assembled program. It  causes a title for the program to print on line 2 of each page of the program listing.

Maximum 60 characters are allowed. Format TITLE text.

TYPE operator tells the assembler to determine the type of specified variable in bytes. For bytes the assembler gives a value 1, for word 2 & double word 4.

55. Define SOP

The segment override prefix allows the programmer to deviate from the default segment

Eg        :           MOV CS : [BX] , AL

56. Define variable

A variable is an identifier that is associated with the first byte of data item. In

assembly language statement: COUNT DB 20H, COUNT is the variable.

57. What are procedures

Procedures are a group of instructions stored as a separate program in memory and it

is called from the main program whenever required. The type of procedure depends on where the procedures are stored in memory. If it is in the same code segment as that of

the main program then it is a near procedure otherwise it is a far procedure.

58. Explain the linking process

A linker is a program used to join together several object files into one large object

file. The linker produces a link file which contains the binary codes for all the combined modules. It also produces a link map which contains the address information about the link files. The linker does not assign absolute addresses but only relative address starting from zero, so the programs are relocatable & can be put anywhere in memory to be run.

59. Explain about passing parameters using registers with example

Procedures  process  some  data  or  address  variable  from  the  main  program,  for

processing it is necessary to pass the address variables or data. This is called passing parameters to procedures. In passing parameters using registers the data to be passed is stored in registers & these registers are accessed in the procedure to process the data.

CODE SEGMENT MOV AL, DATA

CALL PRO1

PRO1 PROC NEAR MOV INPUT, AL

RET

PRO1 ENDP CODE ENDS

60. What is recursive procedures

A recursive procedure is a procedure, which calls itself. Recursive procedures are

used to work with complex data structures called trees. If the procedure is called with N=3, then the N is decremented by 1 after each procedure CALL and the procedure is called until N=0.

61. What are libraries

Library files are collection of procedures that can be used in other programs. These procedures are assembled and compiled into a library file by the LIB program.   The

library file is invoked when a program is linked with linker program. when a library file is linked only the required procedures are copied into the program. Use of library files increase s/w reusability & reduce s/w development time.

62. What are Macros

Macro is a group of instruction. The macro assembler generates the code in the program each time where the macro is called. Macros are defined by MACRO & ENDM

directives. Creating macro is similar to creating new opcodes that can be used in the program

INIT MACRO MOV AX, data

MOV DS MOV ES, AX ENDM

63. How do 8086 interrupts occur

An 8086 interrupt can come from any of the following three sources

•    External signals

•    Special instructions in the program

•    Condition produced by instruction

64. What are the 8086 interrupt types

Dedicated interrupts

•    Type 0: Divide by zero interrupt

•    Type 1: Single step interrupt

•    Type 2:Non maskable interrupt

•    Type 3: Breakpoint

•    Type 4: Overflow interrupt

Software interrupts

•    Type 0-255

65. What is interrupt service routine

Interrupt means to break the sequence of operation. While the CPU is executing a

program an interrupt breaks the normal sequence of execution of instructions & diverts its execution to some other program. This program to which the control is transferred is called the interrupt service routine.

66. Define BIOS

The IBM PC has in its ROM a collection of routines, each of which performs some specific function such as reading a character from keyboard, writing character to CRT.

This collection of routines is referred to as Basic Input Output System or BIOS.

67. Explain PUBLIC

For large programs several small modules are linked together. In order that the modules link together correctly any variable name or label referred to in other modules must be declared public in the module where it is defined. The PUBLIC directive is used

to tell the assembler that a specified name or label will be accessed from other modules. Format PUBLIC Symbol.

68. Explain DUP

The DUP directive can be used to initialize several locations & to assign values to these locations. Format Name Data_Type Num DUP (value)

Example TABLE DW 10 DUP (0). Reserves an array of 10 words of memory and

initializes all 10 words with 0. array name is TABLE.

Procedure Macro Accessed by CALL & RET instruction Accessed during assembly with name given

69. Compare Procedure & Macro

during program execution	to macro when defined
Machine code for instruction is put only once in the memory	Machine code is generated for instruction each time when macro is called
With procedures less memory is required	With macro more memory is required
Parameters can be passed in registers, memory locations or stack	Parameters passed as part of statement which calls macro

70. What is the purpose of segment registers in 8086?

There are 4 segment registers present in 8086. They are

1.  Code Segment (CS ) register

2.  Data Segment (DS ) register

3.  Stack Segment (SS ) register

4.  Extra Segment (ES ) register

The code segment register gives the address of the current code segment. ie. It will points out where the instructions, to be executed, are stored in the memory.

The data segment register points out where the operands are stored in the memory.

The stack segment registers points out the address of the current stack, which is used to store the temporary results.

If the amount of data used is more the Extra segment register points out where the large amount of data is stored in the memory.

71. Define pipelining?

In 8086, to speedup the execution of program, the instructions fetching and

execution of instructions are overlapped each other. This technique is known as pipelining.

In pipelining, when the n th instruction is executed, the n+1 th instruction is fetched and thus the processing speed is increased.

72. Discuss the function of instruction queue in 8086?

In 8086, a 6-byte instruction queue is presented at the Bus Interface Unit

(BIU). It is used to prefetch and store at the maximum of 6 bytes of instruction code from the memory. Due to this, overlapping instruction fetch with instruction execution increases the processing speed.

73. What is the maximum memory size that can be addressed by 8086?

In 8086, an memory location is addressed by 20 bit address and the address

bus is 20 bit address and the address bus is 20 bits. So it can address up to one mega byte (2^20) of memory space.

74. What is the function of the signal in 8086?

BHE signal means Bus High Enable signal. The BHE signal is made low when

there is some read or write operation is carried out. ie . When ever the data bus of the system is busy i.e. whenever there is some data transfer then the BHE signal is made low.

75.What are the predefined interrupts in 8086?

The various predefined interrupts are,

DIVISION BY ZERO  (type 0)  Interrupt. SINGLE STEP            (type 1) Interrupt. NONMASKABLE          (type2)  Interrupt. BREAK POINT           (type 3) Interrupt. OVER FLOW   (type 4) Interrupt.

76. What are the different flag available in status register of 8086?

There are 6 one bit flags are present. They are,

AF - Auxiliary Carry Flag

CF - Carry Flag

OF - Overflow Flag

SF - Sign Flag PF - Parity Flag ZF - Zero Flag

77. List the various addressing modes present in 8086?

There are 12 addressing modes present in 8086. They are,

(a) Register and immediate addressing modes

     Register addressing modes

     Immediate addressing mode

(b) Memory addressing modes.

     Direct addressing modes

     Register indirect addressing modes

     Based addressing modes

     Indexed addressing modes

     Based Indexed addressing modes

     String addressing modes

(c) I/O addressing modes

     Direct addressing mode

     Indirect addressing mode

(d) Relative addressing mode

(e) Implied addressing mode

78. How single stepping can be done in 8086?

By setting the Trace Flag (TF) the 8086 goes to single-step mode. In this mode, after

the execution of each instruction s 8086 generates an internal interrupt and by writing some interrupt  service routine we can display the content of desired registers and memory locations. So it is useful for debugging the program.

79. State the significance of LOCK signal in 8086?

If 8086 is working at maximum mode, there are multiprocessors are

present. If the system bus is given to a processor then the LOCK signal is made low. That  means the system bus is busy and it cannot be given of any other

processors. After the use of the system bus again the LOCK signal is made high. That means it is ready to give the system bus to any processor.

80. What are the functions of bus interface unit (BIU) in 8086?

(a)  Fetch instructions from memory.

(b) Fetch data from memory and I/O ports. (c) Write data to memory and I/O ports.

(d) To communicate with outside world.

(e) Provide external bus operations and bus control signals.

81. What is the clock frequency of 8086?

	8086	8086-2	8086-4
Internal clock Frequency External Clock Frequency	5 MHz 15MHZ	8MHz 24MHZ	4MHz 12MHZ

82. What are the two modes of operations present in 8086?

i.  Minimum mode (or) Uniprocessor system

ii.  Maximum mode  (or) Multiprocessor system

84. Explain the process control instructions

STC – It sets the carry flag & does not affect any other flag

CLC – it resets the carry flag to zero &does not affect any other flag

CMC – It complements the carry flag & does not affect any other flag

STD – It sets the direction flag to 1 so that SI and/or DI can be decremented automatically after execution of string instruction & does not affect other flags

CLD – It resets the direction flag to 0 so that SI and/or DI can be incremented

automatically after execution of string instruction & does not affect other flags

STI – Sets the interrupt flag to 1. Enables INTR of 8086.

CLI – Resets the interrupt flagto0. 8086 will not respond to INTR.

85. Explain REPEAT-UNTIL statements

REPEAT-UNTIL statements allow executing a series of instructions repeatedly until some condition occurs. The REPEAT defines the start of the loop & UNTIL the end of

the loop. UNTIL has a condition when the condition is true the loop is terminated

86. What is multiprogramming?

If more than one process is carried out at the same time, then it is know as multiprogramming.  Another  definition  is  the  interleaving  of  CPU  and  I/O operations among several programs is called multiprogramming.

To improve the utilization of CPU and I/O devices, we are designing to process a set of independent programs concurrently by a single CPU.

This technique is known as multiprogramming

87. Differentiate between absolute and linear select decoding?

Absolute decoding	Linear decoding
All higher address lines are defined to select the memory or I/O device	Few higher address lines are decoded to select the memory or I/O

	device
More h/w is required to design decoding logic	Hardware required to design decoding logic is less
Higher cost for decoding circuit	Less cost for decoding circuit
No multiple address	Has a disadvantage of multiple addressing
Used in large systems	Used in small systems