1. COMPUTER ARCHITECTURE & ORGANISATION - COA

28. Registers 28

29. Computer Register 29

30. Types of Registers in 8085 It has eight addressable 8-bit registers: A, B, C, D, E, H, L, F, and two 16-bit registers PC and SP. These registers can be classified as  General Purpose Registers Temporary Registers: a) Temporary data register b) W and Z registers  Special Purpose Registers: a)Accumulator b) Flag registers c) Instruction register Sixteen-bit Registers: a) ProgramCounter (PC) b) Stack Pointer (SP) 30

31. General Purpose Registers Registers B, C, D, E, H, and L are general purpose registers in 8085 Microprocessor.  All these GPRS are 8-bits wide. They are less important than the accumulator. They are used to store data temporarily during the execution of the program.  It is possible to use these registers as pairs to store 16-bit information.  Only BC, DE, and HL can form register pairs.  When they are used as register pairs in an instruction, the left register is understood to have the MS byte and the right register the LS byte.  For example, in DE register pair, the content of the D register is treated as the MS byte, and the content of E register is treated as the LS byte. 31

32. Temporary Registers  (a)Temporary Data Register - The ALU has two inputs. One input is supplied by the accumulator and other from the temporary data register. The programmer cannot access this temporary data register. However, it is internally used for execution of most of the arithmetic and logical instructions.  (b) W and Z registers - Wand Z registers are temporary registers. These registers are used to hold 8-bit data during the execution of some instructions. These registers are not available for the programmer since 8085Microprocessor Architecture uses them internally. 32

33. Special Purpose Registers  (a) Register A(Accumulator) - Register A is an 8-bit register used in 8085 to perform arithmetic, logical, I/O & LOAD/STORE operations. Register A is quite often called as an Accumulator. An accumulator is a register for short-term, intermediate storage of arithmetic and logic data in a computer's CPU (Central Processing Unit).  (b)Flag Register - It is a 3-bit register, in which five of the bits carry significant information in the form of flags: S (Sign flag), Z (Zero flag), AC(Auxiliary carry flag), P (Parity flag), and CY (carry flag); as shown in Fig. 1.2. 33

50. General Register Organization

51. R1 R2 R3 R4 R5 R6 R7 MUX - AMUX - B Arithmetic Logic Unit (ALU) 3x8 decoder Clock Input Output SEL BSEL A SEL D OPR Load (7 lines) A - busB - bus

52. General Register Organization  Example: R1 R2 + R3  To perform the above operation, the control must provide binary selection variables to the following selector inputs: 1.MUX A selector (SELA): to place the content of R2 into bus A. 2.MUX B selector (SELB): to place the content of R3 into bus B. 3.ALU operation selector (OPR): to provide the arithmetic addition A + B. 4.Decoder destination selector (SELD): to transfer the content of the output bus into R1.  Control Word: SELASELBSELDOPR

53. General Register Organization Binary Code SELASELBSELD 000Input None 001R1 010R2 011R3 100R4 101R5 110R6 111R7 OPR SelectOperationSymbol 00000Transfer ATSFA 00001Increment AINCA 00010A + BADD 00101A – BSUB 00110Decrement ADECA 01000A and BAND 01010A or BOR 01100A xor BXOR 01110Complement ACOMA 10000Shift right ASHRA 11000Shift left ASHLA Encoding of Register Selection Fields Encoding of ALU Operations

54. Stack Organization

55.  A stack is a storage device that stores information in such a manner that the item stored last is the first item retrieved (LIFO).  The register that holds the address for the stack is called a stack pointer (SP) because its value always points at the top item in the stack.  The physical registers of a stack are always available for reading or writing. It is the content of the word that is inserted or deleted.  There are two types of stack organization 1.Register stack – built using registers 2.Memory stack – logical part of memory allocated as stack

56. Register Stack C B A DR FULLEMTY SP 63 3 2 1 0 4 Address

57. Register Stack  A stack can be placed in a portion of a large memory or it can be organized as a collection of a finite number of memory words or registers. Figure shows the organization of a 64-word register stack.  The stack pointer register SP contains a binary number whose value is equal to the address of the word that is currently on top of the stack.  In a 64-word stack, the stack pointer contains 6 bits because 2 6 = 64.  Since SP has only six bits, it cannot exceed a number greater than 63 (111111 in binary).  The one-bit register FULL is set to 1 when the stack is full, and the one- bit register EMTY is set to 1 when the stack is empty of items.  DR is the data register that holds the binary data to be written into or read out of the stack.

58. Register Stack  PUSH Operation SP ← SP + 1 M[SP] ← DR IF (SP= 0) then (FULL ← 1) EMTY ← 0  POP Operation DR ← M[SP] SP ← SP - 1 IF (SP= 0) then (EMTY ← 1) FULL ← 0

59. Memory Stack Program (instructions) Data (operands) Stack DR 1000 Address 2000 3000 3997 3998 3999 4000 4001 PC AR SP

60. Memory Stack  The implementation of a stack in the CPU is done by assigning a portion of memory to a stack operation and using a processor register as a stack pointer.  Figure shows a portion of computer memory partitioned into three segments: program, data, and stack.  The program counter PC points at the address of the next instruction in the program which is used during the fetch phase to read an instruction.  The address registers AR points at an array of data which is used during the execute phase to read an operand.  The stack pointer SP points at the top of the stack which is used to push or pop items into or from the stack.  We assume that the items in the stack communicate with a data register DR.

61. Memory Stack  PUSH Operation SP ← SP - 1 M[SP] ← DR  POP Operation DR ← M[SP] SP ← SP + 1

62. Reverse Polish Notation  The common mathematical method of writing arithmetic expressions imposes difficulties when evaluated by a computer.  The Polish mathematician Lukasiewicz showed that arithmetic expressions can be represented in prefix notation as well as postfix notation. A + B+ ABAB + InfixPrefix or PolishPostfix or reverse Polish A * B + C * DAB * CD * + Reverse Polish

63. Evaluation of Arithmetic Expression (3 * 4) + (5 * 6)3 4 * 5 6 * + 3 4 312 5 6 5 30 1242 34*56*+

64. Instruction Formats

65.  Instructions are categorized into different formats with respect to the operand fields in the instructions. 1.Three Address Instructions 2.Two Address Instruction 3.One Address Instruction 4.Zero Address Instruction 5.RISC Instructions

66. Three Address Instruction  Computers with three-address instruction formats can use each address field to specify either a processor register or a memory operand.  The program in assembly language that evaluates X = (A + B) * (C + D) is shown below.  The advantage of three-address format is that it results in short programs when evaluating arithmetic expressions.  The disadvantage is that the binary-coded instructions require too many bits to specify three addresses. ADDR1, A, B R1 ← M[A]+ M[B] ADDR2, C, D R2 ← M[C]+ M[D] MULX, R1, R2 M[X] ← R1 * R2

67. Two Address Instruction  Two address instructions are the most common in commercial computers. Here again each address field can specify either a processor register or a memory word.  The program to evaluate X = (A + B) * (C + D) is as follows: MOVR1, A R1 ← M[A] ADDR1, B R1 ← R1+ M[B] MOVR2, C R2 ← M[C] ADDR2, D R2 ← R2+ M[D] MULR1, R2 R1 ← R1 * R2 M[X] ← R1 MOVX, R1

68. One Address Instruction  One address instructions use an implied accumulator (AC) register for all data manipulation.  For multiplication and division these is a need for a second register.  However, here we will neglect the second register and assume that the AC contains the result of all operations.  The program to evaluate X = (A + B) * (C + D) is LOADA AC ← M[A] ADDB AC ← AC+M[B] STORET M[T] ← AC LOADC AC ← M[C] ADDD AC ← AC+M[D] MULT AC ← AC*M[T] STOREX M[X] ← AC

69. Zero Address Instruction  A stack-organized computer does not use an address field for the instructions ADD and MUL.  The PUSH and POP instructions, however, need an address field to specify the operand that communicates with the stack.  The program to evaluate X = (A + B) * (C + D) will be written for a stack- organized computer.  To evaluate arithmetic expressions in a stack computer, it is necessary to convert the expression into reverse polish notation. PUSHA TOS ← M[A] PUSHB TOS ← M[B] ADD TOS ← (A+B) PUSHC TOS ← M[C] PUSHD TOS ← M[D] ADD TOS ← (C+D) MUL TOS ← (C+D)*(A+B) POPX M[X] ← TOS

70. RISC Instruction  The instruction set of a typical RISC processor is restricted to the use of load and store instructions when communicating between memory and CPU.  All other instructions are executed within the registers of the CPU without referring to memory.  A program for a RISC type CPU consists of LOAD and STORE instructions that have one memory and one register address, and computational-type instructions that have three addresses with all three specifying processor registers.  The following is a program to evaluate X = (A + B) * (C + D) LOADR1, A R1 ← M[A] LOADR2, B R2 ← M[B] LOADR3, C R3 ← M[C] LOADR4, D R4 ← M[D] ADDR1, R1, R2 R1 ← R1+R2 ADDR3, R3, R4 R3 ← R3+R4 MULR1, R1, R3 R1 ← R1*R3 STOREX, R1 M[X] ← R1

71. Addressing Modes Unit – 5: Central Processing Unit Darshan Institute of Engineering & Technology

72. Addressing Modes  The addressing mode specifies a rule for interpreting or modifying the address field of the instruction before the operand is actually referenced.  Computers use addressing mode techniques for the purpose of accommodating one or both of the following provisions: 1.To give programming versatility to the user by providing such facilities as pointers to memory, counters for loop control, indexing of data, and program relocation. 2.To reduce the number of bits in the addressing field of the instruction.  There are basic 10 addressing modes supported by the computer.

73. Addressing Modes 1.Implied Mode 2.Immediate Mode 3.Register Mode 4.Register Indirect Mode 5.Autoincrement or Autodecrement Mode 6.Direct Address Mode 7.Indirect Address Mode 8.Relative Address Mode 9.Indexed Addressing Mode 10.Base Register Addressing Mode

74. 1. Implied Mode  Operands are specified implicitly in the definition of the instruction.  For example, the instruction “complement accumulator (CMA)” is an implied-mode instruction because the operand in the accumulator register is implied in the definition of the instruction.  In fact, all register reference instructions that use an accumulator and zero address instructions are implied mode instructions.

75. 2. Immediate Mode  Operand is specified in the instruction itself.  In other words, an immediate-mode instruction has an operand field rather than an address field.  The operand field contains the actual operand to be used in conjunction with the operation specified in the instruction.  Immediate mode of instructions is useful for initializing register to constant value.  E.g. MOV R1, 05H instruction copies immediate number 05H to R1 register.

76. 3. Register Mode  Operands are in registers that reside within the CPU.  The particular register is selected from a register field in the instruction.  E.g. MOV AX, BX move value from BX to AX register

77. 4. Register Indirect Mode  In this mode the instruction specifies a register in the CPU whose contents give the address of the operand in memory.  Before using a register indirect mode instruction, the programmer must ensure that the memory address of the operand is placed in the processor register with a previous instruction.  The advantage of this mode is that address field of the instruction uses fewer bits to select a register than would have been required to specify a memory address directly.  E.g. MOV [R1], R2 value of R2 is moved to the memory location specified in R1.

78. 5. Auto-increment or Auto-decrement Mode  This is similar to the register indirect mode expect that the register is incremented or decremented after (or before) its value is used to access memory.  When the address stored in the register refers to a table of data in memory, it is necessary to increment or decrement the register after every access to the table. This can be achieved by using the increment or decrement instruction.

79. 6. Direct Address Mode  In this mode the effective address is equal to the address part of the instruction.  The operand resides in memory and its address is given directly by the address field of the instruction.  E.g. ADD 457

80. 7. Indirect Address Mode  In this mode the address field of the instruction gives the address where the effective address is stored in memory.  Control fetches the instruction from memory and uses its address part to access memory again to read the effective address.  The effective address in this mode is obtained from the following computational: Effective address = address part of instruction + content of CPU register

81. 8. Relative Address Mode  In this mode the content of the program counter is added to the address part of the instruction in order to obtain the effective address.  The address part of the instruction is usually a signed number which can be either positive or negative. Effective address = address part of instruction + content of PC

82. 9. Indexed Addressing Mode  In this mode the content of an index register is added to the address part of the instruction to obtain the effective address.  The indexed register is a special CPU register that contain an index value.  The address field of the instruction defines the beginning address of a data array in memory.  Each operand in the array is stored in memory relative to the begging address. Effective address = address part of instruction + content of index register

83. 10. Base Register Addressing Mode  In this mode the content of a base register is added to the address part of the instruction to obtain the effective address.  A base register is assumed to hold a base address and the address field of the instruction gives a displacement relative to this base address.  The base register addressing mode is used in computers to facilitate the relocation of programs in memory. Effective address = address part of instruction + content of base register

84. A program residing in the memory unit of the computer consists of a sequence of instructions. In the basic computer each instruction cycle consists of the following phases:​  Fetch an instruction from memory.​  Decode the instruction.​  Read the effective address from memory if the instruction has an indirect address.​  Execute the instruction Instruction Cycle

85.  Fetch & Decode PC is loaded with the address of the first instruction in the program. The micro-operations for fetch and decode phases are as follows:

86. Start SC ← 0 AR ← PC IR ← M[AR], PC ← PC + 1 Decode operation code in IR(12-14) AR ← IR(0-11), I ← IR(15) D7D7 II Execute input-output instruction SC ← 0 Execute register-reference instruction SC ← 0 AR ← M[AR] Nothing Execute memory-reference instruction SC ← 0 (Register or I/O) = 1= 0 (Memory-reference) (I/O) = 1= 0 (register) (indirect) = 1= 0 (direct)

87. Register Reference Instruction D 7 I’T 3 = r (common to all register reference instructions) IR(i) = B i [bit in IR(0-11) that specifies the operation] CLArB 11 AC ← 0 Clear AC CLErB 10 E ← 0 Clear E CMArB 9 AC ← AC’ Complement AC CMErB 8 E ← E’ Complement E CIRrB 7 AC ← shr AC, AC(15) ← E, E ← AC(0) Circulate right CILrB 6 AC ← shl AC, AC(0) ← E, E ← AC(15) Circulate left INCrB 5 AC ← AC + 1 Increment AC SPArB 4 If (AC(15) = 0) then (PC ← PC + 1) Skip if AC is positive SNArB 3 If (AC(15) = 1) then (PC ← PC + 1) Skip if AC is negative SZArB 2 If (AC = 0) then (PC ← PC + 1) Skip if AC is zero SZErB 1 If (E = 0) then (PC ← PC + 1) Skip if E is zero HLTrB 0 S ← 0 (S is a start-stop flip-flop) Halt Computer

88. Memory Reference Instructions 1.AND: AND to AC This is an instruction that performs the AND logic operation on pairs of bits in AC and the memory word specified by the effective address. The result of the operation is transferred to AC. D 0 T 4 : DR  M[AR] D 0 T 5 : AC  AC  DR, SC  0

89. Memory Reference Instructions 2.ADD: ADD to AC This instruction adds the content of the memory word specified by the effective address to the value of AC. The sum is transferred into AC and the output carry C out is transferred to the E (extended accumulator) flip-flop. D 1 T 4 : DR  M[AR] D 1 T 5 :AC  AC + DR, E  C out, SC  0

90. Memory Reference Instructions 3.LDA: Load to AC This instruction transfers the memory word specified by the effective address to AC. D 2 T 4 : DR  M[AR] D 2 T 5 :AC  DR, SC  0

91. Memory Reference Instructions 4.STA: Store AC This instruction stores the content of AC into the memory word specified by the effective address. D 3 T 4 :M[AR]  AC, SC  0

92. Memory Reference Instructions 5.BUN: Branch Unconditionally This instruction transfers the program to instruction specified by the effective address. The BUN instruction allows the programmer to specify an instruction out of sequence and the program branches (or jumps) unconditionally. D 4 T 4 :PC  AR, SC  0

93. Memory Reference Instructions 6.BSA: Branch and Save Return Address This instruction is useful for branching to a portion of the program called a subroutine or procedure. When executed, the BSA instruction stores the address of the next instruction in sequence (which is available in PC) into a memory location specified by the effective address. D 5 T 4 :M[AR]  PC, AR  AR + 1 D 5 T 5 :PC  AR, SC  0

94. BSA 0 BSA 135 Next Instruction Subroutine 1 BUN 135 0 BSA 135 Next Instruction Subroutine 1 BUN 135 20 PC = 2121 136 AR = 135 PC = 136 135 21 Memory, PC and AR at Time T 4 Memory and PC after execution

95. Memory Reference Instructions 7.ISZ: Increment and Skip if Zero These instruction increments the word specified by the effective address, and if the incremented value is equal to 0, PC is incremented by 1. Since it is not possible to increment a word inside the memory, it is necessary to read the word into DR, increment DR, and store the word back into memory. D 6 T 4 :DR  M[AR] D 6 T 5 :DR  DR + 1 D 6 T 6 :M[AR]  DR, if (DR = 0) then (PC  PC + 1), SC  0

96. Input-Output of basic computer Keyboard Transmitter Interface INPR AC OUTR FGO FGI Printer Receiver Interface Input-Output terminal Serial communication interface Computer registers and flip-flop =0=1 =0

97. Input-Output of basic computer  A computer can serve no useful purpose unless it communicates with the external environment.  To exhibit the most basic requirements for input and output communication, we will use a terminal unit with a keyboard and printer.  The terminal sends and receives serial information and each quantity of information has eight bits of an alphanumeric code.  The serial information from the keyboard is shifted into the input register INPR.  The serial information for the printer is stored in the output register OUTR.  These two registers communicate with a communication interface serially and with the AC in parallel.

98. Process of input information transfer  Initially, the input flag FGI is cleared to 0. When a key is struck in the keyboard, an 8-bit alphanumeric code is shifted into INPR and the input flag FGI is set to 1.  As long as the flag is set, the information in INPR cannot be changed by striking another key. The computer checks the flag bit; if it is 1, the information from INPR is transferred in parallel into AC and FGI is cleared to 0.  Once the flag is cleared, new information can be shifted into INPR by striking another key.

99. Process of outputting information  The output register OUTR works similarly but the direction of information flow is reversed.  Initially, the output flag FGO is set to 1. The computer checks the flag bit; if it is 1, the information from AC is transferred in parallel to OUTR and FGO is cleared to 0. The output device accepts the coded information, prints the corresponding character, and when the operation is completed, it sets FGO to 1.  The computer does not load a new character into OUTR when FGO is 0 because this condition indicates that the output device is in the process of printing the character.

100. Input-Output Instruction D 7 IT 3 = p (common to all input-output instructions) IR(i) = B i [bit in IR(6-11) that specifies the operation] INPpB 11 AC(0-7) ← INPR, FGI ← 0 Input Character OUTpB 10 OUTR ← AC(0-7), FGO ← 0 Output Character SKIpB 9 If (FGI = 1) then (PC ← PC + 1) Skip on input flag SKOpB 8 If (FGO = 1) then (PC ← PC + 1) Skip on output flag IONpB 7 IEN ← 1 Interrupt enable on IOFpB 6 IEN ← 0 Interrupt enable off

101. Interrupt Cycle R Fetch & Decode instruction Store return address in location 0 M[0] ← PC Execute instruction IEN FGI FGO R ← 1 Branch to location 1 PC ← 1 IEN ← 0 R ← 0 = 0= 1Interrupt cycleInstruction cycle = 1 = 0 = 1 = 0 = 1 = 0

102. Interrupt Cycle  The interrupt cycle is a hardware implementation of a branch and save return address operation.  An interrupt flip-flop R is included in the computer.  When R = 0, the computer goes through an instruction cycle.

103. Interrupt Cycle  During the execute phase of the instruction cycle IEN is checked by the control.  If it is 0, it indicates that the programmer does not want to use the interrupt, so control continues with the next instruction cycle.  If IEN is 1, control checks the flag bits.  If both flags are 0, it indicates that neither the input nor the output registers are ready for transfer of information.  In this case, control continues with the next instruction cycle. If either flag is set to 1 while IEN = 1, flip-flop R is set to 1.  At the end of the execute phase, control checks the value of R, and if it is equal to 1, it goes to an interrupt cycle instead of an instruction cycle.

104. Register transfer statements for Interrupt cycle  The flip-flop is set to 1 if IEN = 1 and either FGI or FGO are equal to 1. This can happen with any clock transition except when timing signals T 0, T 1 or T 2 are active.  The condition for setting flip-flop R = 1 can be expressed with the following register transfer statement: T 0 T 1 T 2 (IEN) (FGI + FGO): R  1  The symbol + between FGI and FGO in the control function designates a logic OR operation. This is AND with IEN and T 0 T 1 T 2.

105. Register transfer statements for Interrupt cycle  The fetch and decode phases of the instruction cycle must be modified and Replace T 0, T 1, T 2 with R'T 0, R'T 1, R'T 2  Therefore the interrupt cycle statements are : RT 0 : AR  0, TR  PC RT 1 : M[AR]  TR, PC  0 RT 2 : PC  PC + 1, IEN  0, R  0, SC  0

106. Register transfer statements for Interrupt cycle  During the first timing signal AR is cleared to 0, and the content of PC is transferred to the temporary register TR.  With the second timing signal, the return address is stored in memory at location 0 and PC is cleared to 0.  The third timing signal increments PC to 1, clears IEN and R, and control goes back to T 0 by clearing SC to 0.  The beginning of the next instruction cycle has the condition RT 0 and the content of PC is equal to 1. The control then goes through an instruction cycle that fetches and executes the BUN instruction in location 1.

107. Demonstration of Interrupt Cycle 0 BUN 1120 Main Program I/O program 1 BUN 0 0 1 PC = 256 255 Before Interrupt 1120 256 0 BUN 1120 Main Program I/O program 1 BUN 0 0 PC = 1 256 255 After Interrupt 1120

108. AR ← PC IR ← M[AR], PC ← PC + 1 Decode operation code in IR(12-14) AR ← IR(0-11), I ← IR(15) D7D7 II Execute input-output instruction SC ← 0 Execute register-reference instruction SC ← 0 AR ← M[AR] Nothing Execute memory-reference instruction SC ← 0 (Register or I/O) = 1= 0 (Memory-reference) (I/O) = 1= 0 (register) (indirect) = 1= 0 (direct) Start SC ← 0, IEN ← 0, R ← 0 R AR ← 0, TR ← PC M[AR] ← TR, PC ← 0 PC ← PC + 1, IEN ← 0, R ← 0, SC ← 0 (Instruction cycle) = 0 =1 (Interrupt cycle) Design of Basic Computer

109. Design of Accumulator Logic  In order to design the logic associated with AC, it is necessary to extract all the statements that change the content of AC. D 0 T 5 : AC  AC  DR, SC  0 AND with DR D 1 T 5 : AC  AC + DR, SC  0 ADD with DR D 2 T 5 : AC  DR Transfer from DR pB 11 : AC(0-7) ← INPR, FGI ← 0 Transfer from INPR rB 9 : AC ← AC’ Complement rB 7 : AC ← shr AC, AC(15) ← E Shift right rB 6 : AC ← shl AC, AC(0) ← E Shift left rB 11 : AC ← 0 Clear rB 5 : AC ← AC + 1 Increment

110. Design of Accumulator Logic Circuit associated with AC Adder and logic circuit Accumulator register (AC) Control gates CLR INRLD Clock From DR From INPR 16 8 To bus

111. Design of Accumulator Logic Gate structure for controlling LD, INR and CLR of AC AC CLR INRLD Clock 16 To bus AND ADD DR INPR CMA SHR SHL INC CLR 16 D0D0 T5T5 D1D1 D2D2 T5T5 p B 11 r B9B9 B7B7 B6B6 B5B5 From Adder & Logic

112. Questions asked in exam 1.Write a detailed note on instruction cycle with neat diagrams. 2.Explain control unit of basic computer and its working with diagram. 3.For the basic computer explain following instructions 1.LDA 2.ADD 3.AND 4.CLA 4.Draw and explain flowchart for interrupt cycle. 5.For the basic computer explain following instructions 1.BUN 2.BSA 3.CIL 4.SZE 6.Explain how Input/Output can be performed using interrupts. 7.State the differences between hardwired control and microprogrammed control.

113. Questions asked in exam 8.A computer uses a memory unit with 256K words of 32 bits each. A binary instruction code is stored in one word of memory. The instruction has four parts: an indirect bit, an operation code, a register code part to specify one of 64 registers, and an address part.1. How many bits are there in operation code, the register code part, and the address part?2. Draw the instruction word format and indicate the number of bits in each part.3. How many bits are there in the data and address inputs of the memory? 9.Draw and explain basic computer instruction formats. 10.Differentiate MRI and non-MRI. 11.Explain Direct and Indirect Addressing. 12.Give an example of register transfer of data through accumulator. 13.What is Interrupt? How it is useful for a system? 14.Explain CLA, ISZ, INP instruction. 15.Explain seven register common bus system.

114. Questions asked in exam 16.Explain with clear diagram, how data can be input to the computer using INP instruction. 17.What is a Program Counter? 18.What is an Accumulator? 19.What is an Instruction Register? 20.What do you understand by Memory Address? 21.What is a Carry Flag? 22.Explain Instruction Fetch. 23.Explain Instruction Decode. 24.Enlist major components of CPU. 25.Effective address.