Computer Organization & Architecture Winter 2022 GTU Paper Solution | 3140707

In this system, four 4-bit registers (A, B, C, and D) share a common bus. The bus allows each register to communicate with other registers and external devices. Each register has an input and output port connected to the bus. When a register needs to read or write data, it sends a request to the bus controller, which then grants access to the bus. The bus controller also ensures that only one register can write to the bus at a time to avoid data collisions.

In a digital circuit, the sequence counter (SC) is a type of counter that is used to generate a sequence of states. The role of the sequence counter in the control unit is to generate control signals for various operations based on the state of the system.

The sequence counter has three inputs: clock input (CLK), clear input (CLR), and enable input (EN). The CLK input provides the clock signal to the counter, which is used to synchronize the counter with other parts of the system. The CLR input resets the counter to its initial state, and the EN input enables the counter to count.

The diagram of a sequence counter with its inputs is shown below:

         +-----+
CLK -->|     |
       |  SC |
CLR -->|     |
       |     |
EN  -->|     |
       +-----+

The sequence counter can be used to generate a sequence of states that can be used to control various operations in the system. For example, the sequence counter can be used to generate a sequence of addresses for accessing memory locations or for executing a series of instructions.

1. Implied Mode: In this addressing mode, the operand is implicitly understood and no specific instruction is needed to specify it. The operand is already known or implied by the instruction itself. This mode is commonly used for instructions that do not require an operand, such as branch or jump instructions. For example, the instruction “BRZ” (branch if zero) in a computer’s assembly language implies that the operand is the current program counter (PC) value, and the instruction jumps to the specified location if the accumulator is zero.
2. Register Mode: In this addressing mode, the operand is stored in a register, and the instruction specifies which register to use. The register is either explicitly named in the instruction or is implied by the opcode. For example, the instruction “ADD R1, R2” adds the contents of register R2 to register R1 and stores the result in R1. In this case, the operand is in register R2, and the instruction specifies the destination register R1. This mode is commonly used for arithmetic and logical operations.

The different phases of the Instruction Cycle are:

1. Fetch: The fetch phase is the first phase in the instruction cycle. In this phase, the instruction is fetched from memory and loaded into the instruction register (IR).
2. Decode: In the decode phase, the instruction in the IR is decoded and the necessary operations are identified.
3. Execute: In the execute phase, the operation specified by the instruction is performed.
4. Store: In the store phase, the result of the operation is stored back in memory or in a register.

Register Transfer for Fetch Phase:

During the fetch phase, the control unit performs the following register transfer operations:

1. The program counter (PC) is loaded with the address of the next instruction to be fetched.
2. The contents of the PC are transferred to the memory address register (MAR).
3. The control unit sends a read signal to memory, and the contents of the memory location specified by the MAR are loaded into the memory data register (MDR).
4. The contents of the MDR are transferred to the instruction register (IR).

The diagram of register transfer for fetch phase is as follows:

A microinstruction is a low-level instruction used by a microprogrammed control unit to perform operations in a computer’s central processing unit (CPU).

There are four different types of 16-bit instruction formats for a basic computer:

1. Register-reference instructions: These instructions specify a register as an operand and reference it in the operation. Example: ADD A, R1
2. Immediate instructions: These instructions specify an immediate data value as an operand. Example: MOV A, #5
3. Direct instructions: These instructions specify a memory address as an operand. Example: MOV A, 2000H
4. Indirect instructions: These instructions specify a register that contains a memory address as an operand. Example: MOV A, (R1)

Three-Address Instructions and Zero Address Instructions are two different types of instruction formats used in computers. Here are some criticisms of each:

Criticisms of Three-Address Instructions:

• They require more memory space to store than other instruction formats, such as one-address or zero-address instructions.
• They can be more complex to decode and execute than other instruction formats.
• They can lead to longer execution times for certain operations that require multiple operands, such as multiplication or division.

Example of Three-Address Instruction: ADD R1, R2, R3 (Adds the values in registers R2 and R3 and stores the result in R1.)

Criticisms of Zero-Address Instructions:

• They can be less flexible than other instruction formats because they only allow operations on the top of the stack.
• They can be slower to execute than other instruction formats because they require more memory access operations to retrieve operands from the stack.

Example of Zero-Address Instruction: POP (Pops the top value from the stack and stores it in a register.)

Overall, the choice of instruction format depends on the specific needs and constraints of the computer system.

The control unit of a computer is responsible for controlling the flow of data between the CPU, memory, and input/output devices. After an instruction has been fetched from memory, the control unit must decode the instruction to determine its type and then execute it. The following flowchart describes how the control unit determines the instruction type after decoding:

1. The instruction is fetched from memory and stored in the instruction register (IR).
2. The opcode portion of the instruction is extracted from the IR.
3. The opcode is used to determine the type of instruction (e.g. arithmetic, logical, branching).
4. The control unit generates a sequence of microinstructions to carry out the instruction.
5. The microinstructions are executed in the appropriate order to complete the instruction.
6. Once the instruction has been executed, the control unit returns to the fetch phase to fetch the next instruction.

For example, consider the following instruction: ADD R1, R2, R3

1. The instruction is fetched from memory and stored in the instruction register (IR).
2. The opcode portion of the instruction (ADD) is extracted from the IR.
3. The opcode is used to determine that this is an arithmetic instruction.
4. The control unit generates a sequence of microinstructions to add the contents of registers R2 and R3 and store the result in register R1.
5. The microinstructions are executed in the appropriate order to complete the addition.
6. Once the addition has been completed, the control unit returns to the fetch phase to fetch the next instruction.

In this way, the control unit determines the type of instruction and generates the appropriate sequence of microinstructions to execute it.

RISC and CISC are two different computer architecture designs that have evolved over time. Here are the main differences between them:

1. Instruction set complexity: CISC processors have a complex instruction set, which means that a single instruction can perform multiple operations. RISC processors, on the other hand, have a reduced instruction set, which means that each instruction performs a single operation.
2. Number of registers: RISC processors have more registers compared to CISC processors. This is because RISC processors rely on register operations rather than memory operations.
3. Memory access: RISC processors have a load/store architecture, which means that all operations are performed on registers, and memory access is only allowed through load and store instructions. CISC processors, on the other hand, allow direct memory access.
4. Pipelining: RISC processors are designed for pipelining, which means that multiple instructions can be executed simultaneously. CISC processors are not as efficient for pipelining because of their complex instruction set.
5. Compiler optimization: RISC processors rely on compiler optimization to achieve better performance. CISC processors, on the other hand, rely on hardware to achieve better performance.
6. Power consumption: RISC processors consume less power compared to CISC processors because they have a simpler instruction set.

Example: A common example of a CISC processor is the Intel x86 architecture, while a common example of a RISC processor is the ARM architecture.

In summary, CISC processors have a complex instruction set, allow direct memory access, and rely on hardware for performance optimization, while RISC processors have a reduced instruction set, rely on compiler optimization, and are designed for pipelining.

Cache memory is a high-speed memory that stores frequently used data or instructions, allowing for faster access than retrieving from the main memory. Direct addressing mapping is a cache mapping technique where each block in the main memory is mapped to exactly one block in the cache memory.

In direct addressing mapping, the address of the data or instruction is divided into three parts: the tag, the index, and the offset. The tag is used to identify which block in the main memory the data or instruction is located in. The index is used to determine which block in the cache memory the data or instruction is mapped to. The offset is used to determine the exact location of the data or instruction within the block.

The following diagram illustrates the direct addressing mapping technique for a cache memory with 4 blocks and a block size of 4 words:

TITLE "TO PRINT THE SUM OF NATURAL NUMBERS FROM 1 TO 50"
.MODEL SMALL
.STACK
.DATA
VAL DB 1
.CODE
MAIN PROC
MOV AX,@DATA
MOV DS,AX
MOV BX,1
MOV CX,50
MOV AX,0

TOP:
ADD AX,BX
INC BX

LOOP TOP
XOR DX,DX
MOV BX,50
DIV BX


AAM
ADD AX,3030H
PUSH DX

MOV DH,AL
MOV DL,AH
MOV AH,02H
INT 21H

MOV DL,DH
MOV AH,02H
INT 21H


POP AX

AAM
ADD AX,3030H
PUSH DX

MOV DH,AL
MOV DL,AH
MOV AH,02H
INT 21H

MOV DL,DH
MOV AH,02H
INT 21H

MOV AH,4CH
INT 21H

MAIN ENDP
END MAIN

The first pass of an assembler involves scanning the source code and creating a symbol table that contains the location of each label and the value of each constant. The assembler reads the source code line by line and performs the following tasks:

1. It reads the source code line and removes any comments.
2. It checks if the line contains a label. If it does, it adds the label to the symbol table along with its location.
3. It checks if the line contains a constant. If it does, it adds the constant to the symbol table along with its value.
4. It calculates the location of the current instruction based on the location of the previous instruction and the length of the previous instruction. This is necessary because the size of each instruction can vary depending on the instruction set architecture.
5. It generates machine code for the current instruction using the opcode and operands.
6. It stores the machine code in memory at the appropriate location.

The first pass of the assembler does not generate the final machine code. Its main purpose is to create a symbol table that will be used in the second pass of the assembler to generate the final machine code. The symbol table contains information about the location of labels and the values of constants, which is required for the assembler to resolve all the symbolic references in the source code.

Here is an example program in Assembly language that moves a block of data from one location to another:

ORG 1000H

; initialize variables
Source DB 12H, 34H, 56H, 78H
Dest DB 0, 0, 0, 0
Count DW 4

MOV CX, Count
MOV SI, OFFSET Source
MOV DI, OFFSET Dest

; loop through the block of data and copy it
LOOP:
  MOV AL, [SI]
  MOV [DI], AL
  INC SI
  INC DI
  LOOP LOOP

END

This program starts at memory location 1000H and initializes three variables: Source, Dest, and Count. Source contains the data to be moved, Dest is the destination for the data, and Count specifies the number of bytes to move.

The program then uses MOV instructions to copy the data from Source to Dest using a loop. The CX register is used to control the loop and is initialized to Count. The SI and DI registers are used as pointers to the source and destination data, respectively. The loop copies each byte from Source to Dest and then increments the pointers. The loop continues until CX reaches zero.

Finally, the program ends with the END directive.

Modes of data transfer in a computer system are:

1. Programmed I/O
2. Interrupt-driven I/O
3. Direct Memory Access (DMA)

Direct Memory Access (DMA) is a technique used to transfer data directly between I/O devices and memory without involving the CPU. This technique is used to reduce the CPU’s workload and increase the overall system performance.

The steps involved in the DMA transfer are as follows:

1. The CPU sets up the DMA controller with the transfer parameters, including the starting memory address, the device address, and the number of bytes to transfer.
2. The DMA controller requests control of the system bus from the CPU.
3. The CPU releases the bus to the DMA controller, which then reads or writes data directly to or from memory and I/O devices.
4. Once the transfer is complete, the DMA controller returns control of the system bus to the CPU.

The DMA controller transfers the data between memory and I/O devices using the Direct Memory Access Transfer Mode. In this mode, the DMA controller directly reads or writes data to or from memory and I/O devices without the intervention of the CPU.

This technique is used in situations where a large amount of data needs to be transferred between memory and an I/O device. It allows the CPU to perform other tasks while the data transfer is taking place, thus increasing system performance.

In instruction pipeline, data dependency conflict occurs when an instruction depends on data produced by a previous instruction that has not yet completed its execution in the pipeline. This dependency creates a conflict as the pipeline cannot move forward until the data is available, leading to pipeline stalls and reduced performance.

There are several solutions to data dependency conflicts in instruction pipelines:

1. Forwarding or data forwarding: This technique involves bypassing the intermediate stages of the pipeline and directly forwarding the data from the output of the earlier stage to the input of the later stage that needs it. This solution reduces pipeline stalls and improves performance.
2. Stalling or pipeline flushing: This technique involves stopping the pipeline until the required data is available. This solution eliminates the possibility of data hazards but can reduce performance.
3. Register renaming: This technique involves duplicating registers to eliminate the dependency between instructions and avoid data hazards. This solution reduces pipeline stalls and improves performance.
4. Speculative execution: This technique involves executing instructions speculatively based on the assumption that the data dependency will not occur. If the assumption is incorrect, the pipeline is flushed, and the instructions are re-executed. This solution can improve performance but requires additional hardware support.
5. Compiler optimization: This technique involves restructuring the code to reduce data dependencies and improve pipeline performance. This solution requires changes to the code and may not always be feasible.

Overall, the choice of solution depends on the design goals and constraints of the pipeline. A combination of these solutions may be used to optimize pipeline performance.

Here is an example of a four-segment instruction pipeline, which is a simplified model of a typical pipeline used in modern CPUs:

1. Instruction Fetch (IF): In this stage, the pipeline fetches the instruction from memory or cache and stores it in the Instruction Register (IR). The program counter (PC) points to the next instruction to be fetched, and the PC value is updated to point to the next instruction.
2. Instruction Decode (ID): In this stage, the pipeline decodes the instruction in the IR and determines the type of instruction and the operands it needs. The pipeline also checks for any data dependencies with previously executed instructions and stalls if necessary.
3. Execute (EX): In this stage, the pipeline executes the instruction using the operands obtained in the previous stage. The execution stage can be divided into several sub-stages, depending on the complexity of the instruction.
4. Write Back (WB): In this stage, the pipeline writes the result of the instruction execution back to the register file or memory. If the instruction was a load or store, the pipeline performs the necessary memory operations.

Let’s consider an example of a simple four-segment pipeline executing the following instructions:

ADD R1, R2, R3
SUB R4, R1, R5
AND R6, R2, R7
OR R8, R9, R10

Here’s how the pipeline would execute each instruction:

Clock cycle 1: IF: Fetch the ADD instruction from memory and store it in the IR ID: Decode the ADD instruction and determine the operands EX: Execute the ADD instruction and compute the result WB: Write the result to the register file

Clock cycle 2: IF: Fetch the SUB instruction from memory and store it in the IR ID: Decode the SUB instruction and determine the operands (stalled as R1 is not ready yet) EX: Wait for the result of the ADD instruction to be available and execute the SUB instruction WB: Write the result to the register file

Clock cycle 3: IF: Fetch the AND instruction from memory and store it in the IR ID: Decode the AND instruction and determine the operands EX: Execute the AND instruction and compute the result WB: Write the result to the register file

Clock cycle 4: IF: Fetch the OR instruction from memory and store it in the IR ID: Decode the OR instruction and determine the operands EX: Execute the OR instruction and compute the result WB: Write the result to the register file

In this example, the pipeline stalls in the second clock cycle because the SUB instruction requires the result of the ADD instruction, which is not yet available. However, the pipeline can still achieve a significant speedup compared to executing the instructions sequentially.

1. Write-through cache and write-back cache:

Write-through and write-back are two techniques used in computer caches to manage data updates. In a write-through cache, every write to the cache is immediately written to the main memory as well. In contrast, in a write-back cache, data is only written back to the main memory when it is evicted from the cache. Here are some key differences between write-through and write-back cache:

• Write-through cache has lower write performance compared to write-back cache since every write operation involves an additional write to the main memory.
• Write-back cache has higher read performance compared to write-through cache since it has more space available to store frequently accessed data.
• Write-through cache is easier to implement than write-back cache because it does not require any additional logic to handle evictions and writebacks.
• Write-back cache is more complex and can be more prone to data inconsistencies if not properly implemented.

2. Spatial locality and temporal locality:

Spatial locality and temporal locality are two principles that describe how data is accessed in computer systems.

Spatial locality refers to the tendency of a program to access data that is located near other recently accessed data. For example, when accessing an array, the program is likely to access the adjacent elements as well, due to the way arrays are stored in memory.

Temporal locality refers to the tendency of a program to access the same data repeatedly over a short period. For example, a loop that repeatedly processes a large array will exhibit high temporal locality, as the same data is accessed multiple times.

Here are some key differences between spatial locality and temporal locality:

• Spatial locality is concerned with accessing nearby data, while temporal locality is concerned with repeated access to the same data.
• Spatial locality is important for cache performance because it allows caching of nearby data to reduce memory access latency.
• Temporal locality is important for cache performance because it allows caching of frequently accessed data to reduce memory access latency.
• Both spatial and temporal locality are important factors in determining cache performance and overall system performance.

Overall, spatial and temporal locality are both important principles for optimizing computer system performance, and understanding them can help in designing more efficient caching and memory systems.

Flynn’s classification scheme is a taxonomy that categorizes computer architectures based on the number of instruction streams and data streams that can be processed simultaneously. The scheme was introduced by Michael J. Flynn in 1966 and is still widely used in computer science.

The Flynn’s classification scheme divides computer architectures into four categories based on two dimensions: instruction stream and data stream. The instruction stream dimension indicates the number of concurrent instruction streams, while the data stream dimension indicates the number of concurrent data streams.

Here are the four categories of computer architectures in Flynn’s classification scheme:

1. SISD (Single Instruction Stream, Single Data Stream): This is the traditional type of computer architecture, in which a single instruction stream controls the flow of operations, and a single data stream provides the operands for the instructions. This type of architecture is used in most traditional von Neumann computers.
2. SIMD (Single Instruction Stream, Multiple Data Stream): In this architecture, a single instruction stream is used to perform the same operation on multiple data streams simultaneously. SIMD computers are optimized for tasks that can be performed using parallelism, such as graphics processing and scientific simulations.
3. MISD (Multiple Instruction Stream, Single Data Stream): In this architecture, multiple instruction streams are executed on a single data stream. MISD computers are not commonly used in practice, but they can be used in safety-critical systems, such as aerospace or nuclear power plants, where multiple redundant processors can check each other’s results.
4. MIMD (Multiple Instruction Stream, Multiple Data Stream): This architecture allows for multiple instruction streams and data streams to be processed simultaneously, providing the highest level of parallelism. MIMD computers can be further divided into two subcategories: shared-memory and distributed-memory architectures. In shared-memory MIMD computers, all processors share a common memory, while in distributed-memory MIMD computers, each processor has its own local memory and communicates with other processors via a communication network.

Flynn’s classification scheme provides a useful framework for understanding the capabilities and limitations of different computer architectures. By categorizing architectures based on their ability to handle instruction and data streams, it helps to identify which architectures are best suited for different types of applications.