Computer System Architecture (CSA) - BCA 2nd YEAR - QUESTIONS (PART-2)

1) Explain 7-Segment LED Display.

An LED or Light Emitting Diode, is a solid state optical pn-junction diode which emits light energy in the form of photons.

But the main advantage of light emitting diodes is that because of their small die size, several of them can be connected together within one small and compact package producing what is generally called a 7-segment Display.

The emission of these photons occurs when the diode junction is forward biased by an external voltage allowing current to flow across its junction, and in Electronics we call this process electroluminescence.

The actual colour of the visible light emitted by an LED, ranging from blue to red to orange, is decided by the spectral wavelength of the emitted light which itself is dependent upon the mixture of the various impurities added to the semiconductor materials used to produce it.

Light emitting diodes have many advantages over traditional bulbs and lamps, with the main ones being their small size, long life, various colours, cheapness and are readily available, as well as being easy to interface with various other electronic components and digital circuits.

The 7-segment display, also written as “seven segment display”, consists of seven LEDs (hence its name) arranged in a rectangular fashion as shown. Each of the seven LEDs is called a segment because when illuminated the segment forms part of a numerical digit (both Decimal and Hex) to be displayed.

2) What are the various registers of 8085 ? and its Function.

(a) General Purpose Registers –

The 8085 has six general-purpose registers to store 8-bit data; these are identified as- B, C, D, E, H, and L. These can be combined as register pairs – BC, DE, and HL, to perform some 16-bit operation. These registers are used to store or copy temporary data, by using instructions, during the execution of the program.

(b) Specific Purpose Registers –

• Accumulator:
  The accumulator is an 8-bit register (can store 8-bit data) that is the part of the arithmetic and logical unit (ALU). After performing arithmetical or logical operations, the result is stored in accumulator. Accumulator is also defined as register A.

• Flag registers: The flag register is a special purpose register and it is completely different from other registers in microprocessor. It consists of 8 bits and only 5 of them are useful. The other three are left vacant and are used in the future Intel versions.These 5 flags are set or reset (when value of flag is 1, then it is said to be set and when value is 0, then it is said to be reset) after an operation according to data condition of the result in the accumulator and other registers. The 5 flag registers are:
• Sign Flag
• Zero Flag
• Auxiliary Carry Flag
•              Note – This is the only flag register in 8085 which is not accessible by user.
• Parity Flag
• Carry Flag
• 
  

(c) Memory Registers –

There are two 16-bit registers used to hold memory addresses. The size of these registers is 16 bits because the memory addresses are 16 bits. They are :-

• Program Counter
• Stack Pointer
• Sign Flag (7th bit)
• Zero Flag (6th bit)
• Auxiliary Carry Flag (4th bit)
• Parity Flag (2nd bit)
• Carry Flag (0th bit)

3)Pin Diagram of 8085 Microprocessor 

1. Address Bus and Data Bus: 
The address bus is a group of sixteen lines i.e A0-A15. The address bus is unidirectional, i.e., bits flow in one direction from the microprocessor unit to the peripheral devices and uses the high order address bus. 

2. Control and Status Signals: 

• ALE – It is an Address Latch Enable signal. It goes high during first T state of a machine cycle and enables the lower 8-bits of the address, if its value is 1 otherwise data bus is activated. 

   

• IO/M’ – It is a status signal which determines whether the address is for input-output or memory. When it is high(1) the address on the address bus is for input-output devices. When it is low(0) the address on the address bus is for the memory. 

   

• SO, S1 – These are status signals. They distinguish the various types of operations such as halt, reading, instruction fetching or writing. 

   

IO/M’	S1	S0	Data Bus Status
0	1	1	Opcode fetch
0	1	0	Memory read
0	0	1	Memory write
1	1	0	I/O read
1	0	1	I/O write
1	1	1	Interrupt acknowledge
0	0	0	Halt

 

• RD’ – It is a signal to control READ operation. When it is low the selected memory or input-output device is read. 

   

• WR’ – It is a signal to control WRITE operation. When it goes low the data on the data bus is written into the selected memory or I/O location. 

   

• READY – It senses whether a peripheral is ready to transfer data or not. If READY is high(1) the peripheral is ready. If it is low(0) the microprocessor waits till it goes high. It is useful for interfacing low speed devices. 
   

3. Power Supply and Clock Frequency: 

 

• Vcc – +5v power supply 

   

• Vss – Ground Reference 

   

• XI, X2 – A crystal is connected at these two pins. The frequency is internally divided by two, therefore, to operate a system at 3MHZ the crystal should have frequency of 6MHZ. 

   

• CLK (OUT) – This signal can be used as the system clock for other devices. 
   

4. Interrupts and Peripheral Initiated Signals: 


The 8085 has five interrupt signals that can be used to interrupt a program execution. 

(i) INTR 
(ii) RST 7.5 
(iii) RST 6.5 
(iv) RST 5.5 
(v) TRAP 

The microprocessor acknowledges Interrupt Request by INTA’ signal. In addition to Interrupts, there are three externally initiated signals namely RESET, HOLD and READY. To respond to HOLD request, it has one signal called HLDA. 




• INTR – It is an interrupt request signal. 

   

• INTA’ – It is an interrupt acknowledgement sent by the microprocessor after INTR is received. 
   

5. Reset Signals: 

 

• RESET IN’ – When the signal on this pin is low(0), the program-counter is set to zero, the buses are tristated and the microprocessor unit is reset. 

   

• RESET OUT – This signal indicates that the MPU is being reset. The signal can be used to reset other devices. 
   

6. DMA Signals: 

 

• HOLD – It indicates that another device is requesting the use of the address and data bus. Having received HOLD request the microprocessor relinquishes the use of the buses as soon as the current machine cycle is completed. Internal processing may continue. After the removal of the HOLD signal the processor regains the bus. 

   

• HLDA – It is a signal which indicates that the hold request has been received after the removal of a HOLD request, the HLDA goes low. 
   

7. Serial I/O Ports: 
Serial transmission in 8085 is implemented by the two signals, 

 

• SID and SOD – SID is a data line for serial input where as SOD is a data line for serial output. 

4) What is the importance of Timing and Control in the design of Control Unit?

We use Timing and Controlling unit in 8085 for the generation of timing signals and the signals to control. All the operations and functions both interior and exterior of a microprocessor are controlled by this unit.

The control unit of the central processing unit regulates and integrates the operations of the computer. It selects and retrieves instructions from the main memory in proper sequence and interprets them so as to activate the other functional elements of the system at the appropriate moment…

5) Explain Instruction Cycle. Give the RTL Statement for each Sub Cycle. How the Instruction cycle is to accommodate the Interrupt from I/O Devices.

Time required to execute and fetch an entire instruction is called instruction cycle. It consists:

• Fetch cycle – The next instruction is fetched by the address stored in program counter (PC) and then stored in the instruction register.
• Decode instruction – Decoder interprets the encoded instruction from instruction register.
• Reading effective address – The address given in instruction is read from main memory and required data is fetched. The effective address depends on direct addressing mode or indirect addressing mode.
• Execution cycle – consists memory read (MR), memory write (MW), input output read (IOR) and input output write (IOW)

RTL- 

The term “Register Transfer” can perform micro-operations and transfer the result of operation to the same or other register. 

Micro-operations : 
The operation executed on the data store in registers are called micro-operations. They are detailed low-level instructions used in some designs to implement complex machine instructions. 

Register Transfer : 
The information transformed from one register to another register is represented in symbolic form by replacement operator is called Register Transfer. 

Replacement Operator : 
In the statement, R2 <- R1, <- acts as a replacement operator. This statement defines the transfer of content of register R1 into register R2. 

There are various methods of RTL – 

1. General way of representing a register is by the name of the register inclosed in a rectangular box. 
    
2. Register is numbered in a sequence of 0 to (n-1). 
    
3. The numbering of bits in a register can be marked on the top of the box. 
    
4. A 16-bit register PC is divided into 2 parts- Bits (0 to 7) are assigned with lower byte of 16-bit address and bits (8 to 15) are assigned with higher bytes of 16-bit address. 

Register Transfer Operations:

The operation performed on the data stored in the registers are referred to as register transfer operations.

There are different types of register transfer operations:

1. Simple Transfer – R2 <- R1

 The content of R1 are copied into R2 without affecting the content of R1. It is an unconditional type of transfer operation. 

2. Conditional Transfer – It indicates that if P=1, then the content of R1 is transferred to R2. It is a unidirectional operation. 

3. Simultaneous Operations – 
If 2 or more operations are to occur simultaneously then they are separated with comma (,). 

6) What are the various types of Interrupts ? How does processor determine which device issues the Interrupt.

When microprocessor receives interrupt signals, it sends an acknowledgement (INTA) to the peripheral which is requesting for its service.

Interrupts can be classified into various categories based on different parameters:

1. Hardware and Software Interrupts –
   When microprocessors receive interrupt signals through pins (hardware) of microprocessor, they are known as Hardware Interrupts. There are 5 Hardware Interrupts in 8085 microprocessor. 
2. Vectored and Non-Vectored Interrupts –
   Vectored Interrupts are those which have fixed vector address (starting address of sub-routine) and after executing these, program control is transferred to that address.
3. Non-Vectored Interrupts are those in which vector address is not predefined. The interrupting device gives the address of sub-routine for these interrupts. INTR is the only non-vectored interrupt in 8085 microprocessor.

  3. Maskable and Non-Maskable Interrupts –
     Maskable Interrupts are those which can be disabled or ignored by the                          microprocessor. These interrupts are either edge-triggered or  level-triggered, 

      so they can be disabled.

Priority of Interrupts –
When microprocessor receives multiple interrupt requests simultaneously, it will execute the interrupt service request (ISR) according to the priority of the interrupts.

7) Discuss the function of DMA controller in data transfer between I/O Memory. Also state different modes of DMA Operation.

Direct memory access (DMA) is a method that allows an input/output (I/O) device to send or receive data directly to or from the main memory, bypassing the CPU to speed up memory operations. The process is managed by a chip known as a DMA controller (DMAC)

DMA controller has four modes for data transfer:

1. 1. Single Byte Transfer Mode/ Cycle Stealing

   • Once the DMAC becomes the bus master, it will transfer only ONE BYTE and return the bus back to the microprocessor. As soon as the microprocessor performs one bus cycle, DMAC will once again take the bus back from the microprocessor.
   • Both DMAC and microprocessor are constantly stealing bus cycles from each other. It is the most popular method of DMA, because it keeps the microprocessor active in the background.
   • After a byte is transferred, the CAR and CWCR are adjusted accordingly. The system bus is returned to the µP. For further bytes to be transferred, the DREQ line must go active again, and then the entire operation is repeated.
   
   

   2. Block Transfer Mode.

   • In this mode, the DMAC is programmed to transfer all the bytes in one complete DMA operation. After a byte is transferred, the CAR and CWCR are adjusted accordingly.

   • The system bus is returned to the µP, only after all the bytes are transferred. i.e. TC is reached or EOP signal is issued. It is the fastest form of DMA but keeps the microprocessor inactive for a long time.

   • The DREQ signal needs to be active only in the beginning for requesting the DMA service initially. Thereafter DREQ can become low during the transfer.

   
   

   3. Demand Transfer Mode

   • It is very similar to Block Transfer, except that the DREQ must active throughout the DMA operation.

   • If during the operation DREQ goes low, the DMA operation is stopped and the busses are returned to the µP.

   • In the meantime, the µP can continue with its own operations. Once DREQ goes high again, the DMA operation continues from where it had stopped.

   
   

   4. Cascade Transfer Mode

   • In this mode, more than one DMACs are cascaded together. It is used to increase the number of devices interfaced to the µP. Here we have one Master DMAC, to which one or more Slave DMACs are connected.

   • The Slave gives HRQ to the Master on the DREQ of the Master, and the Master gives HRQ to the µP on the HOLD of the µP.

8) What are Addressing Modes ? Discuss different types of addressing modes.

The way of specifying data to be operated by an instruction is called addressing mode.

Types of addressing modes – 
In 8085 microprocessor there are 5 types of addressing modes: 

1. Immediate Addressing Mode – In immediate addressing mode the source operand is always data. If the data is 8-bit, then the instruction will be of 2 bytes, if the data is of 16-bit then the instruction will be of 3 bytes. 
2. Register Addressing Mode – In register addressing mode, the data to be operated is available inside the register(s) and register(s) is(are) operands. Therefore the operation is performed within various registers of the microprocessor. 
3. Direct Addressing Mode – In direct addressing mode, the data to be operated is available inside a memory location and that memory location is directly specified as an operand. The operand is directly available in the instruction itself. 
4. Register Indirect Addressing Mode – In register indirect addressing mode, the data to be operated is available inside a memory location and that memory location is indirectly specified by a register pair. 
5. Implied/Implicit Addressing Mode – In implied/implicit addressing mode the operand is hidden and the data to be operated is available in the instruction itself. 

9) Explain all Peripheral devices. Write about each of them.

A Peripheral Device is defined as the device which provides input/output functions for a computer and serves as an auxiliary computer device without computing-intensive functionality.

A peripheral device is a device that is connected to a computer system but is not part of the core computer system architecture. Generally, more people use the term peripheral more loosely to refer to a device external to the computer case.

Classification of Peripheral devices:

It is generally classified into 3 basic categories which are given below:

• Input Devices:

The input devices is defined as it converts incoming data and instructions into a pattern of electrical signals in binary code that are comprehensible to a digital computer.

Example:

Keyboard, mouse, scanner, microphone etc. 

• Output Devices:

An output device is generally reverse of the input process and generally translating the digitized signals into a form intelligible to the user. The output device is also performed for sending data from one computer system to another. For some time punched-card and paper-tape readers were extensively used for input, but these have now been supplanted by more efficient devices.

Example:

Monitors, headphones, printers etc. 

• Storage Devices:

Storage devices are used to store data in the system which is required for performing any operation in the system. The storage device is one of the most requirement devices and also provide better compatibility.

Example:

Hard disk, magnetic tape, Flash memory etc. 

Advantage of Peripherals Devices:

Peripherals devices provides more feature due to this operation of the system is easy. These are given below:

It is helpful for taking input very easily.

It is also provided a specific output.

It has a storage device for storing information or data

It also improves the efficiency of the system.

10) What is Micro-Operations.

Micro-operations (also known as micro-ops) are the functional or atomic, operations of a processor.... They transfer data between registers or between external buses of the CPU, also performs arithmetic and logical operations on registers.

11) Explain CPU Organization with Diagram in detail

• The part of the computer that performs the bulk of data-processing operations is called the central processing unit and is referred to as the CPU.

• The CPU is made up of three major parts, the register set stores intermediate data used during the execution of the instructions. The arithmetic logic unit (ALU) performs the required microoperations for executing the instructions. The control unit supervises the transfer of information among the registers and instructs the ALU as to which operation to perform.

• The CPU performs a variety of functions dictated by the type of instructions that are incorporated in the computer.

• This includes the instruction formats, addressing modes, the instruction set, and the general organization of the CPU registers.

12) What is LED Display. Write advantages of it.

A LED display is a flat panel display that uses an array of light-emitting diodes as pixels for a video display. ... LED displays are capable of providing general illumination in addition to visual display, as when used for stage lighting or other decorative (as opposed to informational) purposes.

Advantages of LED Monitors:

• Slim Design
• Brighter and sharper Images
• Better Color
• Flicker-Free Images
• Better Picture Quality(true black picture)
• No motion delay and lags
• Longer lifespan and less environmental impact
• Lower Power Consumption
• Wider Viewing angle(typically 175 degree)

13) Write  Short Note on-

 

   (a) Function of DMA - Direct Memory Access (DMA) is a common hardware function within a computer system that is used to relieve the processor or coprocessor from the burden of copying large blocks of data.

  (b) Floppy Disk Drive - Floppy disk is the magnetic disk which contains single plastic disk. In the initial days of computer invention it was used as main storage device, later on it was used for carrying data from one computer to another but now-a-days it is almost not used. It requires floppy drive for the operation.

  (c)  ALU Design - The ALU is a digital circuit that provides arithmetic and logic operations. It is the fundamental building block of the central processing unit of a computer. A modern CPU has a very powerful ALU and it is complex in design. In addition to ALU modern CPU contains a control unit and a set of registers.

  (d)  Instruction Format -  The instruction format is defined as standard machine instruction format that can be directly decoded and executed by the central processing unit ( CPU )

  (e) Direct Memory Access (DMA) - Direct memory access (DMA) is a method that allows an input/output (I/O) device to send or receive data directly to or from the main memory, bypassing the CPU to speed up memory operations. The process is managed by a chip known as a DMA controller (DMAC).

                                                or

Stands for "Direct Memory Access." DMA is a method of transferring data from the computer's RAM to another part of the computer without processing it using the CPU. For example, a PCI controller and a hard drive controller each have their own set of DMA channels.

  (f) ACC - Accumulator (ACC) - holds the data being processed and the results of processing.

  (g) ALU - An arithmetic logic unit(ALU) is a major component of the central processing unit of the a computer system. It does all processes related to arithmetic and logic operations that need to be done on instruction words.

  (h) Assembly Language -  An assembly language is a type of low-level programming language that is intended to communicate directly with a computer's hardware. Unlike machine language, which consists of binary and hexadecimal characters, assembly languages are designed to be readable by humans.

  (I) Instruction Cycle - A program residing in the memory unit of a computer consists of a sequence of instructions. These instructions are executed by the processor by going through a cycle for each instruction.

In a basic computer, each instruction cycle consists of the following phases:

1. Fetch instruction from memory.
2. Decode the instruction.
3. Read the effective address from memory.
4. Execute the instruction.

(j) Memory Mapped I/O Scheme - Memory-mapped I/O uses the same address space to address both memory and I/O devices. ... So a memory address may refer to either a portion of physical RAM, or instead to memory of the I/O device. Thus, the CPU instructions used to access the memory can also be used for accessing devices.

(k) Isolated I/O - When the CPU fetches and decodes the operation code of an input or output instruction, it locates the address related to the instruction into the common address lines.

(L) Mnemonic Code -  A mnemonic is a term, symbol or name used to define or specify a computing function. Mnemonics are used in computing to provide users with a means to quickly access a function, service or process, bypassing the actual more lengthy method used to perform or achieve it.

(M) Two pass assemblers - The two pass assembler performs two passes over the source program. ... Basically, the assembler goes through the program one line at a time, and generates machine code for that instruction. Then the assembler proceeds to the next instruction. In this way, the entire machine code program is created.

(N) Micro Processor - Microprocessor is the brain of computer, which does all the work. It is a computer processor that incorporates all the functions of CPU (Central Processing Unit) on a single IC (Integrated Circuit) or at the most a few ICs. Microprocessors were first introduced in early 1970s.

(O) Stack - A stack is a storage device that stores information in such a manner that the item stored last is the first item retrieved. The stack in digital computers is essentially a memory unit with an address register that can count only (after an initial value is loaded into it).

(P) Assembler - The Assembler is a Software that converts an assembly language code to machine code

 (Q) Interrupt - An interrupt in computer architecture is a signal that requests the processor to suspend its current execution and service the occurred interrupt.

 (R) 8085 Architecture -  The 8085 is an 8-bit microprocessor, and it was launched by the Intel team in the year of 1976 with the help of NMOS technology. ... The processor consists of 16-bit and 8-bit address and data lines and so the capacity of the device is 2¹⁶ which is 64KB of memory.

14) Difference between LDA and STA instructions.

LDA Load the accumulator with a data from the the memory in a particular address. It's is a three byte instruction. First byte is the opcode, second & third byte together will give the address from where data is to be moved into accumulator. 

STA , Store the current contents of the Accumulator in to the memory, whose address will be given as part of instruction. Here also first byte is opcode and the second & third byte together give the address. Both are examples of Direct Addressing method.

15) What are the various registers of 8085 and its function in computer system architecture

The 8085 has six general-purpose registers to store 8-bit data; these are identified as- B, C, D, E, H, and L. These can be combined as register pairs – BC, DE, and HL, to perform some 16-bit operation. These registers are used to store or copy temporary data, by using instructions, during the execution of the program.

16) Explain immediate, index and relative addressing modes in brief

Immediate Addressing Mode-

 

In this addressing mode,

• The operand is specified in the instruction explicitly.
• Instead of address field, an operand field is present that contains the operand.

Indexed Addressing Mode-

In this addressing mode,

• Effective address of the operand is obtained by adding the content of index register with the address part of the instruction.

 

Effective Address = Content of Index Register + Address part of the instruction

Relative Addressing Mode-

 

In this addressing mode,

• Effective address of the operand is obtained by adding the content of program counter with the address part of the instruction.

 

Effective Address = Content of Program Counter + Address part of the instruction

NOTE-

 

• Program counter (PC) always contains the address of the next instruction to be executed.
• After fetching the address of the instruction, the value of program counter immediately increases.
• The value increases irrespective of whether the fetched instruction has completely executed or not.

(17) What are the role of CPU in microprocessor

This central processing unit (CPU) of PLC, that is a microprocessor by design and functionality. The main function of this unit is to sense input values via its I/O modules, generate control signals following the input signals and the predefined instruction (stored in the memory unit as program).

(18) Why we use controller in i/o architecture . Write five controller names

I/O controllers are a series of microchips which help in the communication of data between the central processing unit and the motherboard. The main purpose of this system is to help in the interaction of peripheral devices with the control units (CUs). ... It is usually installed on the motherboard of a computer.

(19) Difference between Synchronous and Asynchronous I/O 

Synchronous vs asynchronous I/O

• Synchronous I/O − In this scheme CPU execution waits while I/O proceeds

• Asynchronous I/O − I/O proceeds concurrently with CPU execution

 (20) Differentiate between - 

Direct vs Indirect addressing mode

S.NO	Direct Addressing Mode	Indirect Addressing Mode
1.	Address field contains the effective address of operand	Address field contains reference of effective address
2.	Requires only one memory reference	Requires two memory references
3.	Fast addressing	Slower than direct addressing mode
4.	No further classification	Further classified into two categories
5.	No further calculation is required to perform the operation	Require further calculation to find the effective address

Differentiate between Address bus vs Data bus

(21) Differences between memory mapped I/O and isolated I/O –

Isolated I/O	Memory Mapped I/O
Memory and I/O have separate address space	Both have same address space
All address can be used by the memory	Due to addition of I/O addressable memory become less for memory
Separate instruction control read and write operation in I/O and Memory	Same instructions can control both I/O and Memory
In this I/O address are called ports.	Normal memory address are for both
More efficient due to separate buses	Lesser efficient
Larger in size due to more buses	Smaller in size
It is complex due to separate separate logic is used to control both.	Simpler logic is used as I/O is also treated as memory only.

(22) What are Shift Micro Operations.

Shift micro-operations are those micro-operations that are used for serial transfer of information. These are also used in conjunction with arithmetic micro-operation, logic micro-operation, and other data-processing operations.

(23) What is the Architecture of 8085 microprocessor?  (in Short)

The architecture of the 8085 microprocessor mainly includes the timing & control unit, Arithmetic and logic unit, decoder, instruction register, interrupt control, a register array, serial input/output control. The most important part of the microprocessor is the central processing unit.

  (24)  What is Vector processing ?

Definition: It is basically a central processing unit that has the ability to execute the complete vector input in a single instruction. More specifically we can say, it is a complete unit of hardware resources that executes a sequential set of similar data items in the memory using a single instruction.

  (25) Define micro operations with example

In computer central processing units, micro-operations (also known as micro-ops) are the functional or atomic, operations of a processor. These are low level instructions used in some designs to implement complex machine instructions. They generally perform operations on data stored in one or more registers. They transfer data between registers or between external buses of the CPU, also performs arithmetic and logical operations on registers.

A micro-operation is an elementary operation performed on the information stored in one or more registers. Example: Shift, count, clear and load. The micro-operations in digital computers are of 4 types: Register transfer micro-operations transfer binary information from one register to another.