Questions and Answers
What is the function of the mode input S in a 4-bit adder-subtractor circuit?
The mode input S controls whether the circuit performs addition (S = 0) or subtraction (S = 1).
How does the circuit perform subtraction when S = 1?
When S = 1, the circuit complements B and adds 1 through the input carry, effectively calculating A plus the 2's complement of B.
Describe the operation of a 4-bit binary incrementer.
A 4-bit binary incrementer adds one to the binary number in a register, using half-adders to propagate carries through the bits.
What is the purpose of half-adders in a 4-bit incremental circuit?
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Explain the role of multiplexers in a 4-bit arithmetic circuit.
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What is a circular shift and how does it differ from other types of shifts?
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How is the output generated in a 4-bit binary incrementer?
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Describe the effect of an arithmetic shift-left operation on a signed binary number.
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What is the significance of using a full-adder in the 4-bit adder-subtractor circuit?
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What happens to a signed binary number during an arithmetic shift-right operation?
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What happens to the input carry (C0) when performing subtraction in the adder-subtractor circuit?
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How is the sign bit affected by arithmetic shift operations?
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In a combinational circuit shifter, what do the selection inputs determine?
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What are the required components for constructing a shifter with n data inputs and outputs?
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What is the primary function of the two serial inputs in the shifter design described?
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Explain the term 'rotate operation' in the context of circular shifts.
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What happens when timing signal T4 becomes active?
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Describe the four phases of an instruction cycle.
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What is the significance of the sequence counter SC in the instruction cycle?
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How is the instruction transferred from memory to the instruction register IR?
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Why is it necessary to transfer the address from PC to AR during timing signal T0?
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What is the role of the AND gate in the control function of the sequence counter?
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Explain the purpose of decoding the instruction during the fetch and decode phases.
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What condition causes the instruction cycle to stop?
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What is the role of the input flag FGI in the input-output configuration?
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Describe the sequence of events that occurs when a key is pressed on the keyboard.
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How does the computer determine when to transfer information from INPR to AC?
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What happens to the output flag FGO when the output device is busy printing?
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Explain the significance of the 8-bit input register INPR.
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What is the function of the output register OUTR in the output process?
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What are input and output instructions used for in the context of the system described?
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How does the computer handle new information when the input flag FGI is cleared?
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What role do the selection inputs S3S2 play in determining the type of operation in a digital system?
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What is the significance of the stored program concept in general-purpose computers?
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In the context of instruction codes, what does the operation part signify?
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What happens to the input carry during logic operations as stated in the table?
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How does a computer execute an instruction read from memory?
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What are instruction codes, and why are they important?
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Can all computers execute any operation without limitations, based on instruction codes?
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What is meant by micro operations in the context of computer instructions?
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Study Notes
4-Bit Adder-Subtractor Circuit
- Mode input S alters the function: S = 0 for addition, S = 1 for subtraction.
- Exclusive-OR gates pair the input M with B; when S = 0, B remains unchanged; when S = 1, B's inputs are complemented.
- When subtracting, input carry C0 is set to 1, facilitating the addition of 2's complement of B.
Binary Incrementer
- The increment micro-operation increases a register’s value by one.
- Example: A 4-bit register with binary 1101 increments to 1110.
- Each half-adder in the increment circuit connects outputs, with the least significant half-adder receiving a constant logic-1 input.
- Extending the design allows for n-bit incrementers with multiple half-adders.
Arithmetic Circuit
- A composite circuit can perform various arithmetic micro-operations.
- Basic structure includes parallel adder circuits and multiplexers for operation selection.
- The circuit incorporates two 4-bit inputs (A and B) and one 4-bit output (D).
Circular Shift
- Circular shifting rotates bits in a register without losing any information.
- Outputs re-enter as inputs, allowing for continuous rotation in either direction.
- Notation: cil for circular shift left, cir for circular shift right.
Arithmetic Shift
- Arithmetic shifts adjust signed binary numbers—left shift multiplies by 2; right shift divides by 2.
- The sign bit remains unchanged during shifts, preserving the number's sign.
- A shifter designed with multiplexers caters to various shift operations and controls input direction.
Instruction Codes
- Defines the sequence of micro-operations within a digital system.
- Programs instruct the computer by dictating operations and operand sequences.
- Binary instruction codes composed of operation parts specify actions like addition, subtraction, and shifts.
- Each computer has a distinct instruction set based on the stored program concept.
Instruction Cycle
- Comprises phases: fetching instructions from memory, decoding them, addressing, and executing.
- Upon completion of execution, the process loops back to fetching the next instruction until a HALT instruction is encountered.
Fetch and Decode Phases
- Program Counter (PC) loads the first instruction address.
- Sequence Counter (SC) manages timing signals and increments after each clock pulse.
- Micro-operations include transferring data to the Address Register (AR) and Instruction Register (IR) during specific timing signals.
Input-Output Configuration
- Serial communication transfers data in 8-bit alphanumeric code.
- The input interface transmits data from the keyboard to the Input Register (INPR).
- An input flag (FGI) indicates new information availability; it signals data readiness to be transferred into the Accumulator (AC).
Information Transfer Process
- Initially, FGI is cleared; when a key is pressed, data loads into INPR, setting FGI to 1.
- If the flag is active, data transfers from INPR to AC in parallel, then FGI resets to 0 for new input.
- Output process starts with the output flag (FGO) cleared; if FGO is active, data is sent from AC to Output Register (OUTR) for further processing.
Input-Output Instructions
- Facilitate data transfer to/from the AC register, manage flag bits, and control interrupts.
- Key for ensuring accurate input and output processing in the computing system.
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Description
Explore the fundamentals of a 4-bit adder-subtractor circuit in this quiz. Understand how the mode input S controls the operation between addition and subtraction. Test your knowledge on how exclusive-OR gates and full-adders work in this context.