MIT 6.004 - Computation Structures: Digital Logic & CPU Design Flashcards

Question 1

What is Boolean Algebra?

Accepted Answer

A branch of algebra in which the values of the variables are the truth values `true` and `false`, usually denoted 1 and 0 respectively. It is fundamental for designing digital circuits, using logical operations like AND, OR, and NOT.

Question 2

List the three fundamental Boolean operations and their common symbols.

Accepted Answer

1. **AND**: $A \cdot B$ or $AB$
2. **OR**: $A + B$
3. **NOT**: $ar{A}$ or $A'$

Question 3

Describe the function of an `AND` gate and provide its truth table.

Accepted Answer

An `AND` gate outputs `1` (true) only if all of its inputs are `1`. Otherwise, it outputs `0`.

Truth Table (2-input):
| A | B | Y |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |

Question 4

Describe the function of an `OR` gate and provide its truth table.

Accepted Answer

An `OR` gate outputs `1` (true) if at least one of its inputs is `1`. It outputs `0` only if all inputs are `0`.

Truth Table (2-input):
| A | B | Y |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |

Question 5

Describe the function of a `NOT` gate (inverter) and provide its truth table.

Accepted Answer

A `NOT` gate has a single input and a single output. It outputs the inverse of its input: if the input is `1`, the output is `0`, and vice versa.

Truth Table:
| A | Y |
|---|---|
| 0 | 1 |
| 1 | 0 |

Question 6

What are `NAND` and `NOR` gates, and why are they considered "universal gates"?

Accepted Answer

*   **NAND**: `NOT` AND. Outputs `0` only if all inputs are `1`.
*   **NOR**: `NOT` OR. Outputs `1` only if all inputs are `0`.

They are "universal gates" because any other logic gate (AND, OR, NOT, XOR) can be constructed solely using `NAND` gates or solely using `NOR` gates.

Question 7

Differentiate between combinational and sequential circuits.

Accepted Answer

*   **Combinational Circuits**: Outputs depend *only* on the current inputs. They have no memory or feedback loops. Examples: adders, multiplexers, decoders.
*   **Sequential Circuits**: Outputs depend on both current inputs *and* past inputs (i.e., their current state). They incorporate memory elements (latches, flip-flops) and feedback. Examples: counters, registers, FSMs.

Question 8

Explain the purpose of a `multiplexer` (MUX).

Accepted Answer

A `multiplexer` (MUX) is a combinational circuit that selects one of several input lines and routes it to a single output line. The selection is controlled by a set of select inputs. An $N$-input MUX requires $\log_2 N$ select lines.

Question 9

What is a `latch` in digital logic?

Accepted Answer

A `latch` is a basic memory element in digital circuits that can store one bit of information. It is level-sensitive, meaning its output can change as long as the enable signal is active. `SR latches` and `D latches` are common types.

Question 10

What is a `flip-flop`, and how does it differ from a `latch`?

Accepted Answer

A `flip-flop` is a synchronous memory element that stores one bit of information. Unlike latches, flip-flops are edge-triggered, meaning their output changes only at a specific transition (rising or falling edge) of a clock signal, making them suitable for synchronous systems. Common types include `D`, `JK`, and `T` flip-flops.

Question 11

Define a `Finite State Machine (FSM)` and its two main types.

Accepted Answer

A `Finite State Machine (FSM)` is a mathematical model of computation used to design sequential logic circuits. It consists of a finite number of states, transitions between those states, and actions/outputs associated with states or transitions.

The two main types are:
*   **Moore Machine**: Outputs depend *only* on the current state.
*   **Mealy Machine**: Outputs depend on both the current state *and* the current inputs.

Question 12

What is the `CPU datapath`?

Accepted Answer

The `CPU datapath` refers to the collection of functional units (like the `ALU`, registers, and memory access units) and the buses that connect them, through which data flows and is processed within the CPU. It performs operations on data as directed by the control unit.

Question 13

What is the primary function of the `Arithmetic Logic Unit (ALU)`?

Accepted Answer

The `Arithmetic Logic Unit (ALU)` is a digital circuit within the CPU that performs arithmetic operations (like addition, subtraction, multiplication, division) and logical operations (like AND, OR, NOT, XOR) on integer binary numbers.

Question 14

Explain the role of `registers` in a CPU.

Accepted Answer

`Registers` are small, high-speed storage locations within the CPU. They hold data, instructions, and memory addresses that the CPU is currently processing or will process very soon. They provide the fastest form of memory access to the CPU. Examples include the `Program Counter (PC)`, `Instruction Register (IR)`, and general-purpose registers.

Question 15

What is the `Program Counter (PC)`?

Accepted Answer

The `Program Counter (PC)` is a special-purpose register in the CPU that stores the memory address of the next instruction to be fetched and executed. After an instruction is fetched, the PC is typically incremented to point to the subsequent instruction.

Question 16

What is the function of the `Control Unit` in a CPU?

Accepted Answer

The `Control Unit` is the component of the CPU that directs and coordinates all operations within the processor. It interprets instructions, generates control signals to orchestrate the datapath components (ALU, registers, memory), and manages the flow of data and instructions throughout the CPU.

Question 17

Define `Instruction Set Architecture (ISA)`.

Accepted Answer

The `Instruction Set Architecture (ISA)` is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt, and exception handling. It serves as the interface between hardware and software, defining the set of machine language instructions that a CPU can understand and execute.

Question 18

Describe the basic steps of the `instruction cycle` (Fetch-Decode-Execute cycle).

Accepted Answer

The `instruction cycle` is the fundamental process by which a CPU executes a program:
1.  **Fetch**: The CPU retrieves an instruction from the memory location pointed to by the `Program Counter (PC)`.
2.  **Decode**: The CPU interprets the fetched instruction to determine what operation needs to be performed and which operands are involved.
3.  **Execute**: The CPU performs the operation specified by the instruction, using the `ALU` and registers, and stores the result.

Question 19

Explain the concept of `memory hierarchy`.

Accepted Answer

`Memory hierarchy` is a system design that organizes computer storage into a hierarchy based on speed, cost, and capacity. Faster, more expensive, and smaller memory (e.g., CPU registers, cache) is at the top, while slower, cheaper, and larger memory (e.g., main RAM, secondary storage) is at the bottom. The goal is to provide the CPU with fast access to frequently used data while keeping overall memory costs down.

Question 20

What is `cache memory`, and what principles does it exploit?

Accepted Answer

`Cache memory` is a small, very fast memory component located between the CPU and main memory (RAM). It stores copies of data from frequently used main memory locations. It exploits the principles of:
*   **Temporal Locality**: If an item is referenced, it will tend to be referenced again soon.
*   **Spatial Locality**: If an item is referenced, items whose addresses are close by will tend to be referenced soon.

Question 21

How does low-level software (assembly language) relate to hardware?

Accepted Answer

Assembly language is a low-level programming language that has a very strong correspondence between the language's instructions and the architecture's machine code instructions. Each assembly instruction typically maps directly to a single machine instruction that the CPU's hardware can execute. This direct mapping allows programmers to control hardware resources (like registers and memory) very precisely, bridging the gap between high-level programming and the underlying digital logic.

Question 22

What is the significance of a `clock signal` in synchronous digital circuits?

Accepted Answer

A `clock signal` is a periodic square wave that synchronizes the operations of all components in a synchronous digital circuit. It provides a timing reference, ensuring that state changes in sequential elements (like flip-flops) occur at precise, predictable moments (e.g., on the rising or falling edge of the clock), preventing race conditions and ensuring stable operation.

MIT 6.004 - Computation Structures: Digital Logic & CPU Design

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