2.7

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Questions and Answers

What are fields in C primarily used for within a word?

To pack objects and match interfaces (correct)

To store string literals

To perform floating-point operations

To create arrays of integers

Which of the following operations can isolate a field in a word?

An AND operation and a left shift followed by a right shift (correct)

A logical OR followed by a bitwise NOT

A bitwise XOR followed by incrementing the result

A shift right followed by a bitwise AND

What is a key difference between a computer and a simple calculator?

The ability to perform multiplication

The capacity to store large amounts of data

The proficiency in handling complex numbers

The capability to make decisions based on input data (correct)

What does the RISC-V instruction 'beq' stand for?

Branch if equal Signup and view all the answers

What is the purpose of a conditional branch in programming?

To test a condition and alter the program flow based on its outcome. Signup and view all the answers

Which instruction is used to go to a label if two registers do not hold equal values?

bne Signup and view all the answers

In the given RISC-V code, what will the instruction 'bne x22, x23, Else' do?

Transition to the label 'Else' if i is not equal to j. Signup and view all the answers

What is the minimum bit size of a field in C?

1 bit Signup and view all the answers

Which instruction performs the operation 'f = g + h' when the condition is true?

add x19, x20, x21 Signup and view all the answers

In decision-making, what does the sign of a number determine?

Which routine to execute Signup and view all the answers

How does the RISC-V code handle the termination of the conditional statements?

Through an unconditional branch instruction. Signup and view all the answers

What operation does the 'bne' instruction perform in RISC-V assembly?

It branches if the values are not equal Signup and view all the answers

What does the instruction 'beq x0, x0, Exit' signify?

The program will branch to 'Exit' unconditionally. Signup and view all the answers

Why is it often more efficient to test for the opposite condition in branching?

It requires fewer instructions. Signup and view all the answers

What is the role of labels like 'Else' and 'Exit' in assembly language?

To mark locations in the code for branching. Signup and view all the answers

Which operation is performed in the else section of the if statement provided?

f = g - h Signup and view all the answers

What is the main advantage of high-level programming languages regarding branch and label management?

They eliminate the need for explicit labels and branches. Signup and view all the answers

In a traditional loop in C, what condition is being checked?

If save[i] is equal to k. Signup and view all the answers

How is the index 'i' adjusted within the loop based on the given C code?

It is incremented by 1. Signup and view all the answers

What assembly instruction is primarily used to load the value of save[i] in RISC-V based on the given C code?

Load Instruction. Signup and view all the answers

What is the primary arithmetic operation performed on 'i' to create the address of save[i] in RISC-V?

Multiplication by 4. Signup and view all the answers

In the if statement structure illustrated, which component corresponds to the else part?

The right box. Signup and view all the answers

Which term describes the repetitive execution of code as defined in the content?

Loops. Signup and view all the answers

What is the role of shifting left in the context of accessing array elements in RISC-V?

To multiply the index by 4. Signup and view all the answers

What does the instruction 'slli x10, x22, 2' accomplish?

Shifts the value in x22 to the left by 2 bits Signup and view all the answers

Which operation is performed to calculate the address of save[i]?

adding x10 to the base address stored in x25 Signup and view all the answers

What condition does the instruction 'bne x9, x24, Exit' check for?

If save[i] is not equal to k Signup and view all the answers

What is a basic block in programming?

A sequence of instructions without any branches Signup and view all the answers

What type of comparison tests are less common in loop instructions?

Bitwise tests Signup and view all the answers

What is the significance of the most significant bit in comparisons?

It determines if a number is negative or positive Signup and view all the answers

What happens at the end of the loop labeled 'Exit'?

The next instruction after Exit is executed Signup and view all the answers

Which of the following statements is true about the use of loops in programming?

Loops can use various comparison tests including less than and greater than Signup and view all the answers

What is the purpose of the sign bit in two's complement notation?

It distinguishes positive from negative integers. Signup and view all the answers

How can an unsigned comparison of two integers be used to check for index-out-of-bounds?

By checking if either integer is negative or the first integer is greater than the second. Signup and view all the answers

What programming construct allows selection among multiple alternatives based on a single value?

Case or switch statement Signup and view all the answers

Which of the following best describes a branch address table?

An array containing addresses of alternative instruction sequences. Signup and view all the answers

What role does the indirect jump instruction serve in RISC-V?

It enables an unconditional branch to a specified address. Signup and view all the answers

How is a switch statement typically implemented under the hood?

Using a sequence of conditional tests. Signup and view all the answers

Which instruction in RISC-V is used to branch to an address stored in a register?

jalr Signup and view all the answers

What is a conditional branch at the instruction set level primarily used for?

Implementing decision-making structures Signup and view all the answers

Study Notes

Language of the Computer: Bit Fields

C language supports definition of bit fields within words, enabling efficient memory packing for objects and aligning with external interfaces such as I/O devices.
Bit fields in C must fit within a single word and can be as small as 1 bit.
RISC-V compilers manage bit field insertion and extraction using logical instructions: andi, ori, slli, and srli.

Decision-Making in Computers

A distinguishing feature of computers compared to simple calculators is their ability to make decisions based on computational results.
Decision-making is represented by 'if' statements along with go-to statements and labels in programming languages.
RISC-V assembly language utilizes two primary decision-making instructions:
- beq rs1, rs2, L1 for "branch if equal".
- bne rs1, rs2, L1 for "branch if not equal".

Compiling if-then-else Structures

Example C statement: if (i == j) f = g + h; else f = g - h;
RISC-V code is compiled for efficiency by testing for the opposite condition first using bne to branch to the Else section if i is not equal to j.
Unconditional branches can be created with beq x0, x0, Exit, ensuring code execution always moves to a designated exit point.

Loop Structures

Loops rely on decision-making instructions, such as:
- while (save[i] == k) i += 1;
RISC-V assembly code first calculates the address of save[i] by shifting i left by 2 bits for byte addressing.
Key instructions for the loop include loading data, checking the loop condition, and branching back to the start if the condition holds true.

Basic Blocks

Sequences of instructions leading to a branch are termed basic blocks, which lack branches except possibly at the end.
Compilation involves breaking the program into basic blocks for more manageable code structure and optimization.

Comparison of Values

Various relational tests are common in loops, including less than (<), greater than (≥), equality (==), and inequality (≠).
Signed and unsigned number comparisons are important, particularly in two’s complement notation, where the sign bit affects comparisons.

Index Out of Bounds Check

Code for bounds checking can be optimized to check for negative values and bounds in one instruction using unsigned comparison.
For example: bgeu x20, x11, IndexOutOfBounds checks if x20 is greater than or equal to x11 or if x20 is negative.

Case/Switch Statements

Most programming languages implement case or switch statements, which can be transformed into a series of conditional tests.
More efficient implementations can use a branch address table (a list of instruction addresses) to select alternatives based on a single value.
RISC-V includes the jump-and-link register instruction (jalr) for indirect jumps to addresses specified in registers, enabling efficient branching.

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Description

This quiz covers Chapter 2 of C programming, focusing on the language's ability to define bit fields within words. You'll explore how fields can be packed inside a word and fit an external interface, such as those required by I/O devices. Test your understanding of unsigned integers and their size limitations in this context.

2.7

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