2.7

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.

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.