DDCA Ch6 - Part 21: Signed and Unsigned RISC-V Instructions

Introduction

In this section, the speaker introduces the topic of signed and unsigned instructions in RISC-V architecture, highlighting the differences between them and their impact on operations like multiplication.

Signed vs. Unsigned Instructions

• Signed and unsigned instructions in RISC-V cater to different flavors of operations and overflow detection.

• The speaker explains how signed (mole h) and unsigned (mole h u, mole h s u) multiplication instructions treat operands differently.

• Interpretation of numbers as signed or unsigned affects the outcome of operations.

• Example provided where interpreting numbers differently leads to varied results in multiplication.

• Branches and comparisons have both signed and unsigned versions.

• Discussion on branches interpreting numbers as signed two's complement or in an unsigned manner.

Overflow Detection

This part delves into detecting overflow in both signed and unsigned scenarios within RISC-V architecture.

Detecting Overflow

• Unsigned overflow detection relies on comparing the result with input numbers.

• If the result is smaller than either input number, overflow has occurred.

• Signed overflow detection involves using comparison instructions like set less than.

• Comparing results with inputs helps identify if overflow took place based on sign differences.

• Branching can be used to determine correctness post-operation.