757 lines
28 KiB
Markdown
757 lines
28 KiB
Markdown
---
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title: Integer Encoding
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TARGET DECK: Obsidian::STEM
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FILE TAGS: binary
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tags:
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- binary
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---
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## Overview
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Integers are typically encoded using either **unsigned encoding** or **two's-complement**. The following table highlights how the min and max of these encodings behave:
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Value | $w = 8$ | $w = 16$ | $w = 32$
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-------- | ------- | -------- | ------------
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$UMin_w$ | `0x00` | `0x0000` | `0x00000000`
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$UMax_w$ | `0xFF` | `0xFFFF` | `0xFFFFFFFF`
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$TMin_w$ | `0x80` | `0x8000` | `0x80000000`
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$TMax_w$ | `0x7F` | `0x7FFF` | `0x7FFFFFFF`
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%%ANKI
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Basic
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What is a C integral type?
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Back: A type representing finite ranges of integers.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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Tags: c17
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<!--ID: 1708177246087-->
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END%%
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%%ANKI
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Basic
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In what two ways are C integral types encoded?
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Back: Unsigned encoding or two's-complement.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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Tags: c17
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<!--ID: 1708177246093-->
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END%%
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%%ANKI
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Basic
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What integer values does unsigned encoding represent?
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Back: Nonnegative values.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708177246097-->
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END%%
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%%ANKI
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Basic
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What integer values does two's-complement represent?
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Back: Negative, zero, and positive values.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708177246102-->
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END%%
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%%ANKI
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Basic
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An integral value of $0_{10}$ likely has what encoding?
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Back: Either unsigned or two's-complement.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708177246105-->
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END%%
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%%ANKI
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Basic
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An integral value of $100_{10}$ likely has what encoding?
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Back: Either unsigned or two's-complement.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708177246109-->
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END%%
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%%ANKI
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Basic
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An integral value of $-100_{10}$ likely has what encoding?
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Back: Two's-complement.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708177246114-->
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END%%
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%%ANKI
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Basic
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Which of unsigned encoding or two's-complement exhibit asymmetry in their range?
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Back: Two's-complement.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708453398379-->
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END%%
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%%ANKI
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Basic
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What integral values share the same binary representation in unsigned encoding and two's-complement?
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Back: Nonnegative values $\leq TMax$.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708454709515-->
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END%%
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%%ANKI
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Basic
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According to the C standard, how are `unsigned` integral types encoded?
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Back: Using unsigned encoding.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708455064691-->
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END%%
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%%ANKI
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Basic
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According to the C standard, how are `signed` integral types encoded?
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Back: The C standard leaves this unspecified.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708455064696-->
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END%%
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%%ANKI
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Basic
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According to the C standard, Is `unsigned` underflow/overflow safe?
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Back: Yes
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708551236389-->
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END%%
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%%ANKI
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Basic
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According to the C standard, Is `signed` underflow/overflow safe?
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Back: No
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708551236392-->
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END%%
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%%ANKI
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Basic
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Why is `signed` underflow/overflow considered UB?
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Back: Because there is no requirement on how `signed` integers are encoded.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708551236395-->
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END%%
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%%ANKI
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Basic
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How does $UMax$ relate to $TMax$?
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Back: $UMax = 2 \cdot TMax + 1$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708453398445-->
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END%%
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%%ANKI
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Basic
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*Why* is it $UMax = 2 \cdot TMax + 1$?
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Back: All bit patterns denoting negative numbers in two's-complement are positive in unsigned encoding.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708613447880-->
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END%%
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%%ANKI
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Basic
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What are the binary encodings of $UMax_4$ and $TMax_4$?
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Back: $1111_2$ and $0111_2$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708453398449-->
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END%%
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%%ANKI
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Basic
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Reinterpret $TMax$ in unsigned encoding. What arithmetic operations yield $UMax$?
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Back: Multiply by two and add one.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708453398454-->
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END%%
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%%ANKI
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Basic
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Reinterpret $TMax$ in unsigned encoding. What bitwise operations yield $UMax$?
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Back: One-bit left shift and add one.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708453398459-->
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END%%
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%%ANKI
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Basic
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Reinterpret $UMax$ in two's-complement. What decimal value do you have?
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Back: $-1$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708453398469-->
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END%%
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### Unsigned Encoding
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Always represents nonnegative numbers. Given an integral type $\vec{x}$ of $w$ bits, we convert binary to its unsigned encoding with: $$B2U_w(\vec{x}) = 2^{w-1}x_{w-1} + \sum_{i=0}^{w-2} 2^ix_i$$
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Note we unfold the summation on the RHS by one term to make it's relationship to $T2U_w$ clearer.
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%%ANKI
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Basic
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What does $UMin_w$ evaluate to?
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Back: $0$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708545383256-->
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END%%
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%%ANKI
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Basic
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What does $UMax_w$ evaluate to?
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Back: $2^w - 1$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708545383258-->
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END%%
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%%ANKI
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Basic
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What half-open interval represents the possible $w$-bit unsigned decimal values?
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Back: $[0, 2^w)$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708177246128-->
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END%%
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%%ANKI
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Basic
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What is the binary representation of the smallest $4$-bit unsigned number?
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Back: $0000_2$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708177246133-->
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END%%
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%%ANKI
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Basic
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What is the binary representation of the largest $4$-bit unsigned number?
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Back: $1111_2$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708177246138-->
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END%%
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%%ANKI
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Basic
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What is the decimal expansion of unsigned integer $1010_2$?
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Back: $2^3 + 2^1 = 10$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708177246143-->
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END%%
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%%ANKI
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Basic
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What does the "uniqueness" of unsigned encoding refer to?
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Back: The function used to convert integral types to their unsigned encoding is a bijection.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708177246148-->
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END%%
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%%ANKI
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Basic
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How does Bryant et al. define $B2U_w$?
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Back: $B2U_w(\vec{x}) = 2^{w-1}x_{w-1} + \sum_{i=0}^{w-2} 2^ix_i$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179147785-->
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END%%
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%%ANKI
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Basic
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What is $B2U_w$ an acronym for?
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Back: **B**inary to **u**nsigned, width $w$.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179147791-->
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END%%
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%%ANKI
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Basic
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What is $U2B_w$ an acronym for?
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Back: **U**nsigned to **b**inary, width $w$.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708613447885-->
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END%%
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%%ANKI
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Basic
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What does $w$ in $B2U_w$ represent?
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Back: The number of bits in the integral type being interpreted.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179147795-->
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END%%
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%%ANKI
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Basic
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What is the domain of $B2U_w$?
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Back: Bit strings of size $w$.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179147798-->
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END%%
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%%ANKI
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Basic
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What is the domain of $U2B_w$?
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Back: $[0, 2^w)$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708613447888-->
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END%%
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%%ANKI
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Basic
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What is the range of $B2U_w$?
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Back: $[0, 2^w)$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179147801-->
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END%%
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%%ANKI
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Basic
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What is the range of $U2B_w$?
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Back: Bit strings of length $w$.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708613447891-->
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END%%
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%%ANKI
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Basic
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How is the smallest unsigned integer formatted in hexadecimal?
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Back: As all `0`s.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708453398392-->
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END%%
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%%ANKI
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Basic
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How is the largest unsigned integer formatted in hexadecimal?
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Back: As all `F`s.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708453398403-->
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END%%
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%%ANKI
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Basic
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How does `n` relate to `~n` in unsigned encoding?
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Back: `~n = UMax - n`
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Reference: “Two’s-Complement.” In *Wikipedia*, January 9, 2024. [https://en.wikipedia.org/w/index.php?title=Two%27s_complement&oldid=1194543561](https://en.wikipedia.org/w/index.php?title=Two%27s_complement&oldid=1194543561).
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<!--ID: 1708545383259-->
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END%%
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%%ANKI
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Basic
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Using unsigned encoding, *why* does `n + ~n = UMax`?
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Back: Because this always yields a bit string of all `1`s.
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Reference: “Two’s-Complement.” In *Wikipedia*, January 9, 2024. [https://en.wikipedia.org/w/index.php?title=Two%27s_complement&oldid=1194543561](https://en.wikipedia.org/w/index.php?title=Two%27s_complement&oldid=1194543561).
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<!--ID: 1708545574154-->
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END%%
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%%ANKI
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Basic
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Regardless of word size, what bitwise operations yield $UMax$?
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Back: `~0`
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Reference: “Two’s-Complement.” In *Wikipedia*, January 9, 2024. [https://en.wikipedia.org/w/index.php?title=Two%27s_complement&oldid=1194543561](https://en.wikipedia.org/w/index.php?title=Two%27s_complement&oldid=1194543561).
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<!--ID: 1708545383261-->
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END%%
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### Two's-Complement
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Represents negative numbers along with nonnegative ones. Given an integral type $\vec{x}$ of $w$ bits, we convert binary to its twos'-complement encoding with: $$B2T_w(\vec{x}) = -2^{w-1}x_{w-1} + \sum_{i=0}^{w-2} 2^ix_i$$
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%%ANKI
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Basic
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What does $TMin_w$ evaluate to?
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Back: $-2^{w-1}$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708545383252-->
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END%%
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%%ANKI
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Basic
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What does $TMax_w$ evaluate to?
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Back: $2^{w-1} - 1$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708545383255-->
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END%%
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%%ANKI
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Basic
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How do $TMin$ and $TMax$ relate to one another?
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Back: $TMin = -TMax - 1$
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<!--ID: 1708609869518-->
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END%%
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%%ANKI
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Basic
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What half-open interval represents the possible $w$-bit two's-complement decimal values?
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Back: $[-2^{w-1}, 2^{w-1})$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708177246128-->
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END%%
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%%ANKI
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Cloze
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$[${1:$0$}, {2:$2^w$}$)$ is to unsigned as $[${1:$-2^{w-1}$}, {2:$2^{w-1}$}$)$ is to two's-complement.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179147813-->
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END%%
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%%ANKI
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Basic
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What is the binary representation of the smallest $4$-bit two's-complement number?
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Back: $1000_2$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179649872-->
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END%%
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%%ANKI
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Basic
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What is the binary representation of the largest $4$-bit two's-complement number?
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Back: $0111_2$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179649876-->
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END%%
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%%ANKI
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Cloze
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The {sign bit} refers to the {most significant bit} in two's-complement.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179649881-->
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END%%
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%%ANKI
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Basic
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What is the weight of the sign bit in $w$-bit two's-complement?
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Back: $-2^{w-1}$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179649887-->
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END%%
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%%ANKI
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Basic
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What does the "uniqueness" of two's-complement refer to?
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Back: The function used to convert integral types to two's-complement is a bijection.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179649894-->
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END%%
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%%ANKI
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Basic
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How does Bryant et al. define $B2T_w$?
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Back: $B2T_w(\vec{x}) = -2^{w-1}x_{w-1} + \sum_{i=0}^{w-2} 2^ix_i$
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179649901-->
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END%%
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%%ANKI
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Basic
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What is $B2T_w$ an acronym for?
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Back: **B**inary to **t**wo's-complement, width $w$.
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Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
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<!--ID: 1708179649907-->
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||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
What is $T2B_w$ an acronym for?
|
||
Back: **T**wo's-complement to **b**inary, width $w$.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708613447895-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
What does $w$ in $B2T_w$ represent?
|
||
Back: The number of bits in the integral type being interpreted.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708179649913-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
What is the domain of $B2T_w$?
|
||
Back: Bit strings of size $w$.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708179649921-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
What is the domain of $T2B_w$?
|
||
Back: $[-2^{w-1}, 2^{w-1})$
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708613447899-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
What is the range of $B2T_w$?
|
||
Back: $[-2^{w-1}, 2^{w-1})$
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708179649928-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
What is the range of $T2B_w$?
|
||
Back: Bit strings of length $w$.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708613447903-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
How is the smallest two's-complement integer formatted in hexadecimal?
|
||
Back: With a leading `8` followed by `0`s.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708453398413-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
How is the largest two's-complement integer formatted in hexadecimal?
|
||
Back: With a leading `7` followed by `F`s.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708453398425-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
How is equality $|TMin| = |TMax|$ modified so that both sides actually balance?
|
||
Back: $|TMin| = |TMax| + 1$
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708453398430-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
Which of negative and positive numbers can two's-complement encoding express more of?
|
||
Back: Negative.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708453398435-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
Why is two's-complement's encoding range asymmetric?
|
||
Back: Leading `1`s correspond to negatives but leading `0`s corerspond to nonnegative numbers (which include $0$).
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708453398440-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
What are the median values of two's-complement's encoding range?
|
||
Back: `-1` and `0`
|
||
Reference: “Two’s-Complement.” In *Wikipedia*, January 9, 2024. [https://en.wikipedia.org/w/index.php?title=Two%27s_complement&oldid=1194543561](https://en.wikipedia.org/w/index.php?title=Two%27s_complement&oldid=1194543561).
|
||
<!--ID: 1708545383262-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Cloze
|
||
In two's-complement, the {sign bit} partitions the encoding range into two sets.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708545383265-->
|
||
END%%
|
||
|
||
## Casting
|
||
|
||
Most implementations of C cast an object of one type to another by simply re-interpreting the object's binary representation. This casting may happen implicitly if comparing or operating on e.g. `signed` and `unsigned` objects in the same expression. $T2U$ and $U2T$ reflect this method of casting:
|
||
|
||
$$T2U_w(x) = \begin{cases}
|
||
x + 2^w & x < 0 \\
|
||
x & x \geq 0
|
||
\end{cases}$$
|
||
|
||
$$U2T_w(x) = \begin{cases}
|
||
x & x \leq TMax_w \\
|
||
x - 2^w & x > TMax_w
|
||
\end{cases}$$
|
||
|
||
%%ANKI
|
||
Basic
|
||
How do most implementations of C perform casting?
|
||
Back: As a reinterpretation of the same byte pattern of the object being casted.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
Tags: c17
|
||
<!--ID: 1708615249879-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
What is $T2U_w$ an acronym for?
|
||
Back: **T**wo's-complement to **u**nsigned, width $w$.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708615249883-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
For what values $x$ does $T2U_w(x) = U2T_w(x) = x$?
|
||
Back: $0 \leq x \leq TMax_w$
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708696117167-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
What values $x$ are unaffected when casting from `signed` to `unsigned`?
|
||
Back: $0 \leq x \leq TMax_w$
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
Tags: c17
|
||
<!--ID: 1708615249891-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
What values $x$ are unaffected when casting from `unsigned` to `signed`?
|
||
Back: $0 \leq x \leq TMax_w$
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
Tags: c17
|
||
<!--ID: 1708615249897-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
How are casts implicitly performed in operations containing `signed` and `unsigned` objects?
|
||
Back: `signed` objects are cast to `unsigned` objects.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
Tags: c17
|
||
<!--ID: 1708615249903-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Cloze
|
||
For {$x < 0$}, $T2U_w(x) =$ {$x + 2^w$}.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708615249908-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Cloze
|
||
For {$x \geq 0$}, $T2U_w(x) =$ {$x$}.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708615249914-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
How is $T2U_w$ written as a function composition?
|
||
Back: $T2U_w = B2U_w \circ T2B_w$
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708615249920-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
What is $U2T_w$ an acronym for?
|
||
Back: **U**nsigned to **t**wo's-complement, width $w$.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708615249925-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
How is $U2T_w$ written as a function composition?
|
||
Back: $U2T_w = B2T_w \circ U2B_w$
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708615249930-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Cloze
|
||
For {$x > TMax_w$}, $U2T_w(x) =$ {$x - 2^w$}.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708615249935-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Cloze
|
||
For {$x \leq TMax_w$}, $U2T_w(x) =$ {$x$}.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708615249939-->
|
||
END%%
|
||
|
||
### Expansion
|
||
|
||
For unsigned encoding, use **zero extension** to convert numbers to larger types. For example, $1010_2$ can be expanded to 8-bit $00001010_2$.
|
||
|
||
%%ANKI
|
||
Cloze
|
||
Use {zero} extension to convert {unsigned} numbers to larger types.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708697867799-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
Zero extension is generally used for what type of integer encoding?
|
||
Back: Unsigned.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708697867807-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
*Why* does zero extension of unsigned numbers work?
|
||
Back: The weights of additional bits are zeroed out.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708697867810-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
*Why* does zero extension of two's-complement numbers generally not work?
|
||
Back: A negative value would have its new sign bit be positive.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708697867814-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
How is $\langle x_3, x_2, x_1, x_0 \rangle$ zero extended to 8 bits?
|
||
Back: As $\langle 0, 0, 0, 0, x_3, x_2, x_1, x_0 \rangle$
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708697867818-->
|
||
END%%
|
||
|
||
For two's-complement, use **sign extension** to convert numbers to larger types. This means the additional leftmost bits are set to match the sign bit of the original number. For example, $1010_2$ can be expanded to 8-bit $11111010_2$.
|
||
|
||
%%ANKI
|
||
Cloze
|
||
Use {sign} extension to convert {two's-complement} numbers to larger types.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708697867821-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
Sign extension is generally used for what type of integer encoding?
|
||
Back: Two's-complement.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708697867825-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
*Why* does sign extension of two's-complement numbers work?
|
||
Back: The new sign bit weight is equal to the swing in the previous sign bit weight.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708697867829-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
*Why* does sign extension of unsigned numbers generally not work?
|
||
Back: It actually does, though we technically call it zero extension.
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708697867833-->
|
||
END%%
|
||
|
||
%%ANKI
|
||
Basic
|
||
How is $\langle x_3, x_2, x_1, x_0 \rangle$ sign extended to 8 bits?
|
||
Back: As $\langle x_3, x_3, x_3, x_3, x_3, x_2, x_1, x_0 \rangle$
|
||
Reference: Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
<!--ID: 1708697867839-->
|
||
END%%
|
||
|
||
### Truncation
|
||
|
||
## References
|
||
|
||
* Bryant, Randal E., and David O'Hallaron. *Computer Systems: A Programmer's Perspective*. Third edition, Global edition. Always Learning. Pearson, 2016.
|
||
* “Two’s-Complement.” In *Wikipedia*, January 9, 2024. [https://en.wikipedia.org/w/index.php?title=Two%27s_complement&oldid=1194543561](https://en.wikipedia.org/w/index.php?title=Two%27s_complement&oldid=1194543561).
|