Bits, Bytes and Integers

References

• Slides adapted from CMU

Outline

• Representing information as bits

• Bit-level manipulations

• Integers

  • Representation: unsigned and signed

  • Conversion, casting

  • Expanding, truncating

  • Addition, negation, multiplication, shifting

  • Summary

• Representations in memory, pointers, strings

Everything is bits

• Each bit is 0 or 1

• By encoding/interpreting sets of bits in various ways

  • Computers determine what to do (instructions)

  • … and represent and manipulate numbers, sets, strings, etc.

• Why bits? Electronic implementation

  • Easy to store with bitstable elements

  • Reliably transmitted on noisy and inaccurate wires

Example: Counting in Binary

• Base 2 number representation

  • Represent \(15213_{10}\) as \(11101101101101_{2}\)

  • Represent \(1.20_{10}\) as \(1.0011001100110011[0011] \ldots_{2}\)

  • Represent \(1.5213 \times 10^4\) as \(1.1101101101101_{2} \times 2^{13}\)

Encoding Byte Values

• Byte = 8 bits

  • Binary: \(00000000_{2}\) to \(11111111_{2}\)

  • Decimal: \(0_{10}\) to \(255_{10}\)

  • Hexadecimal: \(00_{16}\) to \(FF_{16}\)

    • Base 16 number representation

    • Use characters ‘0’ to ‘9’ and ‘A’ to ‘F’

    • Typically written in most programming languages with the prefix 0x

Encoding Byte Values

Encoding Byte Values

Example Data Representations

Boolean Algebra

• Algebraic representation of logic

  • Encode “true” as 1 and “false” as 0

  • Developed by George Boole in the 19th Century

• Operations

  • and (&): a & b = 1 when both a = 1 and b = 1

  • or (|): a | b = 1 when either a = 1 and b = 1

  • not (~): ~a = 1 when a = 0

  • xor (^): a ^ b = 1 when either a = 1 or b = 1, but not both

General Boolean Algebras

• Operate on Bit Vectors

  • operations applied bitwise

• Example: \[\begin{align*} & 01101001\\ \texttt{&} \; & 01010101\\ \hline & 01000001\\ \end{align*}\]

• All of the properties of Boolean algebra apply

Example: Representing and Manipulating Sets

• Representation

  • Width \(w\) bit vector represents subsets of \(\{0, \ldots, w-1\}\)

  • \(a_j = 1\) if \(j \in A\)

• Operations

  • &: intersection

  • |: union

  • ^: symmetric difference

  • ~: complement

Example: Representing and Manipulating Sets

• Examples with \(w = 8\)

  • \(x = 01101001 = \{0, 3, 5, 6\}\)

  • \(y = 01010101 = \{0, 2, 4, 6\}\)

  • \(x \; \texttt{&} \; y = 01000001 = \{0, 6\}\)

  • \(x \; \texttt{|} \; y = 01111101 = \{0, 2, 3, 4, 5, 6\}\)

Bit-Level Operations in C

• The operations &, |, ~, and ^ are available in C

  • apply to any “integral” data type: long, int, short, char, unsigned

  • arguments are viewed as bit vectors

  • arguments are applied bitwise

• Examples with char type

  • ~0x41 \(\rightarrow\) 0xBE

  • ~0x00 \(\rightarrow\) 0xFF

  • 0x69 & 0x55 \(\rightarrow\) 0x41

Contrast: Logical Operations in C

• The logical operations in C are &&, ||, and !

  • zero is viewed as “false”

  • any non-zero value is viewed as “true”

  • always return 0 or 1

  • short-circuit evaluation

• Examples with char data type

  • !0x41 \(\rightarrow\) 0x00

  • !0x00 \(\rightarrow\) 0x01

  • 0x42 && 0x55 \(\rightarrow\) 0x01

Shift Operations

• Left shift: x << y

  • shift bit vector x left y positions

  • fill with zeros on the right

• Right shift: x >> y

  • shift bit vector x right y positions

  • logical shift: fill with zeros on the left

  • arithmetic shift: replicate most significant bit on the left

• Undefined behavior: shift amount less than zero or greater than bit vector length

Shift Examples

• x = 01100010

  • x << 3 = 00010000

  • logical: x >> 2 = 00011000

  • arithmetic: x >> 2 = 00011000

• x = 10100010

  • x << 3 = 00010000

  • logical: x >> 2 = 00101000

  • arithmetic: x >> 2 = 11101000

Encoding Integers

• Unsigned

  \[B2U(x) = \sum_{i=0}^{w-1} x_i \cdot 2^i\]

  where \(x\) is the bit vector and \(w\) is the length of the bit vector

• Signed: two’s complement

  \[B2T(x) = -x_{w-1} \cdot 2^{w-1} \sum_{i=0}^{w-2} x_i \cdot 2^i\]

  where \(x\) is the bit vector, \(w\) is the length of the bit vector, and \(-x_{x-1}\) is the sign bit

Example 3 Bit Integer Encodings

Numeric Ranges

• Unsigned values

  • min = 0

  • max = \(2^{w} - 1\)

• Two’s complement values

  • min = \(-2^{w-1}\)

  • max = \(2^{w-1} - 1\)

Example Numeric Ranges

• Values where \(w = 16\)

  	decimal	hex	binary
  unsigned max	65535	FF FF	11111111 11111111
  signed max	32767	7F FF	01111111 11111111
  signed min	-32768	80 00	10000000 00000000
  -1	-1	FF FF	11111111 11111111
  0	0	00 00	00000000 00000000

Unsigned and Signed Numeric Values

• Equivalence

  • Same encodings for non-negative values

• Uniqueness

  • Every bit pattern represents a unique integer value

  • Each representable integer has a unique bit encoding

• Can invert mappings

  • unsigned bit pattern = \(U2B(x) = B2U^{-1}(x)\)

  • two’s complement bit pattern = \(T2B(x) = B2T^{-1}(x)\)

Mapping Between Signed and Unsigned

• Mappings between unsigned and two’s complement numbers: keep the bit representation and reinterpret.

• Two’s complement to unsigned: \(T2B \circ B2U\)

• Unsigned to two’s complement: \(U2B \circ B2T\)

Signed to Unsigned

Unsigned to Signed

Signed vs. Unsigned in C

• Constants

  • By default are considered to be signed integers

  • Unsigned if the suffix is “U”, for example 42U

• Casting

  • Explicit casting between signed and unsigned same as \(U2T\) and \(T2U\)

  • Implicit casting also occurs via assignments and procedure calls

Casting Surprises

• Expression evaluation

  • If there is a mix of unsigned and signed integers in a single expression, then signed values are implicilty cast to unsigned values.

  • Including comparison operations: <, >, ==, <=, >=

• Examples

  Operand 1	Operand 2	Relation	Evaluation
  0	0U	==	unsigned
  -1	0	<	signed
  -1	0U	>	unsigned
  -1	-2	>	signed

Unsigned vs. Signed in C

• Easy to make mistakes

• Example 1

  unsigned i;
  for (i = cnt-2; i >= 0; i--)
      a[i] += a[i+1]

• Example 2

  #define DELTA sizeof(int)
  int i;
  for (i = CNT; i-DELTA >= 0; i -= DELTA)
      ...

Summary: Casting Rules

• Bit pattern is maintained, but reinterpreted

• Can have unexpected effects: adding or subtracting \(2^w\)

• An expression containing signed and unsigned ints implicitly casts the signed ints to unsigned ints

Sign Extension

• Task

  • Given \(w\)-bit signed integer \(x\)

  • Convert it to \(w+k\) bit integer \(x'\) with the same value

• Rule

  • Make \(k\) copies of the sign bit:

  • \(x' = x_{w-1}, \ldots, x_{w-1}, x_{w-1}, x_{w-2}, \ldots, x_{0}\)

• C automatically performs sign extension

Sign Extension Example

• Example of sign extensions from \(w=3\) to \(w=4\)

Truncation

• Task:

  • Given \(k+w\)-bit signed or unsigned integer \(x\)

  • Convert it to \(w\)-bit integer \(x'\) with the same value for “small enough” \(x\)

• Rule:

  • Drop top \(k\) bits:

  • \(x' = x_{w-1}, x_{w-2}, \ldots, x_0\)

Summary: Expanding and Truncating Rules

• Expanding (e.g. short to int)

  • Unsigned: zeros added

  • Signed: sign extension

  • Both yield expected result

• Truncating (e.g. int to short)

  • Unsigned/signed: bits are truncated

  • Result is reinterpreted

  • Unsigned: modulus operation

  • Signed: similar to modulus

  • For small (in magnitude) numbers yields expected behavior

Unsigned Addition

• \(UAdd_{w}(u, v)\)

  • Operands: \(w\) bits

  • True sum: \(w+1\) bits

  • Discard carry: \(w\) bits

• Standard addition function ignores carry output

• Implements modular arithmetic

  \[s = UAdd_w(u, v) = u + v \; \texttt{mod} \; 2^w\]

\(UAdd\) Overflow

• Implements modular arithmetic \(s = UAdd_w(u, v) = u + v \; \texttt{mod} \; 2^w\)

Visualizing Mathematical Integer Addition

• \(Add_4(u, v)\)

Visualizing Unsigned Integer Addition

• \(UAdd_4(u, v)\)

Two’s Complement Addition

• \(TAdd_{w}(u, v)\)

  • Operands: \(w\) bits

  • True sum: \(w+1\) bits

  • Discard carry: \(w\) bits

• \(TAdd\) and \(Uadd\) have identical bit level behavior

\(TAdd\) Overflow

• True add requires \(w+1\) bits; drop off the most significant bit and interpret as 2’s complement integer

Visualizing Two’s Complement Addition

• \(TAdd_{4}(u, v)\)

Integer Multiplication

• Problem: the exact product of \(w\)-bit numbers \(u, v\) might have a result that exceeds \(w\) bits.

  • Unsigned: up to \(2w\) bits

  • Two’s complement min (negative): up to \(2w-1\) bits

  • Two’s complement max (positive): up to \(2w\) bits

• Maintaining exact results

  • would need to keep expanding word size with each product computed

  • is done in software if needed

Unsigned Multiplication in C

• \(UMul_{w}(u, v)\)

  • Operands: \(w\) bits

  • True product: \(2w\) bits

  • Discard \(w\) bits: \(w\) bits

• Implements modular arithmetic

  \[s = UMul_w(u, v) = u + v \; \texttt{mod} \; 2^w\]

Signed Multiplication in C

• \(TMul_{w}(u, v)\)

  • Operands: \(w\) bits

  • True product: \(2w\) bits

  • Discard \(w\) bits: \(w\) bits

• Ignores high order \(w\) bits, some of which are different for signed vs. unsigned multiplication

Power-of-2 Multiply with Shift

• Operation u << k

  • Gives \(u \cdot 2^k\) for both signed and unsigned

  • Operands: \(w\) bits

  • True product \(w+k\) bits

  • Discard \(k\) bits: \(w\) bits

Unsigned Power-of-2 Divide with Shift

• Operation u >> k

  • Gives

    \[\bigg\lfloor \frac{u}{2^k} \bigg\rfloor\]

  • Uses logical shift

Signed Power-of-2 Divide with Shift

• Operation u >> k

  • Gives

    \[\bigg\lfloor \frac{u}{2^k} \bigg\rfloor\]

  • Uses arithmetic shift

  • Rounds wrong direction when \(u < 0\)

Correct Signed Power-of-2 Divide with Shift

• Quotient of negative number power of 2

  • Want

    \[\bigg\lceil \frac{u}{2^k} \bigg\rceil\]

  • Compute as

    \[\bigg\lfloor \frac{u+2^k-1}{2^k} \bigg\rfloor\]

    • In C: (u + (1<<k) - 1) >> k

    • Biases dividend toward 0

Negation: Complement and Increment

• Negate through complement and increment

  ~x + 1 = -x

• Examples

  Value	x	~x	~x+1	Result
  15213	3B6D	C492	C493	-15213
  0	0000	FFFF	0000	0
  TMin	8000	7FFF	8000	TMin

Arithmetic: Basic Rules

• Addition

  • Unsigned/signed: normal addition followed by truncate

  • Unsigned: addition mod \(2^w\)

  • Signed: modified addition mod \(2^w\) (result in proper range)

• Multiplication

  • Unsigned/signed: normal multiplication followed by truncate

  • Unsigned: multiplication mod \(2^w\)

  • Signed: modified multiplication mod \(2^w\) (result in proper range)

Byte-Oriented Memory Organization

• Programs refer to data by address

  • Conceptually envision it as a very large array of bytes

  • An address is like an index into that array, and a pointer variable stores an address

• Note: system provides private address space to each “process”

  • Think of a process as a program being executed

  • So, a program can clobber its own data, but not that of others

Machine Words

• Any given computer has a “word size”

  • Nominal size of integer-valued data

• Until recently, most machines used 32 bits (4 bytes) as a word size

• Increasingly, machines have 64 bit word size

• Machines still support multiple data formats

  • Fractions or multiples of word size

  • Always integral number of bytes

Word-Oriented Memory Organization

• Addresses specify byte locations

  • Address of first byte in word

  • Addresses of successive words differ by 4 (32 bit) or 8 (64 bit)

Byte Ordering

• How are the bytes within a multi-byte word ordered in memory?

• Conventions

  • Big endian: least significant byte has highest address

  • Little endian: least significant byte has lowest address

• Example: 4-byte value of 0x1234567

  • Big endian: 01 23 45 67

  • Little endian: 67 45 23 01

Examining Data Representations

• Code to print byte representation of data

  typedef unsigned char *pointer;
  void show_bytes(pointer start, size_t len) {
    size_t i;
    for (i = 0; i < len; i++) {
        printf("%p\t0x%.2x\n", start+i, start[i]);
    }
    printf("\n");
  }

Representing Strings

• Strings in C

  • Represented by an array of characters

  • Each character is encoded in ASCII format

  • Strings should be null terminated (final character = 0)

• Compatibility

  • Byte ordering is not an issue

Reading Byte-Reversed Listings

• Disassembly

  • Text representation of binary machine code

  • Generated by program that reads the machine code

• Example Fragment

  Address     Instruction code        Assembly Rendition
  8048365:    5b                      pop
  8048366:    81 c3 ab 12 00 00       add     $0x12ab,%ebx
  804836c:    83 bb 28 00 00 00 00    cmpl    $0x0,0x28(%ebx)

Summary

• Representing information as bits

• Bit-level manipulations

• Integers

  • Representation: unsigned and signed

  • Conversion, casting

  • Expanding, truncating

  • Addition, negation, multiplication, shifting

  • Summary

• Representations in memory, pointers, strings