Assumed Knowledge:
Learning Outcomes:
  • Represent integers (including negative integers) in different bases.
  • Convert from one base to other.

1. Representing Information

How do we represent data in a computer? At a fundamental level, a computer is an electronic machine that works by controlling the flow of electrons

It is easy to recognize two scenarios:

  1. presence of current flowing through - call this state “1”
  2. absence of current flowing through - call this state “0”

2. Representing numbers in different bases

An integer in base \(b\) consists of values from 0 to \(b-1\).

Hexadecimal is base-16, so it has values rom 0 to 15. However, beyond 9, the following symbols are used:

ValueSymbol
10a
11b
12c
13d
14e
15f

A number \(n\) in base \(b\) is represented as \(n_b\). That is the base is in the subscript. Absence of subscript means it is a decimal value.

For example,

\(1101_2\) is a base-2 or binary value. \(1304_8\) is a base-8 or octal value. \(E2F_{16}\) is a base-16 or hexadecimal value. \(1603_6\) is is an invalid number since base-6 can only have digits between 0 and (6-1 = 5).

Hexadecimal numbers are often prefixed with 0x. For example 0x17c9 represents the hexadecimal number 17c9. Similarly, binary numbers are often prefixed with 0b. For example 0b1101 represents the binary number 1101.

2.1 Positional number systems

Consider these positions in a number system:

... n3 n2 n1 n0
... d3 d2 d1 d0
\[d_3 \times n^3 + d_2 \times n^2 + d_1 \times n^1 + d_0 \times n^0\]

Consider the number 350 in base 10 (radix 10). That is, n is 10, and thus:

\[3 \times n^2 + 5 \times n^1 + 0 \times n^0\] \[3 \times 100 + 5 \times 10 + 0 \times 1\]

Consider the number 10110 in base 2 (radix 2). That is, n is 2, and thus:

\[1 \times n^4 + 0 \times n^3 + 1 \times n^2 + 1 \times n^1 + 0 \times n^0\] \[1 \times 16 + 0 \times 8 + 1 \times 4 + 1 \times 2 + 0 \times 1\]

or 22 in base 10.

2.2 Converting between number systems

2.2.1 Converting from decimal to base b

  1. Start with result = 0
  2. If number is zero, go to step 5.
  3. Divide the number by \(b\) and put the remainder (a value between 0 and \(b-1\) to the left of result.
  4. Go to step 2.
  5. Binary number is in the result.

In the operation \(\frac{a}{b}\), if \(a \times d + r = b\), we call \(d\) the quotient and \(r\) the remainder. Here \(a\) is being divided by \(b\) and \(a\) is the dividend (numerator) while \(b\) is the divisor (denomenator).

Examples

46 to binary:

Divided Divisor Quotient Remainder Result
46 2 23 0 empty -> 0
23 2 11 1 0 -> 10
11 2 5 1 10 -> 110
5 2 2 1 110 -> 1110
2 2 1 0 1110 -> 01110
1 2 0 1 01110 -> 101110
0 STOP      

73 to binary:

Divided Divisor Quotient Remainder Result
73 2 23 1 empty -> 1
36 2 11 0 1 -> 01
18 2 5 0 01 -> 001
9 2 2 1 001 -> 1001
4 2 1 0 1001 -> 01001
2 2 0 0 01001 -> 001001
1 2 0 1 001001 -> 1001001
0 STOP      

46 to base-3:

Divided Divisor Quotient Remainder Result
46 3 15 1 empty -> 1
15 3 5 0 1 -> 01
5 3 1 2 01 -> 201
1 3 0 1 201 -> 1201
0 STOP      

73 to base-16 (hexadecimal):

Divided Divisor Quotient Remainder Result
73 16 4 9 empty -> 9
4 16 0 4 9 -> 49
0 STOP      

2779 to base-16 (hexadecimal):

Divided Divisor Quotient Remainder Result
2779 16 173 11 (B) empty -> B
173 16 10 13 (D) B -> DB
10 16 0 10 (A) DB -> ADB

2.2.2 Converting from base b to decimal

  1. \[weight = 1, result = 0\]
  2. Start with right-most digit (least significant digit)
  3. Multiply digit by \(weight\) and add to \(result\)
  4. Multiply \(weight\) by \(b\)
  5. If digit exists to the left of current digit, go to step 2
  6. Result holds the decimal value

Examples

\(1101_2\) to decimal:

Current Digit Weight Result
1 1 0 -> 0 + 1*1 = 1
0 2 1 -> 1 + 0*2 = 1
1 4 1 -> 1 + 1*4 = 5
1 8 5 -> 5 + 1*8 = 13 (result)

\(ADB_{16}\) to decimal:

Current Digit Weight Result
B 1 0 -> 0 + B*1 = 0 + 11*1 = 11 (remember, b is 11)
D 16 11 -> 11 + 13*16 = 219
A 256 219 -> 219 + 10*256 = 2779 (result)

\(1201_3\) to decimal:

Current Digit Weight Result
1 1 0 -> 0 + 1*1 = 1
0 3 1 -> 1 + 0*3 = 1
2 9 1 -> 1 + 2*9 = 19
1 27 19 -> 19 + 1*27 = 46 (result)

2.2.3 Converting between arbitrary bases

Say, we need to convert a number \(n\) from base \(b_1\) to base \(b_2\).

  1. Convert the number from base \(b_1\) to decimal.
  2. Convert the decimal version from decimal to base \(b_2\).

Example(s):

Convert \(111001_2\) to base-3.

  1. First convert to decimal: \(111001_2\) = \(57_{10}\)
  2. Convert from decimal to base-3: \(2010_3\)

Convert \(4307_9\) to base-16.

  1. First convert to decimal: \(4307_9\) = \(3166_{10}\)
  2. Convert from decimal to base-3: \(C5E_{16}\)

3. Data Types

3.1 Integers

There are two categories of integers (whole numbers):

  • Unsigned integers
  • Typically, each bit represents decreasing (from left to right) magnitudes of powers of 2
  • Signed integers
  • Signed magnitude
  • 1’s complement
  • 2’s complement

Unsigned integers are all positive, and thus you can use all bits to represent positive numbers.

Signed integers use a bit to represent whether the integer is positive or negative.

3.1.1 Unsigned Integers

Let’s see how many unsigned (non-negative) integers can we store using 4 bits.

NumberBits
00000
10001
20010
30011
40100
50101
60110
70111
81000
91001
101010
111011
121100
131101
141110
151111
  • What is the largest unsigned integer using 16 bits?
  • What is the largest unsigned integer using 32 bits?

3.1.2 Signed Integers

But how can we represent negative numbers?

3.1.2.1 Signed Magnitude

The simplest pattern (at least for humans) might be the “signed magnitude”… just use the left-most bit to mean negative.

NumberBits
-71111
-61110
-51101
-41100
-31011
-21010
-11001
-01000
00000
10001
20010
30011
40100
50101
60110
70111

If we add 1 and -1, we should get zero. Using signed magnitude, it’s 0001 + 1001 = 1010, which is -2. So, no good :(

3.1.2.2 One’s Complement

We can keep the left-most bit for sign and flip the others so n + -n is always 0000000 (Ignoring the sign bit).

NumberBits
-71000
-61001
-51010
-41011
-31100
-21101
-11110
-01111
00000
10001
20010
30011
40100
50101
60110
70111

But this leads to two representations of 0 (positive zero and negative zero) - no good!

3.1.2.3 Two’s Complement

An \(n\)-bit two’s complement represents integers in the range \([-2^{(n-1)}, \hskip 2mm 2^{(n-1)} - 1\)] (That represents all integers from \(-2^{(n-1)}\) to \(2^{(n-1)} - 1\)(inclusive on both sides).

A negative number \(k\) is represented by,

  1. Add 1 to \(k\). Call this \(m\). Note that \(m <= 0\).
  2. Negate \(m\). Call this \(p\). Note that \(p >= 0\).
  3. Flip the bits of \(p\).

This can be represented as

\[bin(k | k < 0) = flip(toBinary(negate(inc(k))))\]

The order of operations should be:

Increment > Negate > To binary > Flip

You can remember it as “INToF”

In a 4-bit system, -6 would be represented by,

  1. adding 1 to get -5,
  2. negating -5 to get 5,
  3. flipping binary representation of 5 (0101) to get 1010.
NumberBits
-81000
-71001
-61010
-51011
-41100
-31101
-21110
-11111
00000
10001
20010
30011
40100
50101
60110
70111

Advantages:

  • no negative zero 👍
  • -1 + 1 = 1111 + 0001 = 0000 (overflow discarded). 👍

What is the range of numbers represented in 16 bits using 2’s complement?

Examples

1
2
3
4
5

-46 in 8-bit binary
I: Increment: -46+1 = -45
N: Negate: -(-45) = 45
To: To binary = 00101101
F: Flip = 11010010

1
2
3
4
5

-793 in 16-bit binary
I: Increment: -793+1 = -792
N: Negate: -(-792) = 792
To: To binary = 0000001100011000
F: Flip = 1111110011100111

3.2 Floating Point Numbers (ADVANCED)

Could be used just to indicate where a decimal place

Used to represent really big numbers and really small numbers.

Consider that we have 32 bits to use. The IEEE Standard for Floating Point Arithmetic defined the following representation:

  • 1 bit for the sign (0 = positive, 1 = negative)
  • 8 bits for exponent (offset by 127)
  • 23 bits for precision (leading 1 assumed)

3.2.1 To IEEE format

Example 1

Convert to IEEE floating-point representation:

\[-6 \dfrac{5}{8}\]

3.2.1.1 Convert to binary representation

n3 n2 n1 n0 Decimal n-1 n-2 n-3 n-4
d3 d2 d1 d0 . d-1 d-2 d-3 d-4

Just as before we see how many column values will go into a number, we continue for \(n^{-1}\)…

\[0 \times n^3 + 1 \times n^2 + 1 \times n^1 + 0 \times n^0 + 1 \times n^{-1} + 0 \times n^{-2} + 1 \times n^{-3}\] \[0 \times 8 + 1 \times 4 + 1 \times 2 + 0 \times 1 + 1 \times \dfrac{1}{2} + 0 \times \dfrac{1}{4} + 1 \times \dfrac{1}{8}\]

or:

-0110.101

3.2.1.2 Normalize

Move the decimal place to left or right to get a single 1 to the left of the decimal place. Count how many places and in which direction:

-0110.101

becomes:

-01.10101

with 2 moves (exponent).

3.2.1.3 Add 127 to exponent, convert to binary

2 + 127 = 129

which is:

100000001

in binary.

3.2.1.4 Combine, leaving off leading 1 of precision

1 100000001 10101

and pad to a total of 32 bits:

1 100000001 10101000000000000000000

Example 2

\[-6 \dfrac{5}{16}\]

3.2.1.1 Convert to binary representation

n3 n2 n1 n0 Decimal n-1 n-2 n-3 n-4 n-5
d3 d2 d1 d0 . d-1 d-2 d-3 d-4 d-5

Just as before we see how many column values will go into a number, we continue for \(n^{-1}\)…

\[0 \times n^3 + 1 \times n^2 + 1 \times n^1 + 0 \times n^0 + 0 \times n^{-1} + 1 \times n^{-2} + 0 \times n^{-3} + 1 \times n^{-4}\] \[=0 \times 8 + 1 \times 4 + 1 \times 2 + 0 \times 1 + 0 \times \dfrac{1}{2} + 1 \times \dfrac{1}{4} + 0 \times \dfrac{1}{8} + 1 \times \dfrac{1}{16}\]

or:

-0110.0101

Rest remains the same.

3.2.2 From IEEE format

Consider:

00111101100000000000000000000000

Do the reverse:

3.2.2.1 First digit is sign

Break into parts:

0 01111011 00000000000000000000000

It is positive!

3.2.2.1 Next 8 bits is exponent, minus 127

Convert next 8 unsigned bits into decimal, and subtract 127:

01111011 is 123

123 - 127 = -4

3.2.2.1 Get precision

Put a one in front of the last 23 bits:

1.00000000000000000000000

and move the decimal spot by the exponent. In this case, 4 places to the left:

0.000100000000000000000000000

Convert to decimal:

\[0 * \dfrac{1}{2} + 0 * \dfrac{1}{4} + 0 * \dfrac{1}{8} + 1 * \dfrac{1}{16} ...\]

3.2.2.1 All together

\[+ \dfrac{1}{16}\]