Octal

Octal became widely used in computing when systems such as the UNIVAC 1050, PDP-8, ICL 1900 and IBM mainframes employed 6-bit, 12-bit, 24-bit or 36-bit words. Octal was an ideal abbreviation of binary for these machines because their word size is divisible by three (each octal digit represents three binary digits). So two, four, eight or twelve digits could concisely display an entire machine word. It also cut costs by allowing Nixie tubes, seven-segment displays, and calculators to be used for the operator consoles, where binary displays were too complex to use, decimal displays needed complex hardware to convert radices, and hexadecimal displays needed to display more numerals.

All modern computing platforms, however, use 16-, 32-, or 64-bit words, further divided into

Octal numbers that are used in some programming languages (C, Perl, PostScript...) for textual/graphical representations of byte strings when some byte values (unrepresented in a code page, non-graphical, having special meaning in current context or otherwise undesired) have to be to escaped as \nnn. Octal representation may be particularly handy with non-ASCII bytes of UTF-8, which encodes groups of 6 bits, and where any start byte has octal value \3nn and any continuation byte has octal value \2nn.

Octal was also used for

Transponders in aircraft transmit a "squawk" code, expressed as a four-octal-digit number, when interrogated by ground radar. This code is used to distinguish different aircraft on the radar screen.

To convert integer decimals to octal, divide the original number by the largest possible power of 8 and divide the remainders by successively smaller powers of 8 until the power is 1. The octal representation is formed by the quotients, written in the order generated by the algorithm. For example, to convert 125₁₀ to octal:

125 = 8² × 1 + 61

61 = 8¹ × 7 + 5

5 = 8⁰ × 5 + 0

Therefore, 125₁₀ = 175₈.

Another example:

900 = 8³ × 1 + 388

388 = 8² × 6 + 4

4 = 8¹ × 0 + 4

4 = 8⁰ × 4 + 0

Therefore, 900₁₀ = 1604₈.

To convert a decimal fraction to octal, multiply by 8; the integer part of the result is the first digit of the octal fraction. Repeat the process with the fractional part of the result, until it is null or within acceptable error bounds.

Example: Convert 0.1640625 to octal:

0.1640625 × 8 = 1.3125 = 1 + 0.3125

0.3125 × 8 = 2.5 = 2 + 0.5

0.5 × 8 = 4.0 = 4 + 0

Therefore, 0.1640625₁₀ = 0.124₈.

These two methods can be combined to handle decimal numbers with both integer and fractional parts, using the first on the integer part and the second on the fractional part.

To convert integer decimals to octal, prefix the number with "0.". Perform the following steps for as long as digits remain on the right side of the radix: Double the value to the left side of the radix, using octal rules, move the radix point one digit rightward, and then place the doubled value underneath the current value so that the radix points align. If the moved radix point crosses over a digit that is 8 or 9, convert it to 0 or 1 and add the carry to the next leftward digit of the current value. Add octally those digits to the left of the radix and simply drop down those digits to the right, without modification.

Example:

 0.4 9 1 8 decimal value
  +0
 ---------
   4.9 1 8
  +1 0
  --------
   6 1.1 8
  +1 4 2
  --------
   7 5 3.8
  +1 7 2 6
  --------
 1 1 4 6 6. octal value

To convert a number k to decimal, use the formula that defines its base-8 representation:

${\displaystyle k=\sum _{i=0}^{n}\left(a_{i}\times 8^{i}\right)}$

In this formula, a_i is an individual octal digit being converted, where i is the position of the digit (counting from 0 for the right-most digit).

Example: Convert 764₈ to decimal:

764₈ = 7 × 8² + 6 × 8¹ + 4 × 8⁰ = 448 + 48 + 4 = 500₁₀

For double-digit octal numbers this method amounts to multiplying the lead digit by 8 and adding the second digit to get the total.

Example: 65₈ = 6 × 8 + 5 = 53₁₀

To convert octals to decimals, prefix the number with "0.". Perform the following steps for as long as digits remain on the right side of the radix: Double the value to the left side of the radix, using decimal rules, move the radix point one digit rightward, and then place the doubled value underneath the current value so that the radix points align. Subtract decimally those digits to the left of the radix and simply drop down those digits to the right, without modification.

Example:

 0.1 1 4 6 6  octal value
  -0
 -----------
   1.1 4 6 6
  -  2
  ----------
     9.4 6 6
  -  1 8
  ----------
     7 6.6 6
  -  1 5 2
  ----------
     6 1 4.6
  -  1 2 2 8
  ----------
     4 9 1 8. decimal value

To convert octal to binary, replace each octal digit by its binary representation.

Example: Convert 51₈ to binary:

5₈ = 101₂

1₈ = 001₂

Therefore, 51₈ = 101 001₂.

The process is the reverse of the previous algorithm. The binary digits are grouped by threes, starting from the least significant bit and proceeding to the left and to the right. Add leading zeroes (or trailing zeroes to the right of decimal point) to fill out the last group of three if necessary. Then replace each trio with the equivalent octal digit.

For instance, convert binary 1010111100 to octal:

001	010	111	100
1	2	7	4

Therefore, 1010111100₂ = 1274₈.

Convert binary 11100.01001 to octal:

011	100	.	010	010
3	4	.	2	2

Therefore, 11100.01001₂ = 34.22₈.

The conversion is made in two steps using binary as an intermediate base. Octal is converted to binary and then binary to hexadecimal, grouping digits by fours, which correspond each to a hexadecimal digit.

For instance, convert octal 1057 to hexadecimal:

To binary:

1	0	5	7
001	000	101	111

then to hexadecimal:

0010	0010	1111
2	2	F

Therefore, 1057₈ = 22F₁₆.

Hexadecimal to octal conversion proceeds by first converting the hexadecimal digits to 4-bit binary values, then regrouping the binary bits into 3-bit octal digits.

For example, to convert 3FA5₁₆:

To binary:

3	F	A	5
0011	1111	1010	0101

then to octal:

0	011	111	110	100	101
0	3	7	6	4	5

Therefore, 3FA5₁₆ = 37645₈.

Due to having only factors of two, many octal fractions have repeating digits, although these tend to be fairly simple:

The table below gives the expansions of some common irrational numbers in decimal and octal.

• Computer number format – Internal representation of numeric values in a digital computer
• Octal games, a game numbering system used in combinatorial game theory
• Split octal, a 16-bit octal notation used by the Heath Company, DEC and others
• Squawk code, a 12-bit octal representation of Gillham code
• Syllabic octal
  , an octal representation of 8-bit syllables used by English Electric