Decimal Number System

The Decimal Number Base Systems

The Decimal Number System uses base 10. It includes the digits from 0 through 9. The weighted values for each position is as follows:

You have been using the decimal (base 10) numbering system for so long that you often take it for granted. When you see a number like "123", you don't think about the value 123. Instead, you generate a mental image of how many items this value represents. In reality, however, the number 123 represents:

Each digit appearing to the left of the decimal point represents a value between zero and nine times power of ten represented by its position in the number. Digits appearing to the right of the decimal point represent a value between zero and nine times an increasing negative power of ten. For example, the value 725.194 is represented as follows:

Binary Number System

The Binary Number Base Systems

Computer systems (including the IBM PC) operate using binary logic. The computer represents values using two voltage levels (usually 0V for logic 0 and either +3.3 V or +5V for logic 1). With two levels we can represent exactly two different values. These could be any two different values, but by convention we use the values zero and one. These two values, coincidentally, correspond to the two digits used by the binary number system.

Since there is a correspondence between the logic levels used by the computer and the two digits used in the binary numbering system, it should come as no surprise that computers employ the binary system. The binary number system works like the decimal number system except the Binary Number System:

uses base 2

includes only the digits 0 and 1 (any other digit would make the number an invalid binary number)

The weighted values for each position is determined as follows:

In the United States among other countries, every three decimal digits is separated with a comma to make larger numbers easier to read. For example, 123,456,789 is much easier to read and comprehend than 123456789. We will adopt a similar convention for binary numbers. To make binary numbers more readable, we will add a space every four digits starting from the least significant digit on the left of the decimal point. For example, the binary value 1010111110110010 will be written 1010 1111 1011 0010.

Number Base Conversion

Binary to Decimal

It is very easy to convert from a binary number to a decimal number. Just like the decimal system, we multiply each digit by its weighted position, and add each of the weighted values together. For example, the binary value 1100 1010 represents:

1*2^7 + 1*2^6 + 0*2^5 + 0*2^4 + 1*2^3 + 0*2^2 + 1*2^1 + 0*2^0 =

1 * 128 + 1 * 64 + 0 * 32 + 0 * 16 + 1 * 8 + 0 * 4 + 1 * 2 + 0 * 1 =

128 + 64 + 0 + 0 + 8 + 0 + 2 + 0 =

202

Decimal to Binary

To convert decimal to binary is slightly more difficult. There are two methods, that may be used to convert from decimal to binary, repeated division by 2, and repeated subtraction by the weighted position value.

Repeated Division By 2

For this method, divide the decimal number by 2, if the remainder is 0, on the side write down a 0. If the remainder is 1, write down a 1. This process is continued by dividing the quotient by 2 and dropping the previous remainder until the quotient is 0. When performing the division, the remainders which will represent the binary equivalent of the decimal number are written beginning at the least significant digit (right) and each new digit is written to more significant digit (the left) of the previous digit. Consider the number 2671.

Binary Number Formats

We typically write binary numbers as a sequence of bits (bits is short for binary digits). We have defined boundaries for these bits. These boundaries are:

In any number base, we may add as many leading zeroes as we wish without changing its value. However, we normally add leading zeroes to adjust the binary number to a desired size boundary. For example, we can represent the number five as:

We'll number each bit as follows:

1. The rightmost bit in a binary number is bit position zero.
2. Each bit to the left is given the next successive bit number.

Bit zero is usually referred to as the LSB (least significant bit). The left-most bit is typically called the MSB (most significant bit). We will refer to the intermediate bits by their respective bit numbers.

The Bit

The smallest "unit" of data on a binary computer is a single bit. Since a single bit is capable of representing only two different values (typically zero or one) you may get the impression that there are a very small number of items you can represent with a single bit. Not true! There are an infinite number of items you can represent with a single bit.

With a single bit, you can represent any two distinct items. Examples include zero or one, true or false, on or off, male or female, and right or wrong. However, you are not limited to representing binary data types (that is, those objects which have only two distinct values).

To confuse things even more, different bits can represent different things. For example, one bit might be used to represent the values zero and one, while an adjacent bit might be used to represent the values true and false. How can you tell by looking at the bits? The answer, of course, is that you can't. But this illustrates the whole idea behind computer data structures: data is what you define it to be.

If you use a bit to represent a boolean (true/false) value then that bit (by your definition) represents true or false. For the bit to have any true meaning, you must be consistent. That is, if you're using a bit to represent true or false at one point in your program, you shouldn't use the true/false value stored in that bit to represent red or blue later.

Since most items you will be trying to model require more than two different values, single bit values aren't the most popular data type. However, since everything else consists of groups of bits, bits will play an important role in your programs. Of course, there are several data types that require two distinct values, so it would seem that bits are important by themselves. however, you will soon see that individual bits are difficult to manipulate, so we'll often use other data types to represent boolean values.

The Nibble

A nibble is a collection of bits on a 4-bit boundary. It wouldn't be a particularly interesting data structure except for two items: BCD (binary coded decimal) numbers and hexadecimal (base 16) numbers. It takes four bits to represent a single BCD or hexadecimal digit.

With a nibble, we can represent up to 16 distinct values. In the case of hexadecimal numbers, the values 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F are represented with four bits. BCD uses ten different digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) and requires four bits. In fact, any sixteen distinct values can be represented with a nibble, but hexadecimal and BCD digits are the primary items we can represent with a single nibble.

The Byte

. A byte consists of eight bits and is the smallest addressable datum (data item) in the microprocessor.Main memory and I/O addresses in the PC are all byte addresses. This means that the smallest item that can be individually accessed by an 80x86 program is an 8-bit value. To access anything smaller requires that you read the byte containing the data and mask out the unwanted bits.

The bits in a byte are numbered from bit zero (b0) through seven (b7) as follows:

Bit 0 is the low order bit or least significant bit, bit 7 is the high order bit or most significant bit of the byte. We'll refer to all other bits by their number.

A byte also contains exactly two nibbles. Bits b0 through b3 comprise the low order nibble, and bits b4 through b7 form the high order nibble. Since a byte contains exactly two nibbles, byte values require two hexadecimal digits.

Since a byte contains eight bits, it can represent 2^8, or 256, different values. Generally, we'll use a byte to represent:

1. unsigned numeric values in the range 0 => 255
2. signed numbers in the range -128 => +127
3. ASCII character codes
4. other special data types requiring no more than 256 different values. Many data types have fewer than 256 items so eight bits is usually sufficient.

Since the PC is a byte addressable machine, it turns out to be more efficient to manipulate a whole byte than an individual bit or nibble. For this reason, most programmers use a whole byte to represent data types that require no more than 256 items, even if fewer than eight bits would suffice. For example, we'll often represent the boolean values true and false by 00000001 and 00000000 (respectively).

Probably the most important use for a byte is holding a character code. Characters typed at the keyboard, displayed on the screen, and printed on the printer all have numeric values. To allow it to communicate with the rest of the world, the IBM PC uses a variant of the ASCII character set. There are 128 defined codes in the ASCII character set. IBM uses the remaining 128 possible values for extended character codes including European characters, graphic symbols, Greek letters, and math symbols.

The Word

NOTE:
The boundary for a Word is defined as either 16-bits or the size of the data bus for the processor, and a Double Word is Two Words. Therefore, a Word and a Double Word is not a fixed size but varies from system to system depending on the processor. However, for our discussion, we will define a word as two bytes.

For the 8085 and 8086, a word is a group of 16 bits. We will number the bits in a word starting from bit zero (b0) through fifteen (b15) as follows:

Like the byte, bit 0 is the LSB and bit 15 is the MSB. When referencing the other bits in a word use their bit position number.

Notice that a word contains exactly two bytes. Bits b0 through b7 form the low order byte, bits 8 through 15 form the high order byte. Naturally, a word may be further broken down into four nibbles. Nibble zero is the low order nibble in the word and nibble three is the high order nibble of the word. The other two nibbles are "nibble one" or "nibble two".

With 16 bits, you can represent 2^16 (65,536) different values. These could be the unsigned numeric values in the range of 0 => 65,535, signed numeric values in the range of -32,768 => +32,767, or any other data type with no more than 65,536 values. The three major uses for words are

1. 16-bit integer data values
2. 16-bit memory addresses
3. any number system requiring 16 bits or less

The Double Word

A double word is exactly what its name implies, two words. Therefore, a double word quantity is 32 bits. Naturally, this double word can be divided into a high order word and a low order word, four bytes, or eight nibbles.

Double words can represent all kinds of different data. It may be

1. an unsigned double word in the range of 0 => 4,294,967,295,
2. a signed double word in the range -2,147,483,648 => 2,147,483,647,
3. a 32-bit floating point value
4. any data that requires 32 bits or less.

Hexadecimal Number System

The Hexadecimal Number Base System

A big problem with the binary system is verbosity. To represent the value 202 requires eight binary digits.

The decimal version requires only three decimal digits and, thus, represents numbers much more compactly than does the binary numbering system. This fact was not lost on the engineers who designed binary computer systems.

When dealing with large values, binary numbers quickly become too unwieldy. The hexadecimal (base 16) numbering system solves these problems. Hexadecimal numbers offer the two features:

• hex numbers are very compact
• it is easy to convert from hex to binary and binary to hex.

Since we'll often need to enter hexadecimal numbers into the computer system, we'll need a different mechanism for representing hexadecimal numbers since you cannot enter a subscript to denote the radix of the associated value.

The Hexadecimal system is based on the binary system using a Nibble or 4-bit boundary. In Assembly Language programming, most assemblers require the first digit of a hexadecimal number to be 0, and we place an H at the end of the number to denote the number base.

The Hexadecimal Number System:

uses base 16

includes only the digits 0 through 9 and the letters A, B, C, D, E, and F

In the Hexadecimal number system, the hex values greater than 9 carry the following decimal value:

This table provides all the information you'll ever need to convert from one number base into any other number base for the decimal values from 0 to 16.

To convert a hexadecimal number into a binary number, simply brake the binary number into 4-bit groups beginning with the LSB and substitute the corresponding four bits in binary for each hexadecimal digit in the number.

For example, to convert 0ABCDh into a binary value, simply convert each hexadecimal digit according to the table above. The binary equivalent is:

To convert a binary number into hexadecimal format is almost as easy. The first step is to pad the binary number with leading zeros to make sure that the the binary number contains multiples of four bits. For example, given the binary number 10 1100 1010, the first step would be to add two bits in the MSB position so that it contains 12 bits. The revised binary value is 0010 1100 1010.

The next step is to separate the binary value into groups of four bits, e.g., 0010 1100 1010. Finally, look up these binary values in the table above and substitute the appropriate hexadecimal digits, e.g., 2CA.

The weighted values for each position is as follows:

Binary to Hex Conversion

It is easy to convert from an integer binary number to hex. This is accomplished by:

1. Break the binary number into 4-bit sections from the LSB to the MSB.
2. Convert the 4-bit binary number to its Hex equivalent.

For example, the binary value 1010111110110010 will be written:

Hex to Binary Conversion

It is also easy to convert from an integer hex number to binary. This is accomplished by:

1. Convert the Hex number to its 4-bit binary equivalent.
2. Combine the 4-bit sections by removing the spaces.

For example, the hex value 0AFB2 will be written:

This yields the binary number 1010111110110010 or 1010 1111 1011 0010 in our more readable format.

Hex to Decimal Conversion

To convert from Hex to Decimal, multiply the value in each position by its hex weight and add each value. Using the value from the previous example, 0AFB2H, we would expect to obtain the decimal value 44978.

40960 + 3840 + 176 + 2 = 44978

Decimal to Hex Conversion

To convert decimal to hex is slightly more difficult. The typical method to convert from decimal to hex is repeated division by 16. While we may also use repeated subtraction by the weighted position value, it is more difficult for large decimal numbers.

Repeated Division By 16

For this method, divide the decimal number by 16, and write the remainder on the side as the least significant digit. This process is continued by dividing the quotient by 16 and writing the remainder until the quotient is 0. When performing the division, the remainders which will represent the hex equivalent of the decimal number are written beginning at the least significant digit (right) and each new digit is written to the next more significant digit (the left) of the previous digit. Consider the number 44978.

As you can see, we are back with the original number. That is what we should expect.

When you use hex numbers in an 8085 program, the Assembler usually requires the most significant hex digit to be 0 even if this number of digits exceed the size of the register. This is an Assembler requirement and your value will be assembled correctly.

• Exercises:

Convert 124 from decimal to binary	Answer 111100
Convert 97 from decimal to binary	Answer 1100001
Convert 10111 from binary to decimal	Answer 23

In summary:

bit

a single binary digit, either zero or one.

byte

8 bits, can represent positive numbers from 0 to 255.

hexadecimal

A representation of 4 bits by a single digit 0..9,A..F. In this way a byte can be represented by two hexadecimal digits

long word

A long word is usually twice as long as a word.

nibble

4 bits, half of a byte.

octal

A representation of 3 bits by a single digit 0..7. This is used much less commonly than it once was (early DEC computers used octal for much of their I/O)

word

Usually 16 bits, or two bytes. But a word can be almost any size, depending on the application being considered -- 32 and 64 bits are common sizes

 

Exercises:

Convert 2000 from decimal to hexadecimal	Answer 7D0
Convert 3C from hexadecimal to decimal	Answer 60
Convert 1010 0111 1011 from binary to hexadecimal	Answer A7B
Convert 7D0 from hexadecimal to binary	Answer 0111 1101 0000