2.3 Module 4: instruction set: characteristics and functions  (Page 4/7)

Three types of numerical data are common in computers:

• Integer or fixed point
• Floating point
• Decimal

2.2 characters

A common form of data is text or character strings. While textual data are most con­venient for human beings, they cannot, in character form, be easily stored or transmitted by data processing and communications systems. Such systems are designed for binary data. Thus, a number of codes have been devised by which characters are represented by a sequence of bits. Perhaps the earliest common example of this is the Morse code. Today, the most commonly used character code in the International Reference Alphabet (IRA), referred to in the United States as the American Stan­dard Code for Information Interchange (ASCII). IRA is also widely used outside the United States. Each character in this code is represented by a unique 7-bit pattern, thus, 128 different characters can be represented. This is a larger number than is necessary to represent printable characters, and some of the patterns represent control characters. Some of these control characters have to do with controlling the printing of characters on a page. Others are concerned with communications procedures. IRA-encoded characters are almost always stored and transmitted using 8 bits per character. The eighth bit may be set to 0 or used as a par­ity bit for error detection. In the latter case, the bit is set such that the total number of binary 1s in each octet is always odd (odd parity) or always even (even parity).

Another code used to encode characters is the Extended Binary Coded Dec­imal Interchange Code (EBCDIC). EBCDIC is used on IBM S/390 machines. It is an 8-bit code. As with IRA, EBCDIC is compatible with packed decimal. In the case of EBCDIC, the codes 11110000 through 11111001 represent the digits 0 through 9.

2.3 logical data

Normally, each word or other addressable unit (byte, half-word, and soon) is treated as a single unit of data. It is sometimes useful, however, to consider an n-bit unit as consisting 1-bit items of data, each item having the value 0 or I. When data are viewed this way, they are considered to be logic data.

There are two advantages to the bit-oriented view:

• First, we may sometimes wish to store an array of Boolean or binary data items, in which each item can take on only the values I (true) and II (fake). With logical data, memory can be used most efficiently for this storage.
• Second, there are occasions when we wish to manipulate the bits of a data item.

3. types of operations

The number of different opcodes varies widely from machine to machine. However, the same general types of operations are found on all machines. A useful and typi­cal categorization is the following:

• Data transfer
• Arithmetic
• Logical
• Conversion
• I/O
• System control
• Transfer of control

3.1 data transfer

The most fundamental type of machine instruction is the data transfer instruction. The data transfer instruction must specify several things.

• The location of the source and destination operands must be specified. Each location could be memory. a register, or the lop of the stack.
• The length of data to be transferred must be indicated.
• As with all instructions with operands, the mode of addressing for each operand must be specified.