2.3 Module 4: instruction set: characteristics and functions  (Page 5/7)

In term of CPU action, data transfer operations are perhaps the simplest type. If both source and destination are registers, then the CPU simply causes data to be transferred from one register to another; this is an operation internal to the CPU. If one or both operands are in memory, then (he CPU must perform some or all of following actions:

1. Calculate the memory address, based on the address mode

2. If the address refers to virtual memory, translate from virtual to actual mem­ory address.

3. Determine whether the addressed item is in cache.

4. If not, issue a command lo the memory module.

Example:

3.2 arithmetic

Most machines provide the basic arithmetic operations of add, subtract, multiply, and divide. These are invariably provided for signed integer (fixed-point) numbers, Often they are also provided for floating-point and packed decimal numbers.

Other possible operations include a variety of single-operand instructions: for example.

• Absolute: Take the absolute value of the operand.

• Negate: Negate the Operand.

• Increment.: Add 1 to the operand.

• Decrement: Subtract 1 from the operand

3.3 logical

Most machines also provide a variety of operations for manipulating individual bits of a word or other addressable units, often referred to as "bit twiddling." They are based upon Boolean operations.

Some of the basic logical operations that can be performed on Boolean or binary data are AND, OR, NOT, XOR, …

These logical operations can be applied bitwise to n-bit logical data units. Thus, if two registers contain the data

(R1) - 10100101 (R2) - 00001111

then

(R1) AND (R2) – 00000101

In addition lo bitwise logical operations, most machines provide a variety of shifting and rotating functions such as shift left, shift right, right rotate, left rotate…

3.4 conversion

Conversion instructions are those that change the format or operate on the format of data. An example is converting from decimal to binary.

3.5 input/output

As we saw, there are a variety of approaches taken, including isolated programmed IO, memory-mapped programmed I/O, DMA, and the use of an I/O processor. Many implementations provide only a few I/O instructions, with the specific actions spec­ified by parameters, codes, or command words.

3.6 system controls

System control instructions are those that can he executed only while the proces­sor is in a certain privileged state or is executing a program in a special privileged area of memory, typically, these instructions are reserved for the use of the oper­ating system.

Some examples of system control operations are as follows. A system control instruction may read or alter a control register. Another example is an instruction to read or modify a storage protection key, such us is used in the S/390 memory system. Another example is access to process control blocks in a multiprogramming system.