Lec 20 - Addressing Modes

1. INTRODUCTION TO INSTRUCTION SET ARCHITECTURE
There are two basic memory operations. These are the memory write and memory read operations. During a memory write operation a word is stored into a memory location whose address is specified. During a memory read operation a word is read from a memory location whose address is specified. Typically, memory read and memory write operations are performed by the central processing unit (CPU). Three basic steps are needed in order for the CPU to perform a write operation into a specified memory location:


1. The word to be stored into the memory location is first loaded by the CPU into a specified register, called the memory data register (MDR).
2. The address of the location into which the word is to be stored is loaded by the CPU into a specified register, called the memory address register (MAR).
3. A signal, called write, is issued by the CPU indicating that the word stored in the MDR is to be stored in the memory location whose address in loaded in the MAR.
It is worth mentioning that the MDR and the MAR are registers used exclusively by the CPU and are not accessible to the programmer.
Similar to the write operation, three basic steps are needed in order to perform a memory read operation:
1. The address of the location from which the word is to be read is loaded into the MAR.
2. A signal, called read, is issued by the CPU indicating that the word whose address is in the MAR is to be read into the MDR.
3. After some time, corresponding to the memory delay in reading the specified word, the required word will be loaded by the memory into the MDR ready for use by the CPU.


2. ADDRESSING MODES
Information involved in any operation performed by the CPU needs to be addressed. In computer terminology, such information is called the operand. Therefore, any instruction issued by the processor must carry at least two types of information. These are the operation to be performed, encoded in what is called the op code field, and the address information of the operand on which the operation is to be performed, encoded in what is called the address field. Instructions can be classified based on the number of operands as: three address, two-address, one-and-half-address, one-address, and zero-address. The convention operation, source, destination will be used to express any instruction. In that convention, operation represents the operation to be performed, for example, add, subtract, write, or read. The source field represents the source operand(s). The source operand can be a constant, a value stored in a register, or a value stored in the memory. The destination field represents the place where the result of the operation is to be stored, for example, a register or a memory location.
The different ways in which operands can be addressed are called the addressing modes. Addressing modes differ in the way the address information of operands is specified. The simplest addressing mode is to include the operand itself in the instruction, that is, no address information is needed. This is called immediate addressing. A more involved addressing mode is to compute the address of the operand by adding a constant value to the content of a register. This is called indexed addressing. Between these two addressing modes there exist a number of other addressing modes including direct addressing, and indirect addressing. A number of different addressing modes are explained below.
2.1. IMMEDIATE MODE
According to this addressing mode, the value of the operand is (immediately) available in the instruction itself. Consider, for example, the case of loading the decimal value 1000 into a register Ri. This operation can be performed using an instruction such as the following: LOAD #1000, Ri. In this instruction, the operation to be performed is to load a value into a register. The source operand is (immediately) given as 1000, and the destination is the register Ri. It should be noted that in order to indicate that the value 1000 mentioned in the instruction is the operand itself and not its address (immediate mode), it is customary to prefix the operand by the special character (#).

2.2. DIRECT (ABSOLUTE) MODE
According to this addressing mode, the address of the memory location that holds the operand is included in the instruction. Consider, for example, the case of loading the value of the operand stored in memory location 1000 into register Ri. This operation can be performed using an instruction such as LOAD 1000, Ri. In this instruction, the source operand is the value stored in the memory location whose address is 1000, and the destination is the register Ri. Note that the value 1000 is not prefixed with any special characters, indicating that it is the (direct or absolute) address of the source operand.
2.3. INDIRECT MODE
In the indirect mode, what is included in the instruction is not the address of the operand, but rather a name of a register or a memory location that holds the (effective) address of the operand. In order to indicate the use of indirection in the instruction, it is customary to include the name of the register or the memory location in parentheses. Consider, for example, the instruction LOAD (1000), Ri. This instruction has the memory location 1000 enclosed in parentheses, thus indicating indirection. The meaning of this instruction is to load register Ri with the contents of the memory location whose address is stored at memory address 1000. Because indirection can be made through either a register or a memory location, therefore, we can identify two types of indirect addressing. These are register indirect addressing, if a register is used to hold the address of the operand, and memory indirect addressing, if a memory location is used to hold the address of the operand.

2.4. INDEXED MODE
In this addressing mode, the address of the operand is obtained by adding a constant to the content of a register, called the index register. Consider, for example, the instruction LOAD X(Rind), Ri. This instruction loads register Ri with the contents of the memory location whose address is the sum of the contents of register Rind and the value X. Index addressing is indicated in the instruction by including the name of the index register in parentheses and using the symbol X to indicate the constant to be added.