What is Index Addressing Mode in Assembly Language?

What is Index Addressing Mode in Assembly Language?

Index addressing mode is a type of addressing mode used in assembly language to reference data stored in memory. This mode allows the programmer to access data through the use of an index register, which specifies an address in memory that the data is located at. The index register can be used to access the data indirectly via the address that it points to, making it easy to access multiple pieces of data at the same time without requiring a unique address for each item.

To use index addressing mode, the assembly language programmer must first set up an index register. This is typically done by loading a constant or a variable into the index register that points to the location of the desired data in memory. Then when the instruction is executed, the data is accessed indirectly via the index register that points to the address in memory.

One of the key advantages of using index addressing mode is that it allows the programmer to access multiple pieces of data with one instruction. This can be especially useful when dealing with large arrays of data that need to be accessed in a specific order, since the index register can be used to easily keep track of which data items are needed and when.

Index addressing mode is widely used in assembly language programming and is an important tool for any programmer who needs to access large amounts of data quickly and efficiently. While it may take some practice to understand how it works, this addressing mode can give programmers a great deal of power over their code.

How Does Indirect Addressing Work?

Indirect addressing is a memory addressing mode used in assembly language programming. In this addressing mode, the operand of an instruction is a memory address that points to another memory location where the actual data is stored.

Here's how indirect addressing works:

1. The instruction specifies a memory address that contains the address of the actual data. This address is often stored in a register or a memory location that the programmer has defined.

2. The CPU reads the memory address specified in the instruction to get the address of the actual data.

3. The CPU then reads the actual data from the memory location pointed to by the address retrieved in the previous step.

For example, let's say we want to load a value stored in memory address 0x100 into the EAX register using indirect addressing. Here's what the program might look like in x86 assembly language:

mov ebx, 0x200   ; assume 0x200 contains the address of 0x100
mov eax, [ebx]   ; move the value at the address pointed to by ebx into eax

In this program, we first load the address of the desired data (0x100) into the EBX register. Then, we use indirect addressing by specifying the value in the EBX register (which contains the address of the actual data) as the operand of the mov instruction. The CPU then reads the value at the memory location pointed to by EBX (0x100), and moves that value into EAX.

Indirect addressing is often used in situations where the address of the data is not known until runtime or where the data is stored in a data structure like an array or a linked list.

Note:

• Source or destination address is the sum of the base address and the Accumulator
• This Addressing Mode is only for code memory read
• Base Address can be or PC

Possiblities

MOVC A,@A+DPTR

MOVC A,@A+PC

Example

MOV DPTR, #4000h
MOV A,#5h
MOVC A,@A+DPTR; A<-M[4005]