In computer organization, addressing modes refer to the different ways in which operands (data) can be accessed and manipulated in memory during the execution of machine instructions. Auto-increment and auto-decrement are two specific addressing modes used in various computer architectures. These addressing modes are commonly used in assembly language programming to simplify the handling of data and memory access.
Auto-Increment Addressing Mode: In the auto-increment addressing mode, the memory location pointed to by a memory address register (MAR) or an index register is first accessed for the operation, and then the register’s value is automatically incremented by a fixed value after the access. This means that the next access will be to the memory location immediately following the current one.
Consider the following hypothetical assembly language instructions:
MOV R1, #100 ; Load a value into register R1
LOAD R2, [R1]+ ; Load the value from memory address pointed by R1 and increment R1 by 1 (auto-increment)
In this example, the instruction LOAD R2, [R1]+ will first load the value from memory address 100 (as R1 contains 100), and then the value of R1 will be automatically incremented by 1. So, the next time you use R1, it will point to memory address 101.
Auto-Decrement Addressing Mode: In the auto-decrement addressing mode, the memory location pointed to by a memory address register (MAR) or an index register is first accessed for the operation, and then the register’s value is automatically decremented by a fixed value after the access. This means that the next access will be to the memory location immediately preceding the current one.
Let’s see an example for this mode as well:
MOV R1, #200 ; Load a value into register R1
LOAD R2, [R1]- ; Load the value from memory address pointed by R1 and decrement R1 by 1 (auto-decrement)
In this example, the instruction LOAD R2, [R1]- will first load the value from memory address 200 (as R1 contains 200), and then the value of R1 will be automatically decremented by 1. So, the next time you use R1, it will point to memory address 199.
Applications:
These addressing modes are particularly useful when accessing arrays, lists, or data structures in memory. For example, in a loop that processes an array of values, you can use auto-increment to access the next element and auto-decrement to access the previous element easily without manually updating the memory address register.
