addressing_modes

Code Breakdown

The .data Section

.data
list:
    .word 4,5,-9,1,0,2,-3

• .data: This directive indicates that we're starting a section for defining data.
• list: This is a label that acts as a pointer to the beginning of our data in memory.
• .word: This directive defines a series of 32-bit values (words). In ARM, a word is 32 bits or 4 bytes.

The .global Directive and _start Label

.global _start
_start:

• .global _start: This line makes the _start label accessible from other files, which is useful when linking multiple files.
• _start: This label marks the entry point of our program where execution begins.

Loading Data with LDR Instruction

    LDR R0,=list
    LDR R1, [R0]
    LDR R2,[R0,#4]!
    LDR R2,[R0],#4

• LDR R0,=list: Loads the address of list into register R0.
• LDR R1, [R0]: Loads the first word from list into register R1 (which is 4).
• LDR R2,[R0,#4]!: Pre-increment load; increments R0 by 4 (to point to 5) and loads that into R2.
• LDR R2,[R0],#4: Post-increment load; loads the word at current R0 (which is -9) into R2 and then increments R0 by 4.

Understanding Addressing Modes

Addressing modes in ARM determine how the processor calculates the effective address of an operand:

• Pre-increment: The register is incremented before it's used for addressing. For example:

  LDR R2,[R0,#4]!

  Here, R0 is incremented first, then list[R0] is accessed.

• Post-increment: The register is used for addressing and then incremented. For example:

  LDR R2,[R0],#4

  Here, list[R0] is accessed first, then R0 is incremented.

Conclusion

Through this guide, you've learned about data declaration in .data, program entry with .global _start, loading data with LDR, and addressing modes. These are foundational concepts that will aid you as you delve deeper into ARM assembly programming.