loops_with_branches

Loops with Branches

This README.md provides a detailed explanation of an ARM assembly code snippet that demonstrates the use of loops with branches. The code calculates the sum of a list of numbers using a loop structure.

Code Explanation

Setup

.global _start
.equ endlist, 0xaaaaaaaa

• .global _start: This line makes the _start label accessible from other files, which is necessary for the linker.
• .equ endlist, 0xaaaaaaaa: This line defines a constant endlist with the value 0xaaaaaaaa, which we'll use as a sentinel value to mark the end of our list.

Entry Point

_start:
	LDR R0,=list
	LDR R3,=endlist
	LDR R1,[R0]
	ADD R2,R2,R1

• _start:: This label marks the entry point of our code.
• LDR R0,=list: Loads the address of the list into register R0.
• LDR R3,=endlist: Loads the sentinel value into register R3.
• LDR R1,[R0]: Loads the first word (number) from the address pointed by R0 into register R1.
• ADD R2,R2,R1: Adds the value in R1 to R2. Since R2 is not initialized, it should be cleared before this operation to avoid undefined behavior.

Loop Structure

loop:
	LDR R1,[R0,#4]!
	CMP R1,R3
	BEQ exit
	ADD R2,R2,R1
	BAL loop

• loop:: This label marks the beginning of our loop.
• LDR R1,[R0,#4]!: Loads the next word from the address pointed by R0 into R1 and then increments R0 by 4 bytes (since our list contains 32-bit numbers).
• CMP R1,R3: Compares the loaded word in R1 with our sentinel value in R3.
• BEQ exit: If the values are equal (meaning we've hit our sentinel value), branch to the label exit.
• ADD R2,R2,R1: If not equal, add the value in R1 to our running total in R2.
• BAL loop: Unconditionally branch back to the beginning of the loop to process the next number.

Exit Point

exit:

• exit:: This label marks our exit point from the loop where we can proceed to further instructions or terminate the program.

Data Section

.data
list:
	.word 1,2,3,4,5,6,7,8,9,10

• .data: This directive indicates that we are starting a data section where we define our list.
• list:: This label marks the beginning of our list in memory.
• .word 1,2,3,...: Defines a series of 32-bit numbers that will be summed by our code.

Flow and Register Values

At each iteration of the loop:

• R0 holds the current address being read from our list.
• R1 holds the current number being added to our sum.
• R2 accumulates our running total sum.
• R3 always holds our sentinel value for comparison.

Initially:

• R0 = address of first element in list (value = 1)
• R1 = first number from list (value = 1)
• R2 = undefined (should be initialized to 0 before starting)

After first iteration:

• R0 = address of second element (value = 2)
• R1 = second number from list (value = 2)
• R2 = sum so far (value = 1)

This pattern continues until R1 equals endlist, at which point we exit the loop.

Remember to initialize R2 to zero before entering _start, as it's used for accumulating our sum. Also note that this code does not handle termination properly; it will just stop executing after reaching the end of the loop without any clean-up or system call to exit.