Question

3. (a) Rewrite the following C language nested for loop using ARM assembly language. Use register 10 to store index i and register rl to store index j. (15 marks) for (i=4; i<10; i++) { for(j=18; j<5; j--) { www

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Answer 1

Given 'C' language code is a nested for loop.

I have used the notations:

• ( )₁₀ to represent number in decimal system.
• ( )₁₆ to represent number in hexadecimal system.
• I have used suffix 'h' after the number in code to represent the number as hexadecimal.

Below I am attaching the logic in both the 'C' language and AVR assmbly language so that your understanding would be better.

Instruction for Logic mov ro, #04h bge done1 mov r1, #12h Logic in 'C' language code Logic in ARM assembly code It is given that ro register stores index 'i'. So load the hexadecimal equivalent of (4)10 Initialize value of 'i' to (4)10- into the ro register. As (4)10 = (4)16. Load the value of (4)16 into ro register. Check if the value of 'i' with (10)10. If i >= 10 then break Compare the contents of ro register with (10)10 = (A)16. If value in ro register is greater the loop and goto the line after the ending of loop. than or equal to (A)16 then branch to the line after the end of loop. Ifi < (10)10 then loop doesn't break and continue with If value in ro register is greater than or equal to (A)16 then continue execution of the the for loop execution. loop without branching. It is given that rı register stores index 'j'. So load the hexadecimal equivalent of (18)10 Initialize value of 'j' to (18)10. into the ri register. As (18) 10 = (12)16. Load the value of (12)16 into r1 register. Check if the value of 'j' with (5)10. If i >= 5 then break Compare the contents of rı register with (5)10 = (5)16. If value in rı register is greater the loop and goto the line after the ending of loop. than or equal to (5)16 then branch to the line after the end of loop. Ifj< (5)10 then loop doesn't break and continue with If value in r1 register is greater than or equal to (5)16 then continue execution of the the for loop execution. loop without branching. Decrement value of 'j' by (1)10- Decrement value of rų register by (1)10 = (1)16. It is done by substracting (1)16 from the r1 register and storing result again in rı register. Branch to the starting of nested loop Branch to the starting of nested loop Increment value of 'i' by (1)10. Increment value of ro register by (1)10 = (1)16. It is done by adding (1)16 to the ro register and storing the result again in ro register. Branch to the starting of main loop | Branch to the starting of main loop bge done2 sub r1, 11, #01h b loop2 add roro, #01h b loop1

The required ARM assembly language code is:

mov Mo, #04h loop1: Cmp Mo, #0Ah bge done1 bropa: mov M,, #12h loop2: cmp M,,#osh bge donea sub r, er, #01h b loop2. donea: add ro ro #01h b loop 1 lone 1:

In the above code, I have used labels as follows:

• loop1 to denote starting of main loop
• loop2 to denote starting of nested loop
• done1 to denote the line of code to be executed after breaking the main loop
• done2 to denote the line of code to be executed after breaking the nested loop

Explaination of code:

• mov r_0, #04h - This instruction will load the value of (04)₁₆ into the r₀ register
• cmp r₀, #0Ah - This instruction will compare the data in r₀ register with value (0A)_h.
• bge done1 - If 'greater than or equal to' condition is satisfied in above comparision then this instruction will make the code to branch (jump) to the line of code labelled as 'done1'. .
• mov r₁, #12h - This instruction will load the value of (12)₁₆ into the r₁ register
• cmp r₁, #05h - This instruction will compare the data in r₁ register with value (05)_h.
• bge done2 -  If 'greater than or equal to' condition is satisfied in above comparision then this instruction will make the code to branch (jump) to the line of code labelled as 'done2'. .
• The dotted lines are to represent any statement inside of nested for loop as given in question.
• sub r₁, r₁, #01h - This instruction will substract the value of (01)₁₆ from the r₁ register and store the result in r₁ register
• b loop2 - This instruction will make the execution of code to branch (jump) unconditionally to the line of code labelled as 'loop2'
• add r₀, r₀, #01h - This instruction will add the value of (01)₁₆ to the r₀ register and store the result in r₀ register
• b loop1 - This instruction will make the execution of code to branch (jump) unconditionally to the line of code labelled as 'loop1'