Loops

COMP124 Lectures 2021-02-17

Iteration can be achieved by jumping backwards in the code.

We need to ensure that we don’t end up in an infinite loop. A while loop simply repeat while a given condition is true.

while Loops

This codes calculates the 1000th Fibonacci number:

while (fib2 < 1000) {
	fib0 = fib1;
	fib1 = fib2; 
	fib2 = fib1 + fib0;
}

This is the equivalent assembly code:

while1:
	mov eax, fib2
	cmp eax, 1000	; compare for condition
	jge end_while	; jump out of loop if condition is met
	mov eax, fib1
	mov fib0, eax
	mov eax, fib2
	mov fib1, eax
	add eax, fib0
	mov fib2, eax
	jmp while1		; jump back to start of loop
end_while:
	...

do-while Loops

This is the same code using a do-while loop:

do {
	fib0 = fib1;
	fib1 = fib2; 
	fib2 = fib1 + fib0;
} while (fib2 < 1000);

And the same code in assembly:

do-while:
	mov eax, fib1
	mov fib0, eax
	mov eax, fib2
	mov fib1, eax
	add eax, fib0
	mov fib2, eax
	cmp eax, 1000
	jl do-while
end_while:
	...

This code is more efficient than the last implementation as fib2 is already in the accumulator for the comparison.

for Loops

A counting loop can be implemented in almost exactly the same way as a while loop:

for (int x = 1; x <= 10; x++) {
	y = y + x;
}

Attempt 1

	mov eax, 1	; use eax as x var
floop:			; start of for loop
	add y, eax	; update y
	inc eax		; x++
	cmp eax, 10
	jle floop	; end when eax = 11

Attempt 2

We can improve by counting in reverse:

	mov eax, 10	; start at 10
floop:			; start of for loop
	add y, eax	; update y
	dec eax		; x--
	jnz floop	; end at zero

Attempt 3 - ecx and loop

The ecx register is the counter register and often used in loops.

In can be used iwth special jump instructions:

• JECXZ - Jump is ecx is zero.
• JCXZ - Jump is cx is zero.

It can also be used with the loop instruction:

loop <lab>	; decrement ecx
			; jump to <lab> if ecx not zero

So, general form to repeat a statement $n$ times:

	mox ecx, n
L1:	code for statement
	loop L1

This would give the following for the example:

	mov ecx, 10	; use ecx as x var
floop:
	add y, ecx	; update y
	loop floop	; loop back to floop if ecx not zero

This saves another instruction.