Example: Sum of Squares

This example shows how machine code is generated for a simple Pascal program. The example program computes

the sum of the squares of the first NUMINTS integers.

The program is written using a while loop instead of a for loop to produce assembly code which more closely resembles the Pascal program. NUMINTS is a constant and the program uses two variables, Count and Squares to perform the calculation. The result is left in a variable called Squares.

program SumSquare;
const
    NUMINTS = 10 {number of integers to sum}
var
    Count, Squares: integer;
begin   {SumSquare}
    Squares := 0;
    Count := 1;
    while ( Count <= NUMINTS ) do
    begin   {while}
        Squares := Squares + (Count * Count);
        Count := Count + 1;
    end;    {while}
end.    {SumSquare}

The MAL squares program uses registers for the variables Count and Squares. The number of integers in the sum is stored in memory at location numints.

#
#	MAL program to sum the squares of 
#	the first 'numints' integers.
#
#	Registers:
#
#	    $2	sum of squares
#	    $3	branch condition
#	    $5	numints
#	    $6	squares
#	    $7	count
#
	.data
numints:	.word	10
	.text
__start:	li	$7, 1
		li	$2, 0
		lw	$5, numints
while:		sub 	$3, $7, $5
		bgtz	$3, end
		mul	$6, $7, $7
		add	$2, $2, $6
		add	$7, $7, 1
		j 	while
end:		done

When the MAL code is assembled, the following TAL program is produced. Note that several MAL instructions must be synthesized from one or more TAL instructions.