Remove:
var tab2[SIZE_WIDTH][SIZE_HEIGHT] = {0}

Replace the whole memory allocation with:
var *tab2 = sys_malloc(SIZE_WIDTH * SIZE_HEIGHT * sizeof(var));

The first initialization block is fine but the (commented) second one is obviously wrong, you are just writing over entry "1 + 0 = 1 = 0 + 1" and so on. What you wanted to do is the following:
tab2[1*SIZE_HEIGHT + 0] = 6;
...

Btw. a for loop would be appropriate here:
for(i = 0; i < 5; i++)
{
tab2[0*SIZE_HEIGHT + i] = i;
tab2[1*SIZE_HEIGHT + i] = i+5;
}