Once we use equations to creat a game, we can't have negative numbers of course.
And the two numbers must be drawn from the random counter.

So we have considered as doing the subtraction, we must do the addition first, for example, after drew two numbers 13 and 1,
it must do the addition first like
13 + 1 = 14

then the answer 14 will become the fellowing to do the subtraction,
14 - 1 = 13 to ensure that the result will not have negative answer.
then the player will see,
14 - 1 = ? and the player need to input the answer 13 and he can get marks.

But I don't really know how to write it to assembly manguage by using internal memory instead of using registers.

Could somebody help? Thank you!