Mult.asm problem

[William DesLauriers]

I ran a mult.asm with a CPU emulator. I have 3 comparison failures at lines 3, 6, and 7.

@R1 	// creates a copy of R1
	D=M 
	@memory
	M=D // end

@R2 	// zeros R2
	M=0 // end

// Calculate!

(LOOP)
	@LOOP
	@R0 	// Calculate R0 times R1
	D=M
	@R2
	M=M+D
	@memory
	D=M 
	D=D-1
	M=D-1
	@LOOP
	D;JGT // end of loop R0 times R1


(END)
	@END
	0;JMP

"Buggy list"
|R0 |R1 |R2 |
|0  |0  |0  |
|1  |0  |1  |
|0  |2  |0  |
|3  |1  |3  |
|2  |4  |6  |
|6  |7  |24 |

Thanks in advance, Wm

[cadet1620]

I see two problems with this code.

The first is that the loop always executes at least one time, so when the program tries to multiply by 0, it ends up multiplying by 1.

The second problem is that the loop decrements 'number' by 2, so the program doesn't go through the loop enough times.

--Mark

[William DesLauriers]

I smashed a bug related to the second problem.

Please rephrase your first problem comment or give me another hint.  Comparison failure at line 3.
Thanks,
Wm

[cadet1620]

Your current code computes prod = a * b as:

prod = 0
do {
    prod = prod + a
    b = b - 1
} while b > 0

So when it computes 1 * 0, it does:

prod = 0
prod = 0 + 1 = 1
b = 0 - 1 = -1

and exits the loop with prod = -1, instead of 0.

--Mark

[William DesLauriers]

I am very sorry.  I did not understand the recent comment.  Please give me another hint.  I haven't take any courses for a Java, a C, a C++, nor a C#.  Wm

[cadet1620]

Basically, there are two types of loop structures: the pretest loop and the posttest loop.

The pretest loop is used when the instructions in the body of the loop are to be executed zero or more times>. The posttest loop executes its body one or more times.

Pretest Loop		if/gotoimplementation LOOP: if (condition) goto EXIT ... goto LOOP EXIT: while implementation while (condition) { ... }
Posttest Loop		if/goto implementation LOOP: ... if (condition) goto LOOP while implementation do { ... } while (condition)

The Mult.asm program needs to use the pretest loop structure because the multiplier can be zero, in which case the loop must exit before it does the addition.

(LOOP)
    @memory
    D=M
    @END
    D;JLE

    @R0     // Calculate R0 times R1
    ...
    @memory
    M=M-1
    @LOOP
    D;JMP
(END)

--Mark

[William DesLauriers]

To reduce my confusion on my own side, I better upload my file here or somewhere else.  I think I got it.  But I changed my code to clarify the program flow.  Now, the message, which is within the code, doesn't match with the email message.  It was my own mistake.

  Multiplier
X Multiplicand
___________
  Product

Sorry, file removed from here.
Thanks,
Wm

[cadet1620]

Congratulations on getting your Mult.asm working.

Please edit your post to remove the working code. We want students to develop their own solutions.

--Mark

[cadet1620]

My misunderstanding this time...there's still a bug.  I'll take a look at it.

--Mark

[cadet1620]

You have multiplier and multiplicand swapped in the body of the loop (the test at the top of the loop is correct.)

multiplicand is what is being summed to make the product; multiplier is the loop counter.


I like the optimization in your loop to break at the end with the

    D;JMP

instead of the normal unconditional jump

    0;JMP

That's a nice insight!

--Mark

[William DesLauriers]

We are very happy that I clarified my program code with just three words; multiplier, multiplicand and product!  It seems help a lot both of us.  With only two words swapped somewhere within the loop, the program now works very beautiful!

Also, I really appreciated very much that you decided to put a flowchart on this part of the forum!

I excused you because we are humans who err once in a while!

I took the program, with its bug surviving, off this forum.

Thanks,
Wm