Mult.asm Comparison Failure

Question

The question I need answered is, why is there a comparison failure at line 9 but not at line 8? I am new to Hack assembly language and am still learning the basics but am confused as to why the CPU Emulator would pass line 8 but not line 9.

2nd question:

How can I could I fix this to support the multiplication of negative values?

The program multiplies R0 and R1 and stores the result in R2. (R0, R1, R2 refer to RAM[0], RAM[1], and RAM[2], respectively.)

Mult.asm:

0   @2
1   M=0
2   @0
3   D=M
4   @8
5   D;JNE
6   @19
7   0;JMP
8   @2
9   D=M
10  @1
11  D=D+M
12  @2
13  M=D
14  @0
15  D=M-1
16  M=D
17  @8
18  D;JGT
19  @19
20  0;JMP

Mult.cmp:

|  RAM[0]  |  RAM[1]  |  RAM[2]  |
|       0  |       0  |       0  |
|       1  |       0  |       0  |
|       0  |       2  |       0  |
|       3  |       1  |       3  |
|       2  |       4  |       8  |
|       6  |       7  |      42  |
|       6  |      -7  |     -42  |
|      -6  |       7  |     -42  |

Answer 1

If memory serves, what that error message is saying is that you are not passing the test in line 9 of Mult.cmp.

So, assuming that the test script in Mult.tst is correct, the problem is that your program is failing when the first number (R0) is initially negative.

In lines 14-18, you are decrementing R0 and jumping back to line 8 if it is >0. This will immediately fail because R0 starts out negative!

You need to have a little extra code at the start to check for this case and make the appropriate adjustments.

Also, a style suggestion. When referring to a memory location, use symbolic notation, ie: @R1 instead of @1, and in particular, use the (LABEL) notation to specify jump targets. This will make your code much easier to read and modify.

Good luck!