The illusive ALU. next stop optimization

Activity 21:

Going to try to get the ALU working now with my custom chips and see if it works the way I want it to.

I think figuring out how to add overflow detection is very important when creating the ALU. Because when I go to create it in real hardware I will need to know when I get an overflow. I did create overflow detection in my adder but for the time being I am using the one the book made. Then if need be I will switch to mine.

This ALU does not do multiplication or division also it cannot do floating point math.

After reading the chapter I am not sure I understand what needs to happen. I first thought I would use a bunch of Mux’s but it doesn’t seem like it’s going to work out.

After working it out I figured most of it out but I still have to figure out how to check if the output is zero or negative.

My issue was the syntax of splitting up the pins. I think the book should cover this more clearly.

You can split the output pin as many times as you want when outputting but can not split the out put and use it for input if that makes since.

Here’s an example

    Mux16(a=AddAnd,b=NEgOut,sel=no,out[0..7]=out3,out[8..15]=out4,out[15]=out5,out=out);

Or8Way(in=out3,out=out1);

Or8Way(in=out4,out=out2);

But you cant

Or8Way(in=out[8..15],out=out);

This blog led me in the right direction.

http://justaguyinphilly.wordpress.com/2011/09/07/chapter-2-activity-5-build-an-alu/#comment-122

With only one Mux input switched around I got it working and now to see on optimization.

This is the one that took the longest by far.

The Work Sheet

F(x,y)=(1) 0001	1010 (-6)	0001 (1)
Zx=1	0000	XXXXXX
Nx=1	1111	XXXXXX
Zy=1	XXXXXX	0000
Ny=1	XXXXXX	1111
X+y	111   1111 + 1111  11110
!out	00001       (1)

F(x,y)=(1) 0001	1010 (-6)	0001 (1)
Zx=1	0000	XXXXXX
Nx=1	1111	XXXXXX
Zy=1	XXXXXX	0000
Ny=0	XXXXXX	0000
X+y (f)=1	111   1111 (x) + 0000 (y)   1111
!out(no)=0	1111       (-1)

F(x,y)=x=0100(4)	0100 (4)	0101 (5)
Zx=0	0100	XXXXXX
Nx=0	0100	XXXXXX
Zy=1	XXXXXX	0000
Ny=1	XXXXXX	1111
X+y(f)=1 XandY(f)=0	(x)0100 (y)1111    0100
!out(no)=0	0100 (4)

F(x,y)=y(0011)	1010 (-6)	0011(3)
Zx=1	0000	XXXXXX
Nx=1	1111	XXXXXX
Zy=0	XXXXXX	0011
Ny=0	XXXXXX	0011
X+y XandY(f)=0	(x)1111 (y)0011    0011
!out(no)=0	0011 (3)

Add/subtract UPDATE added over flow detection

Added an xor to my adder and then added code to the subtractor


//Created By Jeremy King for the hack computer
//The method of creating it cam from
//http://www.play-hookey.com/digital/binary_subtraction.html

/**
 * 16-bit adder/subtractor: a + b = out   if sub == 0
 *                          a - b = out   if sub == 1
 */

CHIP Subtract16 {
    IN a[16], b[16], sub;
    OUT out[16], OvF;

    PARTS:
//does not work
//Accordding to articals I read it should work
    /*Not16(in=b,out=NotB);
Inc16(in=NotB,out=outA);
Mux16(a=b,b=outA,sel=sub,out=outc);
    revAdd16(a=a,outc=outc,sub=sub,out=out); */

//works great
//This is what creats the subtraction
//This xor take a single bit input and a 16bit input
XOr16(a=sub,b=b,out=bout);
//This adder has a third single bit input to start the carry
//Also it now has over flow detection
    revAdd16(a=a,outc=bout,sub=sub,out=out,OvF=OvF);
}

An adder/ subtractor in hardware

Activity 19 E:

A subtractor. I found a schematic for a subtractor. I am attempting to implement it and then create a test script and compare file for the hardware simulator. Using a 16 bit adder and two other 16 bit logic gates I think I have a hardware subtractor. I have tried two approaches neither  one works. The issue was my full adder had to a few changes now it seems to work great. And since it’s not a project in the book I don’t see an issue of posting the code.

//Created By Jeremy King for the hack computer

//The method of creating it came from

//http://www.play-hookey.com/digital/binary_subtraction.html

/**

 * 16-bit adder/subtractor: a + b = out   if sub == 0

 *                          a - b = out   if sub == 1

 */

CHIP Subtract16 {

    IN a[16], b[16], sub;

    OUT out[16];

    PARTS:

       //does not work

       //Accordding to articals I read it should work

    /*Not16(in=b,out=NotB);

       Inc16(in=NotB,out=outA);

       Mux16(a=b,b=outA,sel=sub,out=outc);

    revAdd16(a=a,outc=outc,sub=sub,out=out); */

      

       //works great

       //This is what creats the subtraction

       XOr16(a=sub,b=b,out=bout);

       //This adder has a third single bit input to start the carry

    revAdd16(a=a,outc=bout,sub=sub,out=out);

}