How to Implement an ALU at Gate Level in Verilog?

[alt3r]

Hello guys,
i want ALU implemented in gate level in verilog, please help me

 

[NeutronStar]

That's a pretty standard simple exercise.

Just set up three registers (or bit-level flip-flops) for the number of bits that you wish to deal with.

1. Instruction control register - Usually just 3 or 4 bits wide
2. Register A (for accumulator) - Might be 8, 16, 32, or whatever you want
3. Register B - Must be the same size as register A

Then just write the code based on the contents of the control register.

In other words, if the control register contains say 000 then have Register B AND with register A leaving the result in A.

If the control register contains say 001 then have register B OR with register A leaving the resultes in A.

You can do similar things with various binary codes in your control register.

It's a pretty simple routine. Once you have the three registers defined, the rest is just IF THEN statements of the form:

IF control=000 THEN A = B LOGIC A
IF control=001 THEN A = B LOGIC A
IF control=010 THEN A = B LOGIC A
IF control=011 THEN A = B LOGIC A
IF control=100 THEN A = B LOGIC A
IF control=001 THEN A = B LOGIC A

Where "LOGIC" is something like AND, OR, NOT, ADD, SUB, ect.

And so on. If you need more control make the control register 4-bits wide or wider.

You might also want to implement a carry flag, or other types of flags. Keep going and the next thing you know you'll be designing a microprocessor!

Hope this helps. You'll still have to work out the actual syntax and initial register set up, but if you are taking a course in this that part should be relatively straight-forward.

Here's a link to a similar assignment, it doesn't give the answer but by it might be helpful:

http://www.u-aizu.ac.jp/~yliu/teaching/comparch/lab1.html

 

[NeutronStar]

By the way, using A as both the accumulator and one of the initial registers can sometimes be a bit tricky with some HDL. If you have problems with that you might try adding yet another register say D and write the code as follows

IF control=000 THEN D = B LOGIC A
IF control=001 THEN D = B LOGIC A
IF control=010 THEN D = B LOGIC A
IF control=011 THEN D = B LOGIC A
IF control=100 THEN D = B LOGIC A
IF control=001 THEN D = B LOGIC A

This way you will be guaranteed good results without the possibiliy of causing a closed loop. It's actually been a while since I've done any HDL so maybe others will respond with some better ideas.