delme

DLX CPU VHDL CODE UNIVERSITY

% By Michael Schulte and James Stine in Jan, 1996. 

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\title{An 8 bit Microprocessor Design, with 16 bit Addressing Capabilities, in Max Plus + }
\author{Louis P. Marquette\\
111 Sebring Rd\\
Dickson City, PA 18519, USA\\
[e-mail]\\
http://www.starrtechnologies.com/louis/}

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\begin{abstract}

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\section{Introduction}
     This processor was designed to be simple to create, for that 
reason there are only 4 registers, few basic logic commands, only
the most necessary flags, and only 65k bytes of addressable memory.
This design scheme led to it's being single cycle RISC-like. All 
memory access, besides the normal stepping through the program, 
uses the AB register combination for the memory address.  Because 
of the limited number of registers, this processor is severely 
crippled in performance. This might be the worst part of its design.
The instruction format is the standard "Opcode Operand, Operand". 
Load and Store currently do not operate correctly because of memory
access conflicts with the IFU. The main design file contains the
layout of the microprocessor. ie all the components and their 
interconnections. Each smaller component, the ALU for example, is in its
own design file and might even be itself a collection of smaller units.

\section{Accomplishments}
	This design was first thought up for a computer architecture and
arithmetic class which required a class project. As a result, the 
layout and general principles were learned in that class and not the
result of trial and error or additional research. The layout is very
basic and was designed at the same time that the opcodes were 
designated to try to make them work well together. Although all of 
the architecture principles were learned in class, there was 
research needed to discover how a register operated and how to create
one out of flip-flops. Because of the nature of the language, the 
entire design is structural. ie Everything is detailed down to the 
gate level. In the begining, I attempted to find a sample of some sort
to help my learning of Max Plus +; however, none were to be found. 
This resulted in my doing the entire design from scratch.


\section{Experience}
	This projects represents the first layout for a microprocessor 
that I have done. It took some time getting use to the software and
was interesting to learn that I could create a waveform for stimulus
for the design. There were some snags along the way, including: having 
to use a larger model mode since the design grew so big, dealing with
errors created by timing (this was almost impossible), and trying to 
access memory for the operation and data access in the same cycle (this 
still has not been accompliched and may require either a second out of 
phase clock or a multi-cycle design). There were many times in the process 
of actually laying out what I had envisioned that I realized there was 
an easier way to do something. Hopefully all these painfull realizations
will make my next design better.



\section{Conclusions} 
	A microprocessor with only 4 registers is almost useless. There would
be too much main memory useage, and accessing main memory would require 2 
of the registers for the address and one for data. This only leaves one 
register not involved in memory access. This would need to be addressed if 
this is ever redesigned. 
	Lots of interconect wires running all over the screen can cause much
confusion. Many times I made stupid and hard to find mistakes because I 
tapped into the wrong signal wire. This was especially true in the Control
Logic because of its complexity and massiveness.
	Microprocessor design is much less tedious and much easier when 
done visually then when done in a text based language such as VHDL. I
have used VHDL software to design a much more complex 32 bit RISC 
multi-cycle microprocessor. The Max Plus + work seemed to go much 
faster and it was easier to see where everything was headed because 
I have an actual picture of my design infront of me rather then text
which I had to interprit. However, VHDL allows for not only the 
structural type of designs, which would be described by a picture, 
but by procedural, which are more powerful and complex but cannot be 
turned into a picture. 


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