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Designing The MiniMips – Part 4 Computer Architecture and Data Paths

by SysOps 0 Comments
This entry is part 4 of 4 in the series Designing the MiniMips

Ok, last time we completed the basic design of our ALU. I hope you took the time to build it in Logisim and play with it. It should be trivial to comfirm it works by doing simple operations on paper or a programmer’s calculator and the ALU and comparing the results. Once you have confirmed that your ALU works correctly,  it is time to save it as a sub-circuit in Logisim. We’ll take a short step back to discuss the basic operation of a CPU and support systems and then we’ll answer to question of where our ALU input values and control signals come from and where the output goes.

Computer Architecture

Computers all follow a similar architecture. At a minimum they have Input, Processing, Storage, and Output. These components can take various forms. For example, input could come from a keyboard as with the typical personal computer, or it may come from some type of sensor such as the oxigen sensor in your car. Output could be to a display screen such as the LED monitor or a relay used to control a well pump. Memory can take the form of registers, magnetic disks, solid state disks, integrated circuits, or even paper punch cards. Processing can be either analog, digital, or even quantum devices.  However, the typical computer most people think of is the PC or typical desktop or laptop machine.

Computers systems can be categorized into many categories. Here we will place them into two broad categories based on the system architecture. Systems using seperate stores for instructions and data are said to be of a Hardvard Architecture. These machine read cpu instructions from ROM (Read Only Memory) and execute them. All data is stored in the system RAM (Random Access Memory). A typical microcontroller such as the Atmel AtMega324 is an example of this architecture. Other machines store cpuinstructions and data in the same memory. This allows for programs to become dynamic, being altered or created by the running program. This type of achitecture is known as Von Nueman Architecture. The Von Nueman Architecture is a powerful because program instructions are treated just like data. It does have it’s down sides however. One such short coming is the issue of security. Since any running program can access the program instructions, it can alter not only it’s own code but the code of other programs. So a rouge program could for instance alter the program for calculating an aircraft’s altitude. This could result in personal injury or death to the passengers. Because of this short-coming a lot of work has gone into developing ways to limit programs from having complete access to all programs stored in memory. Modern Von Nueman machnes contain a large amount of circuitry to manage program access to memory so such things wont occur.

Here we will develop a system using the Hardvardarchitecture and use a seperate instruction and data memories. This will allow us to reduce a few complexities and focus on other issues. In the future I may write another article on developing a Von Nueman machine.

CPU Architecture and Data Paths

Ok, let’s talk a bit about how a digital computer system works.  The first thing that occurs in most systems when power is applied is that the major components are set to a predetermined state. This reset also occurs when a “Reset” button is pushed. If you’ve ever had to take a paper click and hold in a tiny button on your router, then you’ve used a reset button. After the system has been reset, the cpu begins to step through it’s cycles. The first cycle is the ‘Fetch’ cycle. In this cycle the CPU fetches the next instruction from the program memory. Next, the instruction decoder decodes the instruction into the appropriate control signals for the various cpu components. The next step is to ‘Execute’ the instruction. The last step is to ‘Store’ the results in memory if required by the instruction. Most cpu instructions follow this pattern. However, this cycle can be shortened or have wait states added to it for complex instructions. In simple processesors some of these instructions are combined. Usually the Fetch and decode cycles are made to occur in the same cycle. In a single cycle cpu are these steps occur in one clock cycle. This is often accomplished by multiplying the clock input to generate the needed signals. However, in some architectures such as the one we will build, these steps can occur in a single cycle without the need for clock multiplication.

Below is the logisim circuit for the SimpleComputer.  This is the basic machine we will be building.  I’ve included some 7-Segment Hex digit displays to allow use to view the the CPU operation. I also included four ports on the ALU that allow us to connect some of those displays directly to the registers in the registerfile. While the circuit looks pretty basic here, it is only because I have used logisim’s ability to can a subcircuit as a component. So in this view all the major components are actually subcircuits that vary in complexity. They are all made out of logic gates of logisim standard components. If you have not used the Subcircuit feature in logisim I recommend you break out the tutorials and learn to use them. OK, let’s describe the circuit from a 10,000 foot level. We’ll being seeing each componet in great detail as we move forward.

The machine must first be reset using the reset button in the lower left of the circuit. Then we can single step through programs by clicking the clock input. We can run programs from the simulation menu in logisim. Once reset, the PC (Program Counter) will be set to 0x00 hexidecimal (hex). This will cause the first program instruction to be placed on the data output of the ROM (Read Only Memory). The ROM instruction code is broken into various parts that control certain aspects of the CPU. For eample, bits 0-3 of the instruction contain the ALU OP code. These four bits are used directly to select the chosen ALU operation. For shift operations bits 4-7 select the shift distance. For example if bits 4 – 7 contain the value 0010b in binary, then the value on the ALU’s A input will be shifted by two bit positions. Many bits in the instruction perform multiple duties. For eaxample, although bits 0-3 and 4-7 are used for the ALUOP code and the SHIFTAMT they also do double duty as the lower order byte of an immediate address in an instruction that contains an address value.

Once the instruction code is broken into it’s constituent parts, the various signals are reouted to other parts of the circuit as needed. A portion of the instruction code defines the type of instruction being executed. This allows the proper signals to be generated to control the parts of the circuit that are not directly controled by the bits in the instruction. Next the if the instruction is a register type instruction, the two source registers and the destination register are selected. The values in those registers and posibly an immediate value are feed into the ALU A and B inputs. The ALU then produces the out for all the operations we provided hardware for. This is done in parallele so producing all output at once isn’t an issue. The ALUOP code selects the desired function results and places them on the ALU output. Depending on the instruction the ALU ouput could be routed back to the destination register selected in the RegFile or it may be stored in RAM or used as a RAM address. It could also be used to set condition flags the determine if a branch will be taken. Lastly, it can be used as the input to the PC though that portion of the circuit is not shown here.

Look over the circuit and trace through the signal paths. See if you can figure out how the various parts work to control the circuit. Next time we’ll talk about the Instruction Set and continue on about the architecture. Until then, I hope you all have a happy new year!



Designing The MiniMips – Part 2 Subtraction and Negative Numbers

by SysOps 0 Comments
This entry is part 2 of 4 in the series Designing the MiniMips

Last time we discussed binary addition and how we could use a few logic gates to do arithmetic. I showed you a few simple circuits and left you with a simple assignment to use the full adder circuit to build a multi-bit full adder. I told you I’d show you a solution to the exercise. So here it is:

As you can see the full adders are simply cascaded by connecting the carry-out of the previous stage to the carry-in on the next stage. This cascading can be continued until the propagation delay of the carry signal become intolerable. If that happens there are techniques for improving the circuit. Checkout the Carry Lookahead Adder circuit.

Binary subtraction

We’ve seen how binary adders can be built from simple logic gates. Now let’s look at binary subtraction. The binary subtraction rules are very simple:

0-0 = 0 1-0=1 1-1 = 1 10-1 = 1

Binary subtraction is similar to decimal subtraction with the difference that when 1 is subtracted from 0, it is necessary to borrow 1 from the next higher order bit. That bit is reduced by 1 and the remainder is 1. The one thing that confuses most people when doing a binary borrow is that when you borrow in decimal you add it as 10 to the previous column. In binary we are using powers of 2. So when we borrow we must add the borrow as two bits of half the original value:

1  1  1
1  1
1 0 0 0
0 1 0 1
0 1 1

In the above, starting from the left most bit. First we can’t subtract 1 from 0 so we try to borrow 1 from the next column to the right (the tow’s column). However, it too is 0. We can’t borrow 1 from 0 so we move on to the third column left (the fours column). Next, in the fourth column left we find a 1 in the eights column. Now this one represents an 8 in decimal. So when we move it to the fours column. We have to split that 8 into two 4s. So we place two 1’s above the fours column. Next we borrow one of them and move it to the two’s column as two 2s. Again, we borrow a two from the 2s column and move it as two ones and place them in the 1s column. Now we can subtract the 1 in the right most the two ones we placed there. This leaves a one which we bring down.  In the two’s column we can now subtract 0 from the remaining 1 and get 1. We then move left to the fours column and subtract 1 from the remaining 1 we carried over and get 0. The eights column is now 0 and being a leading 0 we don’t really need to bring it down as it adds no value to our answer. Now to check our answer we can see that our original problem was: 8-5=3. And indeed 11 in binary is 3 in decimal.

Now let’s look at the logic needed to do a single bit binary subtraction. The truth table for subtraction is:

A B Diff Barrow
0 0 0 0
1 0 1 0
1 1 0 0
0 1 1 1


Looking at the table we can see that the difference is computed using an XOR gate. The borrow out however is a slightly more complex. Borrow out is only 1 when A is 0 and B is 1. This looks like and AND gate with an inverter on B.  Let’s try it out in logisim. Build the following circuit in logisim and see if it matches the truth table above:

Now this circuit is called a half subtractor. Just as the half adder is missing a carry in, the half subtractor is missing a borrow in. However, a similar cascading trick like that used for the half adder can be used to build a full subtractor. Let’s try this out. Build the circuit below in logisim and give it a try:

Negative Numbers

Now this kind of subtractor is great if you are only working with positive numbers. But if we ever need to work with a negative value as an operand or a resulting value, we are out of luck. So how do we handle negative numbers? Well, the most typical way is to reserve the left-most bit for use as a sign bit. The sign bit is set to 1 to indicate a negative value and 0 to indicate a positive value.  So if we have 8 bits we can represent -127 through +127.  There is an issue however, To demonstrate it let’s reduce this to a bit bit signed number. So we can represent -7 to +7 with the three bits know as the magnitude and the fourth bit as the sign bit.


Value Sign 4 2 1
0 0 0 0 0
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
-0 1 0 0 0
-1 1 0 0 1
-2 1 0 1 0
-3 1 0 1 1
-4 1 1 0 0
-5 1 1 0 1
-6 1 1 1 0
-7 1 1 1 1

Notice anything odd about this arrangement? Look closely, there are two zero values. Zero and negative zero. Who ever heard of negative zero? Sign magnitude presents it’s own issues. A program or hardware would have to check for two values of zero if this was implemented in our microprocessor. So what can we do about this? How about flipping the bits and using one’s complements. This too has it’s own issues. The issues with negative numbers is solved by something called two’s-complement. I found a Computerphile video that I think gives one of the best explanations available. Watch the video to learn more about binary math and negative numbers:


Now I am sure you have a good grasp on the problems and solutions to binary mathematics. I’ll leave you with this: How can we modify our multi-bit adder for use in both addition and subtraction? I’ll show you my solution next time. Until then, play around with your own ideas in logisim and try to build an adder/subtractor circuit using 2’s-complement.


Desgning the MiniMips A Simple 16 bit Microprocessor

by SysOps 0 Comments
This entry is part 1 of 4 in the series Designing the MiniMips

A while back I designed and built my own 16-bit microprocessor. I wanted to be able to use my processor in my own projects. My requirements were very simple. It had to be easy to understand, have an instruction set that is simple to use, as this makes it easy to generate a simple compiler for the instruction set. My desired instructions were pretty simple. I wanted something akin to the MIPS instruction set. Instructions should take one or two source operands, and a destination register or memory address. In the case of comparative instructions, it should simply discard the results. The processor should include a hardware stack. It should contain at least 16 general purpose registers and at least four I/O ports. I would prefer instructions to take a single cycle. But If I include a multiply and divide instruction this isn’t practical.  My last requirement was that I should be able to use free and open-source tools. I did a basic design in Logisim and then moved the design to VHDL and place it on an FPGA.

The project started off great but hit a few bumps in the road along the way. I eventually worked them out and got everything working. But I never really documented the project. So that’s what I’ll attempt to do here. I’ll step back and walk through the process I took; errors and all.

Designing a microprocessor isn’t a task for the faint of heart, however, it isn’t a task that should be impossible either.

This serries of blog posts will be more a record of how I proceeded with this project. I’ll include my mistakes, faux pas, and hopefully, the solutions I found, so readers may amuse themselves at my expense… If you follow along and find these posts interesting, have helpful hints or information, or if you build the MiniMips let me know in the contact form. I’d love to hear from you.

So how do we get started? Well, I’ll assume like myself, you have a grasp of digital logic basics. So let’s start by writing some requirements for the ALU. The ALU is the core of the processor. It’s the part that provides the Arithmetic and Logic functions. The ALU has at least three inputs: Operand 1, Operand 2, and the operation code. It should produce at least two outputs; the result of the operation, and a set of status indicators or flags.The ALU will perform the following operations:

  • Binary Addition
  • Binary Two’s Complement Subtraction
  • Binary Bitwise Inversion
  • Logical And of two binary values
  • Logical Or of two binary values
  • Logical XOR of two binary values
  • Logic Shift Left
  • Logical Shift Right
  • Logical Rotate Left
  • Logical Rotate Right

So if I count correctly that’s ten ALU operations we need circuitry for. Let’s start with the arithmetic first, Add and Subtract:

Let’s first review the laws of binary arithmetic:

Addition 0 + 0 = 0 0 + 1 = 1 1 + 0 = 1 1 + 1 = 0
Subtraction 0 – 0 = 0 1 – 0 = 1 1 -1 = 0 10 – 1 = 1

Now let’s look at a couple simple logic operations:

AND Operation OR Operation XOR Operation
1 AND 1 = 1   1 OR 1 = 0  1 XOR 1 = 0
0 AND 1 = 0   1 OR 0 = 1  1 XOR 0 = 1
1 AND 0 = 0  0 OR 1 = 1  0 XOR 1 = 1
0 AND 0 = 0  0 OR 0 = 0 0 XOR 0 = 0

You might notice that adding to binary digits produces the same results as XORing two binary digits. So to add to binary digits together we only need to XOR them. However, when we try to do arithmetic operations on multi-digit numbers we have the issue of carrys and barrows. Let’s just look at addition for the moment. When we add two two-bit binary numbers and have a one in the same column in both numbers we must carry a one to the next column. Just as we do in decimal arithmetic.

Carries 1 1
1 0 1 1
+ 0 0 0 1
Results 1 1 0 0


If you look closely you will notice that the carry happens anytime there are two ones in the same column. This is basically the AND logic operation.

To follow along download Logisim or Logisim-Evolution and create the circuit show below. (I reccomend you open the Logisim help and locate the tutorial and complete that first if you have little or no experience with logisim.)

Above you see a simple XOR gate. Play with it and test it to see if the output is high only when A+B = 1 and low when A+B <> 1. These condition only hold true if only A * -B | B * -A is true.Remember if A and B are both 1 or high, then the addition results in the output being low and a 1 carried over to the second bit condition. However, here we have no second bit to handle the carry out. So let’s see how to add the carry out to our simple 1-bit adder circuit.

Above you can see the addition of a simple AND gate. The AND gate only generates a 1 on it’s output when both inputs are high. So in this circuit it makes the perfect carry out generator. This type of adder is known as a Half Adder because it only does half the job…

Ok, that’s fine and good you may say however, what about adding multi-bit numbers? How can we do that?

Adding multi-bit numbers requires more than a carry out signal… We also need a carry-in signal. The carry-in allows the carry-out from the previous column to be added into the result of A + B. So our new circuit needs to include the carry-in as part of the sum. To do this, we just need to add the carry-in to the result of A + B. We can do this with another Adder:

So here’s two Half-Adders cascaded together to allow for a Carry-in signal from the prevous column. There is an issue however. We now have two carry-outs. The original and the carry out from the second half adder used to add the carry-in bit. As it turns out, you can’t have both carry-outs active at the same time. To see why simply follow the logic;

  • If A=1, B=1 Cin=1: Sum 1 = 0 and Carry-out 1 = 1, Sum-out 2 = 1 and Carry-out 2 = 0
  • If A=1, B=1, Cin=0: Sum 1 = 0 and Carry-out 1 = 1, Sum-out 2 = 0 and Carry-out 2 = 0

Check the rest of the conditions by building a truth table. You’ll see that only one carry-out bit can be active at a time. So we simply need to detect when one or the other is active. That’s a perfect job for an OR gate:

OK, build these circuits in Logisim and play with them. Work out the a truth table for them all and prove to yourself they work as expected.

With the full adder you can create N-bit Adders by simply feeding the Carry-out of the previous column to the Carry-in of the next column. As an exercise use our full adder circuit to create an 8-Bit Adder.  Next time I’ll show you my 8-Bit Adder and then we’ll look at subtraction.








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