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.
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!