![]() To connect multiple 1-bit adders to form a wider adder, we just need to connect the carry out of the previous bit to the carry in of the current bit. It is active when either A and B are both on at the same time, or if there is a carry-in and one of A or B is on. The Sum output is on if either A or B is on but not both, or if there is a carry-in signal and either A and B are both on or both off. Modern designs improve on this by optimizing some of the logic and carry signals, but the fundamentals are still the same. The basic design uses five logic gates and they can be linked together to create any size adder you want. It takes three inputs - A, B, and Carry-In, and produces two outputs - Sum and Carry-Out. The only difference is that a modern chip has billions of transistors.įor a quick example, we'll look at a basic adder 1-bit full adder. ![]() The process of combining individual components to create a working design is exactly what is used today to create modern chips. You can then connect many of these devices to form something that performs a slightly more advanced function. This design process involves combining several gates to create a small device that may perform a simple function. ![]() With building blocks as simple as logic gates, it can be difficult to see how they are transformed into a functioning computer. Connecting transistors to form simple networks like this is the same process used to design more advanced logic gates and other circuitry inside processors. Looking at the NAND gate, we see that it requires four transistors and that the output will be on as long as at least one of the inputs is off. Since we said that pMOS devices conduct when the input is off and nMOS devices conduct when the input is on, it is easy to see that the signal at Out will always be the opposite of the signal at In. The pMOST transistors are drawn with a small circle connecting to their gate. In the inverter, there is a pMOS transistor on top connected to the power line and an nMOS transistor on the bottom connected to ground. There are other gates with different logic functionality like OR, NOR, XOR, and XNOR.īelow we can see how two basic gates are designed from transistors: an inverter and a NAND gate. We can combine these two to create a NAND or not-and gate which turns its output on if and only if none of the inputs are on. An inverter or NOT gate will turn its output on if the input is off. For example, an AND gate will turn its output on if and only if all the inputs to the gate are on. We won't get into the nitty gritty details of how transistors physically work in this article, but we'll touch on it in Part 3 of the series.Ī logic gate is a simple device that takes inputs, performs some operation, and outputs a result. By combining these types of transistors in a complementary way, we can create CMOS logic gates. ![]() An nMOS transistor allows current to flow when the gate is charged or set high, and a pMOS transistor allows current to flow through when the gate is discharged or set low. There are two main types of transistors used in modern processors: pMOS and nMOS. Just like the light switch on your wall, but much smaller, much faster, and able to be controlled electrically. When the gate is off, current can't flow. When the gate is on, electricity is allowed to flow through the transistor. The simplest way to think of a transistor is of a controllable switch with three pins. (Sea of Accelerators, 3D integration, FPGAs, Near Memory Computing)Īs you probably know, processors and most other digital technology are made of transistors. Part 4: Current Trends and Future Hot Topics in Computer Architecture Part 3: Laying Out and Physically Building the Chip (schematics, transistors, logic gates, clocking) (instruction set architectures, caching, pipelines, hyperthreading) Part 1: Computer Architecture Fundamentals
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