In our previous blog post, “Binary Additions using Logic Gates”, we investigated how logic gates can be used together to create a circuit used in the ALU (Arithmetic & Logic Unit of the CPU) to add two binary numbers together.

In this blog post we will investigate how logic gates are used to create the RAM (primary memory), in other words how logic gates can be used to store volatile information.

Random Access Memory

Random Access Memory (RAM) is volatile memory that sits next to the CPU. (Volatile means that it is wiped out when the computer is switched off). It is used to store instructions and data currently used by the CPU.

RAM consists of billions of Data Cells, each data cell being able to store one bit of information. For instance a 2GB RAM can store 2,000,000,000 Bytes of information = 16,000,000,000 bits of information and hence consists of 16,000,000,000 data cells.

D-Type Flip-Flop Circuits

Each data cell consists of a D-Type Flip-Flop circuit that is built using four NAND logic gates connected as follows:


We represent a D-Type Flip-Flop Circuit as follows. You can change the input values D and E by clicking on the corresponding buttons below to see the impact on the outputs Q and Q.

You can also test the behaviour of a D-Type flip-flop circuit using our online simulator:
Click on the above circuit to open in a new window.

A D-Type Flip-Flop Circuit is used to store 1 bit of information. It has two input pins (Called D (Data) and E (Enabler) and two output pins (Q and Q = NOT Q).

The truth table of a D-Type Flip-Flop circuit is as follows:

When the enabler input E is set to 1, the output Q can be set to the Data input D.
When the enabler input E is set to 0, the output Q cannot be changed. It remains as its previous value. In other word it retains its value. This is why this circuit is used to create memory cells (e.g in the RAM).

Clock Signal and Delaying Effect

The enabler input E is often connected to another circuit called the clock (e.g. CPU clock). The clock signal constantly and regularly alternate between two states: 0 and 1, similar to a heart beat. Inside the CPU the clock signal controls the execution of the FDE cycle.

When the clock signal is applied to the Enabler input (in this case also called the clock input), the flip-flop output Q can only change values when triggered by the clock signal. The value of the flip-flop is held or delayed until the next clock signal. This delaying effect is also called a latch. This is why we call this circuit a D-Type flip flop where D stands for Delay.

In other words, the change of input (D) is not applied immediately (to the output Q) but is applied at the next “tick of the clock”. There are many applications to this delay such as the ability to create a frequency divider. (divide the clock frequency by a multiple of 2).

In our previous blog post “from transistors to processors” we found out that the CPU consists of logic gates, which are made using transistors.

In this blog post we are looking at how these logic gates can be combined to create an integrated circuit used by the ALU (Arithmetic and Logic Unit of the CPU) to add two 8-bits binary numbers together.

First let’s recap on how a binary addition works:

Half-Adder Circuit

A half-adder circuit is used to add two bits of data together and is based on the following Truth Table.

A half-adder circuit consists of two logic gates as follows:

You can test this circuit by clicking on the picture below:

Full-Adder Circuit

A full-adder circuit is used to add three bits of data together and is based on the following Truth Table.

A full-adder circuit consists of two half-adder circuits and an OR gate connected as follows:

You can test this circuit by clicking on the picture below:

Full Binary Addition

By connecting an half-adder circuit with 7 full-adder circuits we can create a circuit to implement a full binary addition of two 8-bits binary numbers:

Full circuit:

Vacuum Tubes and Transistors:

Many consider the transistor to be one of the most important inventions of all time.

Though the precursors of the transistor were invented in 1907 (at the time they were not transistors yet, they were vacuum tubes called valves), these were soon replaced by smaller components called transistors. These are still the key components of modern computers nowadays.

So what is a transistor?
A transistor is an electronic component with three pins. Basically, a transistor is a switch (between two of the pins: the collector and the emitter) that is operated by having a small current in the third pin called the base.

Use the checkboxes below this transistor to understand how applying a voltage to the base of a transistor is equivalent to turning on a switch.

Apply voltage to: Base   –    Collector

Transistors are made by layering three types of materials: conductors, insulators and semiconductors.

Logic Gates?

Logic Gates are made by combining transistors. They enable to apply logic to small currents which are either turned on or off and represent binary information inside a computer. Computers are made by combining logic gates together.

Use the tabs below to see how some of the key logic gates are built using transistors:

Integrated Circuits?

An integrated circuit (also referred to as a chip, or a microchip) is a set of electronic circuits on one small flat piece (or “chip”) of semiconductor material, normally silicon. The integration of large numbers of tiny transistors into a small chip results in circuits that are smaller, cheaper, and faster than those constructed of discrete electronic components.

More complex integrated circuits include binary adders (half-adder, full adder used to perform binary additions) and flip-flop circuits used to implement volatile memory.

List of 7400 series integrated circuits:
https://en.wikipedia.org/wiki/List_of_7400_series_integrated_circuits

Mirco-Processors?

A microprocessor is a computer processor which incorporates the functions of a computer’s central processing unit (CPU) on a single integrated circuit (or at most a few integrated circuits). The microprocessor is a multipurpose, clock driven, register based, digital-integrated circuit which accepts binary data as input, processes it according to instructions stored in its memory, and provides results as output.

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5 Generations of Computers

1st Generation Computers: Vacuum Tubes

Back in the 1950s, computers consisted of vacuum tubes called valves (the precursors of transistors). These valves were quite bulky, like electric bulbs, and produced a lot of heat. The installations used to fuse frequently.

Punch cards, paper tape, and magnetic tape were used as input and output devices. 1st Generation Computers were programmed using machine code.

1st Generation Computers were very expensive and only large organisations were able to afford them.

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2nd Generation Computers: Transistors

In the early 1960s, 2nd Generation computers used transistors to replace the vacuum tubes of 1st generation computers. Therefore 2nd Generation computers were cheaper, consumed less power and were more compact in size. They were also more reliable and faster. More transistors could be used to create more complex computers.

Magnetic tape and magnetic disks were used as secondary storage devices as well as punched tapes which were still used.

2nd Generation Computers were programmed using assembly language and high-level programming languages such as FORTRAN or COBOL. 

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3rd Generation Computers: Integrated Circuits

In the second half of the 1960s, integrated circuits were used by 3rd Generation Computers. An integrated circuit has many transistors, resistors, and capacitors along with the associated circuitry. This development made computers smaller in size, more reliable and efficient.

3rd Generation Computers were programmed using High-level languages (FORTRAN, COBOL, PASCAL, BASIC, ALGOL-68 etc.).

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4th Generation Computers: Micro-Processors

In the 1970s, Computers of 4th generation used Very Large Scale Integrated (VLSI) circuits. VLSI circuits having about 5,000 transistors on a single chip called a micro-processor.

Fourth generation computers became more powerful, compact, reliable, and affordable. They started the Personal Computer (PC) revolution.

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5th Generation Computers: Nowadays

The period of fifth generation is 1980-to date. In the fifth generation, VLSI technology became ULSI (Ultra Large Scale Integration) technology, resulting in the production of microprocessor chips having ten million electronic components.

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Moore’s Law

The rate at which transistor counts have increased generally follows Moore’s law, which observed that the transistor count doubles approximately every two years. As of 2016, the largest transistor count in a commercially available single-chip processor was over 7.2 billion.