Sequential circuits and memory

Question

I read some books to understand why we need to memorise bits in electric circuits and found the sequential logic circuits are designed to achieve that goal.

The basic element is the SR Latch which can store one bit of information (0 or 1) and erase it. It is made of two cross-coupled NOR gates, that means many relays.

But we can achieve the same operation of memorising by using one relay: if the switch of the relay is closed the bit (1) is memorised and if it’s opened the bit is erased.

My questions are:

1. Why we use sequential circuits to memorize bits instead of simple relays?
2. Can the processor make arithmetic operations without using memory?

Answer 1

"Sequential circuits" is an abstract idea. A Set-Reset latch made of cross-coupled NOR is just one example of a way to realize a sequential circuit. Could also be built from vacuum tubes or op-amps (with positive feedback and hysteresis) or some types of mechanical relays or even mechanical cogs and gears. Same basic design principles apply to all sequential circuits, regardless of the underlying technology. Technology changes. You're not studying a technology, you're studying the underlying design principles.

Arithmetic operations (such as unsigned 2's complement addition and subtraction) can be made from Combinational circuits (gates). Putting combinational circuits together with sequential circuits makes finite state machines, which lead to more complex machines like computers.

Abstract ideas are powerful but can be hard to grasp without a specific example, that's why your coursework is focusing on SR latch as a specific example of a simple sequential circuit element. It's something you could reasonably test in the lab and it's pretty close to what you will likely see in practice.

• Side note: yes, at one point in history computers were built out of relays. Sometimes debugging required scooping out an actual insect: https://www.computerworld.com/article/2515435/moth-in-the-machine--debugging-the-origins-of--bug-.html

Answer 2

In combinational logic the output is a function of present input/output status.

In sequential logic the output is a function of both past and present input/output status. Thus sequential logic requires data storage (memory). How the data storage is done depends on the technology used (electromagnetic relays, magnetic memory, semiconductor memory etc.).

Basic arithmetic operations can be performed without using data storage.

Answer 3

Dynamic RAM uses a capacitor and a single transistor to store a bit. As you wonder, storing a bit does not require all of the transistors in two NOR gates. Your assumption that a SR latch was the simplest form of storage was not correct.