Memorie și stocare: curs intensiv Informatică #19

CompuNew Memory: From Punch Cards to Magnetic Tape

CompuNew Memory: From Punch Cards to Magnetic Tape

Hi, I’m Carrie Anne, and welcome to Crash Course Computer Science! We’ve talked about computer memory several times in this series, and we even designed some in Episode 6. In general, computer memory is non-permanent. If your xbox accidently gets unplugged and turns off, any data saved in memory is lost. For this reason, it’s called volatile memory. What we haven’t talked so much about this series is storage, which is a tad different. Any data written to storage, like your hard drive, will stay there until it’s over-written or deleted, even if the power goes out. It’s non-volatile. It used to be that volatile memory was fast and non-volatile storage was slow, but as computing technologies have improved, this distinction is becoming less true, and the terms have started to blend together. Nowadays, we take for granted technologies like this little USB stick, which offers gigabytes Of memory, reliable over long periods of time, all at low cost, but this wasn’t always true.


The earliest computer storage was paper punch cards, and its close cousin, punched paper tape. By the 1940s, punch cards had largely standardized into a grid of 80 columns and 12 rows, allowing For a maximum of 960 bits of data to be stored on a single card. The largest program ever punched onto cards, that we know of, was the US Military’s Semi-Automatic Ground Environment, or SAGE, an Air Defense System that became operational in 1958. The main program was stored on 62,500 punchcards, roughly equivalent to 5 megabytes of data, that’s the size of an average smartphone photo today. Punch cards were a useful and popular form of storage for decades, they didn’t need power, plus paper was cheap and reasonably durable. However, punchcards were slow and write-once, you can’t easily un-punch a hole. So they were a less useful form of memory, where a value might only be needed for a fraction of a second during a program’s execution, and then discarded. A faster, larger and more flexible form of computer memory was needed.


An early and practical approach was developed by J. Presper Eckert, as he was finishing work on ENIAC in 1944. His invention was called Delay Line Memory, and it worked like this. You take a tube and fill it with a liquid, like mercury. Then, you put a speaker at one end and microphone at the other. When you pulse the speaker, it creates a pressure wave. This takes time to propagate to the other end of the tube, where it hits the microphone, converting it back into an electrical signal. And we can use this propagation delay to store data! Imagine that the presence of a pressure wave is a 1 and the absence of a pressure wave is a 0. Our speaker can output a binary sequence like 1010 0111. The corresponding waves will travel down the tube, in order, and a little while later, hit the microphone, which converts the signal back into 1’s and 0’s. If we create a circuit that connects the microphone to the speaker, plus a little amplifier to Compensate for any loss, we can create a loop that stores data. The signal traveling along the wire is near instantaneous, so there’s only ever one bit of data showing at any moment in time. But in the tube, you can store many bits! After working on ENIAC, Eckert and his colleague John Mauchly, set out to build a bigger and better computer called EDVAC, incorporating Delay Line Memory.


However, a big drawback with delay line memory is that you could only read one bit of data from a tube at any given instant. If you wanted to access a specific bit, like bit 112, you’d have to wait for it to come around in the loop, what’s called sequential or cyclic-access memory, whereas we really want random access memory, where we can access any bit at any time. It also proved challenging to increase the density of the memory, packing waves closer together meant they were more easily mixed up. In response, new forms of delay line memory were invented, such as magnetostrictive delay lines. These delay lines use a metal wire that could be twisted, creating little torsional waves that represented data. By forming the wire into a coil, you could store around 1000 bits in a 1 foot by 1 foot square. However, delay line memory was largely obsolete by the mid 1950s, surpassed in performance, reliability and cost by a new kid on the block: magnetic core memory which was constructed Out of little magnetic donuts, called cores.


The first big use of core memory was MIT’s Whirlwind 1 computer, in 1953, which used a 32 by 32 core arrangement. And, instead of just a single plane of cores, like this, it was 16 boards deep, providing roughly 16 thousand bits of storage. Importantly, unlike delay line memory, any bit could be accessed at any time. This was a killer feature, and magnetic core memory became the predominant Random Access Memory technology for two decades, beginning in the mid 1950s even though it was typically woven by hand! Although starting at roughly 1 dollar per bit, the cost fell to around 1 cent per bit by the 1970s. Unfortunately, even 1 cent per bit isn’t cheap enough for storage. By 1951, Eckert and Mauchly had started their own company, and designed a new computer called UNIVAC, one of the earliest commercially sold computers. It debuted with a new form of computer storage: magnetic tape. This was a long, thin and flexible strip of magnetic material, stored in reels. The UNIVAC used half-inch-wide tape with 8 parallel data tracks, each able to store 128 bits of data per inch. With each reel containing 1200 feet of tape, it meant you could store roughly 15 million bits – that’s almost 2 megabytes! Although tape drives were expensive, the magnetic tape itself was cheap and compact, and for this reason, they’re still used today for archiving data. The main drawback is access speed. Tape is inherently sequential, you have to rewind or fast-forward to get to data you…

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