Hi, I'm Carrie Anne and welcome to Crash Course Computer Science. Over the last few episodes, We've talked a lot about the mechanics of how computers work. How they use complex circuits to save and retrieve values from memory, and perform operations on those values, like adding two numbers together.
We've even briefly talked about sequences of operations, which is a computer program. What we haven't talked about is how a program gets into a computer. You might remember in episode 7 and 8 , when we stepped through some simple example programs for the CPU that we had created.
For simplicity, we just waved our hands and said that the program was already magically in memory. But in reality, programs have to be loaded into a computer's memory. It's not magic.
It's computer science. [Theme Music] The need to programme machines existed way before the development of computers. The most famous example of this was in textile manufacturing.
if you just wanted to weave a big red tablecloth, you could simply feed red thread into a loom and let it run. But what about if you wanted the cloth to have a pattern like stripes or plaid? Workers would have to periodically reconfigure the loom as dictated by the pattern, but this was labor intensive which made patterned fabrics expensive.
In response, Joseph Marie Jacquard developed a programmable textile loom, which he first demonstrated in 1801. The pattern for each row of the cloth was defined by a punched card. The presence or absence of a hole in the card determined if a specific threat was held high or low in the loom.
Such that the cross thread called the weft passed above or below the thread. To vary the pattern across rows these punch cards were arranged in long chains, forming a sequence of commands for the loom. Sound familiar?
Many consider Jacquard's loom to be one of the earliest forms of programming. Punched cards, turned out to be a cheap , reliable, fairly human readable way to store data. Nearly a century later, punch cards were use to help tabulate the 1890 newest census which we talked about in episode 1.
Each card held an individual person's data. Things like race, marital status, number of children, country of birth, and so on. For each demographic question, a census worker would punch out a hole of the appropriate position.
When a card was fed into the tabulating machine, a hole would cause the running total for that specific answer to be increased by one. In this way you could feed the entire county's worth of people and at the end you'd have running totals for all of the questions that you asked. It is important to note here that early tabulating machines were not truly computers as they can only do one thing-tabulate.
Their operation was fixed and not programmable. Punished cards stored data, but not a program. Over the next 60 years, these business machines grew in capability, adding features to subtract, multiply, divide, and even make simple decisions about when to perform certain operations.
To trigger these functions appropriately, so that different calculations could be performed, a programmer accessed a control panel. This panel was full of little sockets, into which a programmer would plug cables to pass values and signals between different parts of the machine. For this reason they were also called plug boards.
Unfortunately, this meant having to rewire the machine each time a different program needed to be run. And so by the 1920s, these plug boards were made swappable. This not only made programming a lot more comfortable, but also allowed for different programs to be plugged into a machine.
For example, one board might be wired to calculate sales tax, while another helps with payroll. But plug boards were fiendishly complicated to program. This tangle of wires is a program for calculating a profit loss summary, using an IBM 402 accounting machine, which were popular in the 1940s.
And this style of plug board programming wasn't unique to electromechanical computers. The world's first General-Purpose electronic computer, the Eniac, completed in 1946, used a ton of them. Even after a program had been completely figured out on paper, physically wiring up the Eniac and getting the program to run could take upwards of three weeks.
Given the enormous cost of these early computers, weeks of downtime simply to switch programs was unacceptable and the new, faster, more flexible way to programme machines was badly needed. Fortunately by the late 1940s and into the 50s, electronic memory was becoming feasible. As costs fell, memory size grew.
Instead of storing a program as a physical plug board of wires, it became possible to store a program entirely in a computer's memory. Where it could be easily changed by programmers and quickly accessed by the CPU. These computers were called stored-program computers.
With enough computer memory, you could store not only the program you wanted to run, but also any data your program would need. Including new values it created along the way, Unifying the progrmming data into a single shared memory is called the von Neumann architecture. Named after John von Neumann, a prominent mathematician and physicist, who worked on the Manhattan project and several early electronic computers.
And once said, "I'm thinking about something much more important than bombs, I'm thinking about computers". The hallmarks of a von Neumann computer are a processing unit containing an arithmetic logic unit, data registers, an instruction register, and an instruction address register. And finally, a memory to store both data and instructions.
Hopefully, this sounds familiar, because we actually built a von Neumann computer in episode 7. The very first von Neumann architecture stored program computer was constructed in 1948 by the University of Manchester, nicknamed "Baby". And even the computer you are watching this video right now uses the same architecture.
Now electronic computer memory is great and all, but you still have to load the programming data in to the computer before it can run. And for this reason, punch cards were used. Let's get to the Thought Bubble.
Well into the 1980s, almost all computers had a punch card reader. Which could suck in a single punch card at a time and write the contents of the card into the computer's memory. If you loaded in a stack of punch cards, the reader would load them all into memory sequentially, as a big block.
Once the programming data were in memory, the computer would be told to execute it. Of course, even simple computer programs might have hundreds of instructions, which meant that programs were stored as stacks of punch cards. So if you ever have the misfortune of accidentally dropping your program on the floor, it could take you hours, days, or even weeks to put the code back in the right order.
A common trick was to draw a diagonal line on the side of the card stack called striping, so you'd have at least some clue how to get it back into the right order. The largest program ever punched into punch cards was the US Air Force's sage air defense system, completed in 1955. At its peak, the product is said to have employed 20% of the world's programmers.
Its main control program was stored on a whopping 62,500 punch cards, which is equivalent to roughly 5 megabytes of data. Pretty underwhelming by today's standards. And punch cards weren't only useful for getting data into computers, but also getting data out of them.
At the end of a program, results could be written out of computer memory and onto punch cards by, well, punching cards. Then this data could be analyzed by humans or loaded into a second program for additional computation. Thanks, Thought Bubble.
A close cousin to punch cards was punched paper tape, which is basically the same idea, but continuously instead of being on individual cards. And of course, we haven't talked about hard drives, CD-Roms, DVDs, USB thumb drives, and other similar goodies. We'll get to those more advanced types of data storage in a future episode.
Finally, in addition to plug boards and punch paper, there was another common way to program and control computers pre-1980: panel programming. Rather than having to physically plug in cables to activate certain functions, this could also be done with huge panels full of switches and buttons. And there were indicator lights to display the status of various functions and values in memory.
Computers of the 50s and 60s often featured huge control consoles that look like this. Although it was rare to input a whole program using just switches, it was possible. And early home computers made for the hobbyist market used switches extensively, because most home users couldn't afford expensive peripherals like punch card readers.
The first commercially successful home computer was the Altair 8800, which sold in two versions: preassembled and as a kit. The kit, which was popular with amateur computing enthusiasts, sold for the then unprecedented low price of around $400 in 1975 or about $2,000 in 2017. To program 8800 you'd literally toggle the switches on the front panel to enter the binary Op codes for the instruction you wanted.
Then you press the deposit button to write that value into memory. Then in the next location in memory, you toggle the switches again for your next instruction, deposit it and so on. When you had finally entered your whole program into memory, you would toggle the switches, move back to memory address zero, press the run button, and watch the little lights blink.
That was home computing in 1975, wow. Whether it was plug board switches or punched paper Programming these early computers was the realm of experts. Either professionals who did this for a living, or technology enthusiasts.
You needed intimate knowledge of the underlying hardware, So things like processor Op codes and register wits to write programs. This meant programming was hard and tedious. And even professional engineers and scientists struggle to take full advantage of what computing could offer.
What was needed was a simpler way to tell computers what to do; a simpler way to write programs. And that brings us to programming languages which we'll talk about next episode. See you next week.
Crash Course Computer Science is produced in association with PBS Digital Studios. At their channel you can check out a playlist of shows like Braincraft, Coma Niddy and PBS Infinite Series. This episode was filmed at the Chad and Stacey Emigholz studio in Indianapolis, Indiana And it was made with the help of all these nice people and our wonderful graphics team is Thought Cafe.
Thanks for the Random Access Memories, I''ll see you next time.
