3809/2801: CPU as a Calculator, RAM as Paper, and Display as Output Device

2801
CPU as a Calculator,
RAM as Paper,
and Display as Output Device

Prologue

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"I told you guys I wanted to let you choose which microcomputer we build as a class, but, after doing more research over the weekend, I'm at a loss." Mr. Mori, our teacher, cleared his throat as he turned on the overhead projector. "Too many options that I'm not sure even I understand. No way I can explain enough for you guys to make a decision this term, and we need to start building it before next term. Preferably get it built and running before next term."

Rick looked at Hec, Hec looked at me, and I shook my head and shrugged. We looked around at the other students.

Bob spoke up from one of the lab tables behind us. "So just make the decision for us?"

"It won't hurt you to get an overview of the possibilities, but I'm not going to be able to tell you enough that you can make a real informed decision."

Hec said, to no one in particular, "Chicken and egg. We can't really know until we know."

"The school board is pressuring me to make the 'safest bet'. That would be the Altair 8800, according to at least one of them. Yes, Scott?"

"That's the one my dad likes."

"So I'm voting against that one," Barry grumbled from a table on the other side of the room. 

Bob snorted. "Intel invented the microprocessor."

Chuck laughed. "My dad was telling me rumors about some unknown company delivering microprocessors to the military in late '69. Top secret stuff that no one is supposed to know about."  

Karl let out a derisive "Huh. If it's top secret and your dad knows something about it, how come he's telling you?" 

Ted volunteered, "TI almost beat Intel with the 0100." 

(For the record, this page: https://www.righto.com/2015/05/the-texas-instruments-tmx-1795-first.html has a fairly decent timeline and discussion of who was -- not really --first. Pretty much everybody in the industry knew where we were heading and was trying hard to get there. Intel just got the almost-single-chip microprocessor on the general market a few months before anyone else, and bet their farm on it -- in our timeline and this.) 

"There's a company called Apple selling kits for an apocalyptic price," volunteered Harvey. 

Mr. Mori grinned wryly and waited a moment, but no one asked what Harvey meant by apocalyptic. 

There were fourteen of us spread between the lab tables at the front of the room in two rows of three tables that could each seat four. Ordinary desks where we could study on our own were in the back of the room. Everything about the lab was new.

"Guys. I'm glad there's some enthusiasm here. But we're technically one warm body short of making the class, so I'm probably going to go with the board recommendation. Don't want to give them a reason to cancel the class."

Mark commented, sardonically. "One short, even with Joe and Rick coming over from the crosstown trashpit."

"Hey!" Rick interjected with a smirk.

Everybody laughed.

"Sorry." Mark gave us a lopsided grin. "With them coming over from our esteemed crosstown rivals, I doubt the school board is going to just drop the class if we choose another one." He turned and grinned at us. "Thanks, guys." 

Mr. Mori sighed. "I really appreciate you guys working out your differences outside of class."

Hec replied, "Some of us went to Hood with Joe and Rick. We're good."

(Real world: John B. Hood Junior High School is now Wilson and Young Medal of Honor Middle School. Happened in 2015. I'm not sure how I feel about that.)

Lots of grunts in agreement, and Mark gave us a thumbs-up. 

Rick and I tilted our heads and gave him a lopsided grin back.

"Anyway, let's get a look at the CPUs I think are possible for us to get." Mr. Mori laid a transparency on the projector, showing a register diagram of the 8080 on the screen.

There was a knock at the open door. Everyone looked over. 

Cyndy was standing there, carrying her books. She glanced at Hec and then at me before addressing the teacher. I lifted my fingers in a half-wave, but she didn't respond.

"May I come in, Professor Mori?"

"You decided to join us after all! Of course. Come on in." Mr. Mori grinned broadly. "Guys, say hello to our fifteenth man."

The class erupted in applause, wolf whistles, and a few cat-calls, and Cyndy looked a little embarrassed.

"Knock it off, guys." Mr. Mori chastised.

Bob called out, "She can sit by me, Teach!"

Mr. Mori looked at Cyndy. "I think, just for today, anyway, you should sit with the table that was not being rowdy. Joe, Rick, Hec, no complaints?

"Nah!" Rick was quick to respond.

"Huh, uh." I shook my head.

"Nope." Hec stood up and pulled a seat out for her at our lab table.

She frowned, but came over and put her books down. "I can handle my chair myself, Hector," she muttered. Hec ducked his head apologetically and sat down.

"Mori just doesn't want any more interruptions today," Rick apologized.

"You can come to our table anytime!" Mark offered.

Cyndy gave him a coquettish grin as she took her class transfer form to Mr. Mori.

"Okay. Yep. Entered in the roll. Cynthia Lucinda Mills. Miss Mills, would you care to tell the class what you told me several weeks ago about why you want to take this class instead of the regular electronics class?"

"Dad thinks microprocessors are going to be the core of everything in the future. He's told me some of the things he thinks they can do." 

And she sat down.

Several of the guys clapped slowly, then we all applauded.

"Thank you. I think we are all in the same frame of mind." Mr. Mori cleared his throat again and drew our attention to the screen at the front of the room, which was showing the register set layout for the 8080:

Cyndy gave Hec and me both a stern glare. "So don't get any ideas." Then she turned to Rick and said, "And you, neither, uh ...?"

"Rick. Rick Stanton." Rick blinked with a straight face. "Don't worry. My mind is a blank already." 

"Mine too," Hec chimed in.

She gave them both dirty looks before turning on me. 

I gave her a "Who, me?" look back. 

Mr. Mori cleared his throat again. "May we proceed?"

"Sorry. Please," she shuffled through her books. "What book are we in?" 

Hec lifted his notebook. "Notebook. We're taking notes."

"Oh. Thanks." 

"Thank you Hec. The 8080 is the brains, so to speak, of the Altair 8800."  

Mr. Mori paused. 

"I really don't like calling the CPU the brains of the computer, but it sure is a common analogy. Oh, I'd better do this, first."

He moved to the whiteboard and proceeded to draw --

"We see this diagram a lot. 

"RAM -- random access memory -- is a place where data and results are kept. But the data in it usually disappears when the power gets turned off.

"I/O -- input/output -- devices are devices like printers and keyboards and such for people, and thermometers and mechanical relays and other transducers for measuring and controlling things, and tape and disk storage for memory that is more permanent than RAM. 

"And the CPU -- central processing unit -- does the calculations, and moves data around in memory and between the computer and I/O devices. 

Let's look at it this way:" He redrew the diagram to the side, with different labels:

"We can think of RAM as magic addressable paper. If we remember where we put data, we can find it again. And we can overwrite it with new data as often as we want without wearing a hole in the paper.

"And we can think of the CPU as the magic pen with a calculator inside that writes in the RAM, but that's pushing the metaphor too far. 

"Well that's not all exactly accurate, and it's ignoring a lot of applications, like drilling equipment controllers and such. And I don't think we'll be able to hook our overhead projector up to our computer as a useful display device this year or next year. But maybe it's a little easier to visualize, for as incomplete and futuristic and metaphorical as it is. Any questions?"

"Calculator, not brains -- so, you're saying computers don't really think?" Cyndy asked.

"That's definitely part of what I'm saying. We'll need to talk more about it, but I think this gives us enough of a background to talk about different CPUs." 

"How long do you think it will be before we can display computer output on an overhead projector?" Bill asked.

Mr. Mori thought for a moment. "It shouldn't be too hard to rig a CRT display under the lamp and mirror of an opaque projector . I think something like that has been done, but it wasn't very pleasant to look at. And it probably takes a lot of tending while you're using it." 

Cyndy spoke up again, "CRT?"

"Cathode Ray Tube," Mr. Mori replied.

I added, "Like a TV tube, or oscilloscope screen." 

Frank volunteered, "CRT displays are another type of output device." 

"Oh." Cyndy blinked and wrote in her notebook.

"Anything else?" 

We didn't seem to be asking questions, so he said, "Let's get a look at the insides of several versions of that robot calculator CPU." 

I snuck a peek at Cyndy's notes, and they looked better than mine. 

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