Thursday, September 25, 2025

3809/2801: Register Sets for 8080, 6800, and 6502

2801

CPU, RAM, I/O
TOC

(I know some of my audience will think this chapter is a little dry, but important stuff happens in here, so give it a read anyway.) 

Mr. Mori turned back to the projector. 

"The 8080 derives from their initial 8008 CPU, which is based on the Computer Terminal Corporation's Datapoint series controller implemented at lower scale integration -- the central processor is on a printed circuit card rather than a chip. 

"This is the published register set for the 8080:" 

8080 Register Set 

"The 8080 has 7 sort-of general purpose 8-bit registers plus a dedicated status register that can sometimes be treated in pairs as 16-bit registers -- sort-of. It also has a 16-bit stack pointer that you can use to keep track of data in memory, and a 16-bit instruction pointer that keeps track of which instruction the CPU will perform next. 

Cyndy raised her hand.

"I'm sorry to ask, but I'm not quite sure what 8-bit and 16-bit mean. Is that the number of binary digits the register can, uhm, hold?"

"Correct. Barry, what's the biggest number register A can hold?"

"Signed or unsigned?"

"Yes."

"Unsigned, 0 to 255. Signed, wait. Does the 8080 run in two's complement?"

"Yes."

"Signed values range from minus 128 to 127."

"Very good. Chuck, what's the range for H and L together as a 16-bit pair?"

"Uhm," Chuck stalled with a lopsided grimace. "From zero to two to the 16th power minus one for unsigned numbers, and," he slowed, but continued carefully as if he were checking facts in his mind as he recited them, "for signed, minus two to the fifteenth to two to the fifteenth minus one."

512
1,024
2,048
4,096
8,192
16,384
32,768
65,536

I watched over Cyndy's shoulder as she started writing out powers of two from 512 in a margin. Hec and Rick noticed, too.

"You're having trouble remembering these numbers?" Mr. Mori asked with a patient smile.

"Thirty-two thousand something and sixty-five thousand something."

There was a bit of snickering from various members of the class.

"Close enough for now," Mr. Mori nodded. The snickering quieted.

Cyndy interjected, "Okay, the exact ranges are 0 to 65,535 and minus 32,768 to 32,767, right?" 

Eleven pairs of eyes and the heads that owned them turned and stared.

Bob complained, "If you knew, why did you ask?"

Hec replied for her. "She just worked it out right now." 

"Once Chuck confirmed it for her in the power of two forms, she understood, I think. Right, Miss Mills?"

"Yes, Mr. Mori."

"Remind the class what this means, Rick?"

"Why not ask Merry?" Rick grumbled under his breath, giving me a sideways look before answering. "Eh, when you put two registers together, uhm, that'll increase the number of columns, and the maximums are based on the power of two to the number of columns, adjusted because of zero or because of the minus range. Did I get that right, Joe?"

"Yeah, that's the way I understand it."

Rick tilted his head back without looking at me. 

"Mary?" Cyndy questioned under her breath.

None of the three of us in hearing range elucidated. 

"Everybody comfortable with that?" asked Mr. Mori.

There was a chorus of yahs.

"Okay, moving on, A is the register that is primarily used for recording arithmetic and logic results. It's called the accumulator because results tend to accumulate there. But for 16-bit arithmetic, especially address arithmetic, the H:L pair can often be used. B:C and D:E can hold addresses for moving data around, and H:L can also hold addresses -- point to operands for more general purposes than just moving data. None of which probably makes sense yet, but it will when we look at the instruction sets.

"One particular 8-bit register doesn't have a name because it can't really be directly accessed. It can't be used as a general purpose register because it records the condition, or status, codes -- information about the most recent operations, such as whether a carry or borrow has occurred in arithmetic; whether the current result is zero or not; whether it has the most significant bit, the sign bit in two's complement, set or not; and whether the current operation resulted in even or odd parity. We'll get to which operations set what, when, and what it all means later. 

"The condition codes are paired with the accumulator as the program status word, PSW, because they usually represent the current status of computation in the CPU. The entire PSW is basically accessed when you save it or restore it with a push or pop.

"And pushes and pops in the 8080 happen in pairs -- the register pairs shown."

I raised my hand.

"Yeah, Joe?"

"I don't think 16 bits is enough for floating point numbers, is it?" I asked.

"Depends on the floating point precision, but if you are talking about Hec's calculator, no, it's definitely not."

Hec had a programmable Hewlett Packard calculator his dad had given him in payment for some work he had done for his dad. I think it was his dad's old HP-65. It had the magnetic card reader, and he had been studying how to program it.

He volunteered, "The user's manual says it uses 64 bits, I think."

"That sounds right," Mr. Mori continued. "But this cheap 4-function calculator on my desk probably uses just forty bits. Supercomputers might use 128 bits or more."

"What about the 8080?" asked Bill.

"The 8080 doesn't do floating point unless you program it to, so it depends on the floating point software. But 32, 40, and 48 bits are common sizes."

"Is the 8080 slow?" Karl asked.

"Depends on the software. It's likely faster at floating point than many calculators. But some calculators actually use the 8080, often at slower clock speeds than a computer would. The others we will look at today, also, either have been or could be used in calculators." 

Cyndy commented, "The total number of bits in the 8080's data registers isn't enough to do floating point in registers, is it?"

"That's right," Mr. Mori nodded, "pretty much. 32-bit floating point -- you might fit one operand in B:C and D:E together, but it might not help." 

"So floating point numbers have to be stored in RAM, right?"

"Good point. Not just floating point, but strings of text and many other kinds of complex data will be stored in RAM, and pieces of them will be brought into the registers to work on.

"By the way, when you store 16-bit quantities in memory or load them from memory on the 8080, the less significant byte is at the lower address. You may feel that is backwards. I think so. But Intel thought it would save them some critical nanowatts or nanoseconds or development time or something, and, as long as the CPU is consistent with itself, the CPU doesn't care what humans think."

"One more thing, some of the registers in the 8080 tend to have specific purposes, according the instructions that operate on them, and are less useful outside those purposes. That is, there are certain operations that can only be done with certain registers. That allowed Intel's engineers to supporting more functionality using fewer of the available op-codes. 

"And we want to talk more about instructions and addressing when we examine some actual code. But I think we want to get a look at the register sets of some of other CPUs that are reasonably available first." 

He reached for his box of transparencies. 

I looked over at Cyndy's notes again and grinned. "Your notes are much better than mine."

"You want to copy them, Joe?" She was being coquettish again.

"That's okay. I'm not much for taking notes."

"Yeah, Joe doesn't really need to take notes," Rick commented. "He has a mind like a trap. Sometimes it's really hard to get him out of that trap, too." He grinned sardonically.

I gave him a lopsided grin. "I can't argue with that." 

"You can compare notes with me," Hec offered.

"I'm not interested, Hector." 

Mr. Mori cleared his throat, and the conversations at all the lab tables died down. 

"Here's the register set for the 6800." 

Karl snorted. "That's a register set? What can you do with that?"

Mr. Mori chuckled. "Compared to most minicomputers and mainframes, the 8080 has a pretty limited register set. Yeah, you could accuse the Motorola team of trying to find a minimal useful register set, and I think they got close. But if you think this is too few, wait until we get a look at the 6502 and the 9900. Anyway let's look at what the 6800 has to offer."

Chuck said, "It has a stack pointer, like the 8080." 

"Right. We'll look at the differences later."

Cyndy said, "The program counter is the equivalent of the instruction pointer, right?" 

"Very good. Maybe I can let you guys teach the class." Mr. Mori grinned.

Bill volunteered, "The index register X would be like the 8080's H:L pair?"

"Right. But the 6800 handles indexing differently -- which we'll discuss more when we discuss operators and addressing. And the differences do make up somewhat for the dearth of registers."

"Two accumulators?" Mark asked. "That's not so minimal?" 

"Having two accumulators allows one to be used as a counter or such while the other is accumulating results in a loop of repeated operations."

"I could see that," Mark nodded. 

Barry asked, "Can A and B be paired for 16-bit math?"

"Sometimes it's useful to use them that way, but Motorola didn't provide a single instruction that will do it in the 6800. I wish they had."

(In the real world, Motorola was working at the time on MACSS, which gave us the 68000 -- and 6809 as a byproduct. The 6801, which would fix several small issues like this with the 6800, but retain compatibility with the 6800, would come out of the work on the 6809, but would end up released before the 6809. And if you are interested in such things and haven't looked at the 6805, you should.) 

Rick glanced over at my notes. "Joe thinks CCR is the 6800's PSW."

Mr. Mori raised his eyebrows. "What do you think, Rick?" 

I added a couple of marks to my notes where Rick could see them. 

"Condition codes, program status, sounds like the same thing, uhm," he check the marks, "but without the accumulator?"

"That's why I don't let people copy my notes," I grumbled.

We exchanged lopsided grins. 

"Right. There's also a bit in the 6800 condition codes that is not present in the 8080 condition codes, used to disable some interrupts. And we'll talk more about that later, too." 

Out of the corner of my eye I watched Cyndy checking my notes, then Ricks, and then Hec's.

"We'll need to talk more about accessing the condition code register on the 6800 later," Mr Mori continued. 

"And this next part is actually about addressing modes, but this is something that is relevant in considering the dearth of registers -- the 6800 has a direct page shortened addressing mode that treats the first 256 bytes of RAM as special locations in memory, making them almost as useful as registers, but not quite."  He swapped out transparencies:

"We almost might want to call the first 256 bytes pseudo-registers."

Frank laughed. "That's a lot of almost."

"True," Mr. Mori replied with a wry grin. "But I don't want to spend much time on why I say almost until we talk about addressing."

Barry asked, "Would this direct page be a good place to work on floating point numbers?"

"Good question. I'm not sure I know a good general answer to that. One problem with that would be what would happen if a program doing floating point calculations got interrupted and some other code tried to use the same locations."

Barry nodded. "I could see that screwing up calculations."

"We need to talk about saving stuff before you use it, too, later. 

"Oh, and while we are talking about data in memory on the 6800, it stores and reads 16-bit values in memory with the more significant byte first, which I think would get my eyes crossed less. I just want y'all to know." 

He sighed. 

"But right now I think we have enough time to talk about the registers in the 6502 before we take a break." He swapped transparencies again: 

There were noticeable vocalized expressions of surprise from various parts of the room. 

"I think you can see the influence of the 6800, which isn't really surprising, since the team that designed the 6502 is basically a group of the 6800 team engineers that left Motorola to do so. They worked on a shoe-string budget, and the design is rather minimal, but quite powerful." 

Frank said, "So the stack is located at a specific place, and is limited in size to 256 bytes. That might be reasonable. But, ..."

I pursed my lips. "Two index registers looks good, maybe a good tradeoff for one of the accumulators. What's the trick for getting 16-bit addresses out of them?"

(Yeah, Joe is a bit ahead of me, catching on to this so quickly. But the whole class is a bit ahead, too.)

"X and Y are used in different ways, which we will talk about when we talk about addressing modes. But, yeah, the 6502 provides powerful addressing modes which compensate for the width of X and Y and even somewhat for having so few registers.

"In the 6502, the first 256 bytes of address are called the Zero Page. They are very clearly intended to be used as pseudo-registers, and are supported in that use by the instruction set and addressing modes. And the 6502 instructions are a bit more cycle-efficient than the 6800 instructions -- a bit faster, so that the 6502 can keep up with or even beat the 6800 and 8080 at the same memory speeds.

"And, just for the record, the 6502 is, like the 8080, little endian. It stores the less significant byte first for 16-bit values." 

Mr. Mori took the transparency sheet off the projector and turned it off.

"We're too close to the bell to pick up the COSMAC 1802 and the TI TMS9900, so I think I should let you guys check your notes, discuss them in your groups, and ask questions until the bell." 

Not that I should have noticed, but Cyndy was looking a little uncomfortable.

I leaned against the table and stared at the whiteboard, and subvocalized to myself, "I wonder if it would be too way out there for any other girls to decide to join our class, too." 

It shouldn't have been loud enough for anyone to hear. But Rick responded, beneath his breath, "I know who you're thinking of. Wishful thinking man." 

Cyndy shouldn't have been able to hear, either. I wasn't really watching her, but I saw, out of the corner of my eye, her face show a reaction I couldn't read. 

Rick said out loud, to no one in particular. "Jokes aside, should we compare notes?" 

Cyndy relaxed a bit, and we did exactly that.

While we were comparing notes, Bob came over and grabbed an unused chair and tried to get Cyndy's attention. 

"Hey, anytime you want to join our table, Scott's willing to give up his chair."

"Hey!" Scott picked up his books and moved them to the neighboring empty table. "Cyndy, you can join me at my new table."

Cyndy replied, "Guys, give me a minute and I'll come over and look at your notes."

Scott grabbed his notes and came over and Bob went back for his, and pretty soon the whole class was gathered around our table comparing notes -- and joking around a bit, of course. 

When the bell rang, Cyndy excused herself and left, and the rest of us broke up for the ten minute break.

I wandered up to the shelves at the front of the room, and picked up the datasheet for the 6800, somewhat at random. Rick and joined me, picking up a copy of Radio Electronics and flipping through it.

Some of the other guys joined us, as well. Harvey, in particular, seemed to be searching for something.

Rick showed me an ad for the SWTPC 6800 computer and the terminal they were selling at the time. "I could pay for this with a couple of month's wages."

"Looks interesting." I looked over his shoulder as he continued thumbing through the magazine. A page advertising the MEK6800D1 evaluation kit popped up.

"Wait." I grabbed a pencil and piece of paper and wrote down the contact information for Motorola, then we started looking for ads for evaluation kits for the other processors Mr. Mori had mentioned. We had contact information for Motorola, MOS Technology, Intel, RCA, and TI by the time the Cyndy came back. 

Cyndy didn't say why, but remained at our table. 

TOC
Next


Sunday, September 14, 2025

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
TOC

"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 prepared 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 okay."

(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. And you go by Cindy?"

"That's me." 

"Okay, Miss Mills, would you care to tell the class what you told me two weeks ago about why you want to take this class instead of the regular electronics class?"

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

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

"Thank you. I think we are all in the same frame of mind. But what are you sacrificing for it?"

"Yeah, I'm going to have to keep my participation in cheer dance to after school."

"Oh, man, that's tough," I tried to commiserate in the silence that ensued.

She gave me an unreadable look and sat down.

Hec asked, "Is your dad really okay with this?"

She glared, but nodded. "Mom, too, if it's any of your business." 

"Sorry." 

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.

"So don't get any ideas." Cyndy included me in her stern glare. Then she turned to Rick and said, "And you, neither, uhm ...?"

"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 think 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. 

"But 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 neither of those diagrams are exactly accurate, and they're ignoring a lot of potential 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 that, 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?" Mr. Mori asked.

We didn't seem to be asking more 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 she was doing a good job on them. 

TOC
Register Sets for 8080, 6800, and 6502


Thursday, September 4, 2025

3809 -- A Tale of MCUs and Time, Part 1, 2801

The bell rang the end of the last period of the first day of classes, and I pulled the practice sheet of paper from the platen of the Selectric typewriter in front of me, slipped it into the class folder, and tried not to be obvious about watching Sapphire collecting her books in the front row.

It didn't look like there was going to be a gap in her conversation with the girl sitting next to her, so I put my books together, stood up, and moved between the desks, making sure I would pass her desk.

She stood up and glanced my way as I approached. 

"Uh, Hi, Sapphire." I tried to sound and act natural, but I'm sure I didn't.

Before I could say, "You don't remember me, do you?", she smiled and said, 

"Hi, Joey. How did you manage to get into my typing class?"

"Ah, uhh, it was the only section that fit in my schedule."

"Oh?" 

I couldn't read whether she was playing with me or not. 

She collected her books, and the girl she had been talking to collected her books and stood as well. 

"Yeah," I almost stammered. "Typing is supposed to be a useful skill for working with computers."

"Oh, computers." She grinned a wry grin. "More math."

"Uh, Yeah."

"Oh, this is Cyndy. I don't think you've met."

"Hello Cindy. Nice to meet you."

Cyndy smiled. "Nice to meet you," she paused, her smile turned sly, and she put just a little too much emphasis on "too," and she smirked, but then paused again. "Uhm, ..."

I grinned back. "Joe."

"Not Joey?" 

"Well, either way." 

Sapphire turned and moved with her friend toward the door. Then she casually glanced back at me. "So how is it you're taking typing here?"

"Industrial electronics." I followed them toward the door. "The Industrial Electronics class here is doing microcomputers this year." 

"Always such a complete nerd. So smart."

"Eer ... uhm ...." High school senior and still not sure how to respond when girls called me smart.

At the door, they paused, and I moved through first, checking hall traffic.

"And cute." 

They followed after me. 

"We're heading this way." Saphire nodded one direction down the hall.

"Ah, I gotta go meet my buddy for my ride home." I glanced down the other direction, where Rick was just stepping into the hall from his class. He saw me and gave me a doubtful look.

Sapphire responded with a quizzical look.

"Rick Stanton. Maybe you remember him. He's also doing half-day here for the electronics class. We're splitting the lunchtime driving duties across town."

"Oh. I see." She paused and gave me a bright cheerleader smile that could have meant anything or nothing. "Well, I guess we'll see you next week."

"Yeah, I guess so." 

Cyndy nudged her as they walked away and she nudged back and they both giggled.

When I caught up to Rick, he shook his head. 

"You're reaching way too far out of your league."

”Yah, I know. But she's in the class. I had to say hi."

We made our way to the parking lot and got in his car. He started the engine and shoved Queen's Night at the Opera into the 8-track, clicked through the tracks, and "Death on Two Legs" blared from his speakers as we headed toward the exit. 

"7400, 8080, 6800, 6502, LSI, you got any idea what Mori's talking about?" Rick paused to focus on navigating the intersection at the parking lot exit.

"Probably no more than you. Lots of circuitry in a small package."

Rick looked down at his 8-track. "You think they can pack the circuit boards for my 8-track in one of them li'l ol' chips?"  

"The power stuff has to be separate, I think. And I'm not sure how the digital stuff fits with the analog stuff."

Rick slowed down for the red light at the next intersection, signaling and moving into the left turn lane. "Yeah, yeah, tell me again the difference between digital and analog." 

We waited for the light to change. 

"Digital is about discrete values and analog is about continuous values. You know that."

"Mmm, huh. Switches versus pots." He shook his head. "You think we can figure all that out this year?"

The light changed, and he moved the car forward. 

"Isn't that what college is for?"

Rick snorted. "You're always just along for the ride. Nothing fazes you." 

Oncoming traffic cleared, and we turned left.

 

Author's Preface

This may appear to be a pseudo-memoir. It isn't. Yeah, the world is (mostly) as it was at the beginning of my senior year in high school. And most of the characters are (more-or-less, loosely) based on (composites of) people I knew when I was a kid, names changed, etc. But the events of the plot, and their effect on the world, are almost completely whole-cloth fabrication. 

The narrator is not me. 

I definitely did not have the courage to talk to the young woman on whom Sapphire is sort of drawn on that first day in typing class, and Joe is way ahead of me on other fronts, as well.

He managed to avoid wasting a summer in junior high on Tom Swift Jr. fantasies, opting somehow to read instead from Heinlein and Clarke a bit earlier than I did, and he made time for the Book of Ether that summer, as well. He even read War and Peace and 1984 and other classics, along with the rest of his scriptures, during his sophomore and junior years in high school. (Well, I did read 1984 during my junior year, for the English class thesis. But not Animal Farm.) Way ahead of me.

Sapphire -- I have to admit, although I sort of carried a torch, for several years, for (the image in my mind of) the young woman who was the primary inspiration for Sapphire, I never really got to know her well enough to know whether I've steered clear enough away from portraying her. If I got too close, Sapphire, I apologize.

Rick, Hec, Cyndy, Mr. Mori, all the rest are also composites, constructed for the convenience of the story, and I apologize in advance to the violence I've done to their characters, as well. 

But this is really about the second microprocessor revolution that never happened, not Sapphire or me or our friends. And about the illusion we call the butterfly effect.

This belongs squarely in the class of alternate history fiction. 

* * * * *

 

3809

A Tale of MCUs and Time, Part 1

2801

Marion Joseph Reeves

Copyright 2025 Joel Matthew Rees 

Table of Contents 

 





3809/2801: Register Sets for 8080, 6800, and 6502

2801 CPU, RAM, I/O TOC (I know some of my audience will think th...