Episode: 3812
Title: HPR3812: PeePaw's computer does nothing
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3812/hpr3812.mp3
Transcribed: 2025-10-25 05:49:13

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This is Hacker Public Radio Episode 3812 for Tuesday, the 14th of March 2023.
Today's show is entitled, He Paws Computer Does Nothing.
It is hosted by Brian in Ohio and is about 25 minutes long.
It carries a clean flag.
The summary is a Z80 no-op test.
Hello, Hacker Public Radio, Brian in Ohio here.
I'm out from under my rock doing part two of the Z80 computer build, and as you can
tell by the title, today's show is, Pee Paws Computer Does Nothing.
What that means, of course, if you don't know who Pee Paws is, that's who my two-year-old
grandson calls me Pee Paws, and he's the impetus, the reason that I'm doing this project,
and chronicling it here on HPR, and that's so that maybe one day in the future, he or
turns out now his brother who's in the oven, or maybe his cousin who's also in the oven.
Maybe one of those three, or maybe all three, or who knows, will someday in the future,
one or all of those kids will want to hang out with their old grandpa and learn how
a computer works by building one themselves.
And here I am learning to do myself beforehand, getting ready.
And so in the last episode, I put together the one piece of test equipment that I needed,
which is a logic probe, because I'm going to initially spool up this Z80 system using
the facilities of an Arduino Mega Board to emulate ROM and RAM, and also to provide
that some test equipment, like a logic probe, so I can probe around the system when
I provide the required things to see if it's up and running.
And so what's required initially is some power, clock signal, and some data.
And the data I'm going to give the Z80 is provided by the Arduino Mega Board, which
will be the machine code, which has the opcode of zero.
So the Z80 has about 180 instructions that it recognizes.
They're all hexadecimal coded, and it's 0-0 to the processor when it sees that.
It's a no op.
It means do nothing and move on to the next instruction.
And in this case, because the address lines aren't going to be hooked up to anything, it's
not hooked up to any memory, it's just going to think that it's gone from one memory
location to another, and it's just going to find another no op.
And so that's why this is called a no op test.
You're just getting the computer up and running and having it do exactly nothing.
So a couple of things is to make sure, just to clarify, I'm using an Arduino Mega Board,
but it's not the Arduino environment.
I'm running Flash 4th on it, and why would I use Flash 4th to do this instead of the
Arduino environment, because Flash 4th is interactive, without being a huge application
in itself.
There's other interactive languages that run on Arduino boards, things like circuit
Python, but they're huge compared to the size of a 4th virtual machine.
And also, it's 4th is really geared towards manipulating things at the bare metal level
of being able to dig into the registers and control whatever computer it's on at a
very foundational level without any abstraction layers in between, except the ones you build
yourselves by defining words.
And why am I using an Arduino Mega Board instead of the Arduino Luna Board?
It's because the Arduino, which has the Mega 328 chip, and that has to do with the fact
that the Arduino Mega Board has oodles and oodles of IO.
It's got tons of ports, tons of available ports that I can manipulate to interact with
the Z80 chip.
It should be a snap to do it with this board.
So the micro control board, like I said, is going to give the 5 volts the clock signal
and the data to get the Z80 up and running.
It's also going to provide the logic probe so we can test to see that it is running.
So I wired up a proto shield that plugged into the board.
First thing I did, and I've got pictures in the show notes to show all this stuff, is
I hot glued a solderless breadboard onto the Arduino Mega Pro to shield, and then I plugged
in the Z80, and the first wire that's shown by itself is a white wire, and that's the
actual logic probe.
That's from the last episode.
That's, I can plug that in onto the different pins of Z80 and see if things are happening
or not.
The next picture just shows power and ground being connected.
I split the power wire so I can have basically a rudimentary on-off switch.
The next picture, there's a picture, picture number three is with the blue wire that's
a clock signal, and then the next one, picture number four has got the, shows their yellow
lines, that are the control lines, the interrupt, the non-mask will interrupt, the weight signal,
the busy, the bus request, and the reset signals.
Those are all signals that are inputs to the Z80 to help control what's going on.
Then, orange wires are all the data bus.
They go up to port A on the Arduino Mega Board, and the address lines then are, I put
a little silver dot on the chip to show where pin one is, and so from where the clock chip
is, which is pin six, up and around clock wise, those unconnected pins are on the Z80
chip, are all address lines, and the one that we're most interested in is the one at A0,
which is next to the ground line, next to that black wire, and that's the one we'll be
probing once we start this, because as the Z80 goes through memory space, it's going
to put each individual starting at zero, and working up all the way up to 65,000, binary
16-bit data addresses, and so that pin should be cycling between zero and one the most
of all those pins, and so if I see activity on that pin, then I'm pretty confident that
the chip is up and running.
Now, so that's the hardware side of the test we're going to do today.
On the software side, I fixed, so some of the fourth words, and the last, the code that
I presented in the last episode, it's kind of good, HPR is always, it's good, it helps
you to, when you post something, and then even after the fact, I was looking it over
and I saw that I made some mistakes, and I refactored some of the words, broken apart,
and I defined some new words, and that listing is in the show notes too.
The fix I made was that the data processing word, as defined, was getting, when it distanced
that it was a static either high or low signal, it was probing the wrong point, it was probing
the output of pin H3, digital six, I think it is, on the, yeah, digital six on the, on
the, are doing a mega board, and what I really wanted to see was, look at was, was the input
into pin E, the logic probe, that white wire, so I changed that, I changed the output
there too, on the data processing word, I really don't care what frequency it is, I just,
so I just changed the, the word to, to just output, if it's pulsing, to just say it's pulsing,
just, put on the serial terminal pulse, the word pulse, because I'm not interested in
specific frequencies, I split the clock in the logic, init words, so that I can initialize
the clock registers, and on the, on the AVR chip, and I could, and the same for the logic
probe, just, split in two words. I added some words to control the reset line, those
words are reset and run, reset takes the, the pin, the reset pin from a high level state,
which is in the run mode, and takes it to a low, low, low state, so almost all control
signals on the Z80 are active low, so when they are driven to ground, driven to zero voltage,
they become active and do what they're supposed to do, so when you take the reset line and,
and drive it to ground, then the, the chip is, is in a reset mode, and I added one word,
to, to be able to single step the clock, so I can either, I can start and stop the clock
with those words, start clock and stop clock, and I can change the frequency of the clock
by, by putting a number, and, and then calling the word set frequency, but I also wanted
to be able to single step the, the, the, the, the, the Z80 chip, so that, we can probe
around and look to see some other signals, to see what's happening, because the, the,
like all computers, this, Z80 has a very systematic way of functioning, and what it does is it,
it, it, initially it reads in from the data, looking, looks for, and it interprets that
first bit of data as, as some kind of instruction, and then if that instruction needs more data,
it goes and fetches it, and then it processes it, and so the, the, the, you can, there's different
signals that come alive, in that process, because each instruction for the Z80 takes between,
I think it's like four and twenty clock cycles, depending on how long and how much data
it needs. So we, I've got the, I guess, enough explanation I can do the test here. I've
got the board plugged in, I've got, and I've got my serial term, I've got a, a little,
a serial term with, serial terminal with flashforth running, I've got the flashforth prompt,
and the first thing I'll do is, I'll, I'll, I'll get the logic probe up and running by typing the
word, logic probe, init, and when I do that, it says, okay, and then I am going to get the micro-controller
board set up the ports on it. So that's, I'm going to be, let me look at my code. I don't remember,
exactly. A Z80 port, and basically what this does, is it sets up those, those interconnections,
between the Z80 and the, and the, are doing a mega board, and what I'm going to do is,
connections between the Z80 and the are doing a mega board with all those wires it all
those things that it's connected to it's setting up them them up correctly either inputs
or output ports and so that is the word Z80 ports in it and hit that it's okay and then
add some power to the Z80 and so I just plugged in five volts on the board and now I should
be able to the clock should be started so I should be able to take my white logic probe
and put it where the blue wire is and I should be able to type the word sample in and I'm
expecting to see the word pulse output and I do and I can double check that that's actually
working I can go stop clock I'm gonna stop the clock and now I'm gonna do another sample
of it of the pin and I'm in the wrong place it's here put this into the right place
I should be able to get okay so I had to I never initialize the clock I never typed in clock
in it so now I typed in clock in it which initializes the clock and now I can I can I can start the
clock and I can sample and oops I can type quickly sample it comes up pulse and then I can stop the
clock and I can sample and now it shows that it's a low it's it's not pulsing it's a low signal
and I can start the clock again and now the next thing to probe is I'm gonna take a the logic
probe wire the white wire out from where the clock is the blue line and I'm gonna put it over in
a zero not line which is that empty pin right next to the ground wire and I am going to sample
there and I need and I get a high signal so what I need to do is I need to properly reset the the
board which I'll just type in the words reset and run and then I should be able to sample and I
get a pulse so the so on the a zero pin it's pulsing and if I stop the clock oops stop if I type
it correctly stop the clock and now I sample it's just a high because it's because it's not the clock
stopped and you need a functioning clock for it to work so that to me is a good indication that
the Z80 chip is behaving properly that it's that the wiring and all everything is connected
correctly and so now one other experiment one other thing I can check is I can check the M1 line
M1 line is right next to the I should make sure I've got I've got a image of the of the
got the image of the Z80 chip the pin out of the chip in the show notes and yeah M1 is actually
right next to reset so what over so the the the the way is the the order of things that happen when
the Z80 starts functioning when you first reset it so what happens is you bring power up you reset
the the the Z80 puts it in a no good state it puts the program counter to zero it puts address zero
on the address bus and then it it then draws M1 low and M1 is just a signal it it draws the M1
signal low and that's the indication and it's starting the first part of what they call a
machine cycle machine cycle is everything it takes to complete to get an instruction whatever data
needs and then finish it and then and then it signals all this by first off bringing them the M1
line low and so the way to check that that's actually happening is to go back to that the terminal
and the clock is right now stopped so I'm not going to start the clock I'm going to single step
the clock and what I'm going to do is I'm going to go back to reset I'm going to single step the
just steps are step I'm a single step and then sample the pin and I should see it high
and I'll do that a couple times because it takes three clock cycles for the chip to be properly reset
so here we go I'll do that one more time and now I'm going to put the
reset line into the run mode and now I'm going to single step again and sample
and the the M1 signal pin is still high that clock signal and now one more clock signal
and now it went low it's the beginning of a machine cycle and I click clock at a couple of times
it'll go back to high and so I know that the the the M1 signal is working and so with the aid the
address zero pulsing that it that it has busy that it has activity on the address bus and with
the M1 signal cycling the way it is I'm pretty confident that this is the 80 chip is is good and
that it was a good a good no-op test for that for that chip and so I think that covers
my notes here yeah oh there was one more thing we can do and and then that'll be the end
to be the last the last the last signal to probe and this one is going to be the hot signal when
when the when the z80 interrupts when the z80 encounters a halt signal it actually
halts this the processor and does no ops internally until it receives an interrupt signal
and the halt signal again is active leave it's active below also and it's on pin number 18 of the chip
so I'm going to put a logic probe into pin 18 and do the same exact process I'm going to reset the chip
so start with by putting it I put it in reset and then I will cycle the clock
and sample a few times here and now this time what's going to be different is I'm going to put
the hex address of 76 hex 76 on the data bus and so so when I start when I start to run the
program instead of finding on the data bus at address 0 a no-op a machine instruction 0 it's
going to the z80 is going to find on the data bus the machine instruction 76 which is halt
and when it does that I should we should see when I sample that pin the the halt signal it should
go low so let's do that let's put 76 into the data port and I do that by typing in 76 data
dollar sign 76 which is the hex notation in fourth for hex numbers dollar sign 76 and then
the word data which is the data port and I'm going to do c store which is c in an exclamation point
that'll store the number 76 onto the data port and I got the wrong thing what's it called maybe
got the wrong word the wrong thing it's called um what's it called oh it's capitals uh
case sensitive fourth is this fourth is case sensitive so 76 capital DATA c store
c store which will store the values the hex value 76 into the data port and I get okay and now I'm
going to uh sample and uh step and sample the clock and because it's nothing there okay now I'm
going to put the the z80 into run mode and now I'm going to step and sample and do it a few times
because it'll take about four or five or six to get there and there now the pin uh 20 the signal
on pin uh 18 went from high to low and that to me because it found that 76 on the uh on the
data bus it executed the instruction halt and now this the the um the z80 is in a halt mode so
if I if I start the clock now
okay I'm going to do a nice sample it should stay low it should stay low the now completely
and a little stay low until I reset it uh or give it an interrupt signal which I'm not going to do
and so there we go uh looks like I've got a functioning uh good the z80 chip I have learned a little
bit about how to um manipulate and a couple of instructions the no op instruction and the
halt instruction and put them on the data bus and and get the uh z80 to respond accordingly
and I was able to see uh um see all those things by using the logic probe that's that it's built
into the software on the Arduino mega board that's running flash forth so consider consider
this a successful experiment um I think I'm not sure where to go from here I think the next thing
to do would be to maybe do a more um a more a longer a set of instructions a small program
and uh maybe even I might have to add some leds or something we'll see or at least put out some
numbers on to the serial ports like we can see get some output that way and um and that'll require
emulating emulating some RAM right now I'm excuse a ROM emulating some read only memory so
I'll be the next uh the next step in this project so um thanks for listening um if you have any
questions or comments go ahead and leave them in the show notes or you can email me and if you
or if you're interested in retro computers are doing something like this yourself uh pick up a mic
and start recording a show and upload it to hacker public radio and if you um
um and anyway thanks for listening I appreciate your attention and I just like to remind everybody
that it's not how much an item costs it's how much you save goodbye
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