In the previous video I showed my PDP
1173 that came quite late in the history of PDP 11
Whereas what we brought in here is a replica a modern replica based on a Raspberry Pi
Although much earlier PDP 11 the pdp-11 70 and these were much more visually interesting
They had sort of flashing lights on the front and toggle switches that you can you know
Pipe up and down in order to program the machine
So it comes as a kit we have been setting this not an advert for them
But I do think is a pretty cool kit. So we have a bag full of switches
We have LEDs and a couple of chips and some diodes and some resistors. This is the front panel. Let's see how quickly I can
peel off the
Yeah
very brief overview
These are the LEDs showing the data in the currently selected address the LEDs showing the current address
the few status LEDs to show kind of what mode the pdp-11 is in and then you can switch these
Addresses and data to different registers and things with the rotary switches on the right and then we have this board
So we're going to solder up some of this today. Maybe not all of it
And on the back here, we put a Raspberry Pi so
May not have been very sensible. It is to Hilo so I think I might trap these bits in
The case
alright, so let's start with the smallest components that's going to be
Sort of this row of diodes across here
These diets do have to go in the right way. I think I'm only gonna do two or something is the wrong way around
Try not to fix hold on your lens as well
See how they've done that quite nice actually only three diodes but
Yeah, we're not actually gonna build this entire thing right now
It took me about three hours and I had a sandwich in the middle so
quite a bit smaller than a
actual
Pdp-11 70 so pdp-11 70 would meet some 19 inches. I'm using
Imperial because that's how they measure the racks
And it would be I don't know how
So you have sort of the control panel which is of this big and then under that you have the card cage
So quite a lot larger than this thing, but most people don't want a full pdp-11 70 in the house
Although if you want to send me one
That's fine
So let's talk a little bit about
Pdp-11 in general and then we can kind of explain the console a little bit more
The PDP series of computers was basically all based around
octal so
all of the word sizes of these PDP's was
in multiples of six pre ASCII and in order to represent
Kind of the full character set you it needs six bits
So your word length of the machine would be sort of
12 or 36 or you know 18, so that was convenient as well because we could think about
instructions or code listings in octal so each digit would be
Three switches on the front of here the pdp-11 though was late 60s early 70s
When ascii was really sort of taking off and you know, all computers were going to be using ASCII
So the side effect of that is that we wanted 8-bit word length
Machines or 16, you know multiples of 8
So an interesting side effect of dec wanting to stick with this optimal notation while also switching to ASCII and 8-bit based
Word lengths is that we have sort of three switches and three switches and three switches
So, you know zero through two seven on each of these with in at the very end this 21 bits the most significant bit on
The machine is just on its own. It's all alone
So you can see they were clinging on to octal and they didn't want to give it up
I think we should probably enter a small program you're using the switches will chase a light across the screen
that's generally like sort of the hello world when you don't have a terminal with you when you first
set up the PI DP 11 and first boot up it will kind of do this idle LED pass on a
Actual PDP 1170 would knit on this we would just wait and do nothing
So I have on my phone here a short program, which is I think commonly called chasing light
So what we're gonna do is we're going to halt and reset the machine
Well, we're going to make sure we put the right address in. So we're starting at 1,000 load address
and we're going to say
so 5,000 deposit
5 2 0 0
So each time you do this you're selecting with something that's like writing a number and then you're saying write put this in a register
Yeah, you're actually putting it straight into
That's yeah, that's just the default so once you deposit it increments the address register so you don't have to you know
manually insert the address first and load it and then you I mean
this is a lot of work so you wouldn't want to do this for
you wouldn't want to sort of yeah type in the address and then also toggle in the
Data that you want in there. So I'm gonna put this back to 0 and then 1000 which is our starting address
load that and
unhold and then continue
What they done wrong
If not that
See the thing is I grew up in an age where we had to type in in
basic new program listings and that felt like it was really hard work and now I'm seeing the
Well, yeah, so this was 20 years before yeah. Yeah. So the Busey was 82 and this was sort of late 60s. Yeah
Yeah
Probably we're gonna I'm gonna reset a whole machine
which is there's a secret kind of reset button on here on the PI DP 11, so
Let's see
It's finally running we tried it a couple of times to enter it I think sometimes because there's a simulator running in the background
Interrupting it from what it's currently doing and trying I think it introduces an extra layer of sort of problems. Whereas a actual pdp-11
You would enter the program and hopefully overdrawn probably the first time but we got it running so you can see
There is an LED chasing directly across the screen at the moment
We can actually hold this machine and then we can run it one instruction at a time. So you see the address register
Incrementing by one each time and every few instructions. This will jump across to the left until it wraps back around
Yeah overflows and comes back to its zero this machine is so much faster than an actual pdp-11
The speed at which it's chasing across the screen is one not screen
But you know the console is office and it would be on on a an actual pdp-11
Essentially what this program is doing is we clear the register zero
We increment it by one so that it has
you know a bit in it and then we rotate that register to the left once we
reset the bus which just adds a little bit of a delay so that we can actually see the LEDs changing and
Then finally we jump back to instructions
So they rotate so when this bit gets to the end of the sixteen bits, it wraps around back two bit zero
So we we see this bit moving very quickly from address zero to sixteen
I would ideally like my pdp-11 to be on 24/7 at home, but that would cost me about three hundred pounds a year
whereas this is five watts and
That's about five pounds a year
When this is a home I have it now running a web server
Maybe I can give Shawn a link and we can see if we can crash it with some
swarms of people
That'll be fun. I mean, that's that's the other thing because it simulated you
you can just take it back up of your SD card and
If something goes wrong, you can go straight back to it. So
When you're actually playing with hardware, there's so many things that can go wrong from like, you know disks crashing on our lo2 drives
Power supplies failing it's always a fun repair but in this I mean
It depends if you're more interested in the software or the hardware. I think I'm more interested and I like them both
Send me a few fuel Evan
I'm gonna disconnect it from
from the mains
I think it should be fine anyway, but
I will spin it around. I'll just remove these ribbon k
We probably wouldn't get one when the mouse is moved