The design for the protected mode Z80 computer is complete!

hello hackers it's Andy here now I want to make a quick update video about the state of the hack project because I think he has reached a milestone that I should make a video about so as you can see this is the sketch Matic of the hack project right here I have divided down to four different parts this is the CBU this is IO and this is protection which does all the protected mode stuff and this is memory which contains not only the memory chips themselves but also the memory paging circuit so we can go into one of them and have a look so this is the CPU part of the circuit as you can see a large chunk of it is occupied by this circuit which is entirely optional this circuit allow the hack to display some kind of post code during the boot process and it uses nothing but the CPU itself a latch register and a decoder now now remember the hack is a new design it's not like the commander x16 which is based on the Vic 20 so any part of the circuit can go terribly wrong therefore having a debu circuit that relies on as little component working as possible is good for the future development of the project and next let's look at protection this part does not contain a lot of gates but it is actually the most complex part to design because there are tons of of edge cases raise conditions and just general bugs to avoid take the protection Force as an example the hack can generate a fault on an instruction fetch on non data Pages it can also generate a fault during a write to a non-writable page a read or a non-readable page for example pages that contain sensitive system information and it will generate an protection fult if a user program try to access an external IO device directly this is the io protection for that everyone knows and love if you play close attention to them you'll find that although all four fourth generation circuit involves a 74 LS 74 the flipflop with some external circuit no two fourth generation circuit are alike despite you may think about while they are generating for on similar conditions and that is due to the difference in the timing of the z80 for different types of memory access one lesson I learned when designing the hack is that the devil is in the timing it is always in the timing a similar story also happens to this circuit which generates the cyst signal which indicates whether the motherboard is in a user mode or a system or kernel mode but I will leave the story of this circuit to another video because it's way beyond what I can cover in this quick and short video next let's look at memory you have your main memory chips here and the paging memory chips here and finally on the bottom right you have a simple data latge which selects the process ID while generating a CIS gate signal that signal basically means whether that application can enter sismo with a restart instruction and this is used to emulate system that uses the restart instruction for other purposes and finally let's look at IO every logic gate you are seeing here is revision two or three and some of them are revision four or five because some of the logic here are really nasty and hard to get right the expansion bus on the hack is a core part of the entire design I can easily make a half an hour long video just explaining every signal on this bus why they are there and why many of the signals commonly found on other buses for other Z8 machines are not here another challenge for this circuit is actually this part which generates the weight State for the I/O devices the challenge here is that the hack is clocked really high I'm targeting at 16 mahz and it may go even as high as 20 MHz the dip switch here is used to select between 1 to 16 w States this is a feature exclusive to the development version of the board it will allow the developers to kind of squeeze the timing of their card to test out exactly how fast their peripherals can go this with a ton of other features can really help developers develop hardware and software for the hack now if you don't understand any of this it's fine the takeaway is the circuit design is complete and the sketch meic is BAS basically down there are some minor touchups some signal needs connecting I will add a keyboard controller but that's basically it but on the PCB side it's a different story yeah the PCB is not complete as you can see and that's because I have to confess although I have designed projects in Kat before I had really little experience in doing so and placing and routing a deeply entangled system like the hack is kind of a nightmare for me and I've been grinding almost nonstop to get the sketch magic to its Finish Line things like the protection circuit it has been changed numerous times sometimes entirely overhauled just because a new timing Edge case is found and to be honest I'm I'm feeling a little bit tired so I'm announcing that I'm taking a small break from this project don't worry the project is still going you just won't be seeing like two or three commments every day for the following week and I think I can really use some help from the community here if you have experience in designing retro related or just any kind of PCB circuit in Kat and you want to help please leave a comment in the comment section below you can contact me vi patreon and you can join our Discord server using the link in the description below but I think that's it for for this update I will see you in the next one bye-bye

Channel: Andy Hu

Views: 541

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Id: rpTXft1GNQ0

Length: 6min 35sec (395 seconds)

Published: Thu Mar 21 2024