How WozMon works
Added 2023-06-30 22:43:34 +0000 UTCWell you asked for it. After my last video there was a lot of interest in delving deeper into the Wozmon code and explaining how it works. So here you go! I walk through the code line-by-line explaining how it works and some of the tricks Woz used to get it to fit in the 250-byte constraint.
As always, thanks for your continued support and let me know if you see any issues with this video before I release it publicly. I may also do a shorter followup video talking about some of the changes I made to get it to work on the breadboard 6502 with serial interface. This video was getting long enough and I thought it better to just have a video focused on Wozmon itself.
Thanks again!
-Ben
Comments
Late to get to watch this as I've been away; but I really wanted to say that I thought this was a *fantastic* video. Content like that is exactly why I'm proud and happy to sponsor you here. Thankyou for making and sharing this.
Auctoris
2023-07-12 09:20:17 +0000 UTCAnother neat and very "Woz" choice that I don't recall being mentioned in the video is that SETADR loop. My first instinct would probably be just a simple LDA L, STA STL, STA XAML followed by LDA H, STA STH, STA XAMH. But by going to the trouble of making it a loop (even though it only cycles twice), Woz shaved off three bytes: One with the loop itself compared to the unrolled version, and two more by getting a "free" zero in X which he uses later at XAMNXT. Brilliant!
Jeff Jetton
2023-07-10 18:33:02 +0000 UTCYeah, at first I though maybe Woz tried different characters for "store" before eventually settling on ":", and just never updated the comments, which might explain the other comment error too. But then I realized that 7B is impossible no matter what character you start with, since 7B is odd, and ASL will always result in an even number.
Jeff Jetton
2023-07-10 18:20:12 +0000 UTCYep, that's right.
Ben Eater
2023-07-06 22:19:30 +0000 UTCI think so, I had the same thought
Cooper Harasyn
2023-07-05 00:31:33 +0000 UTCThanks, Ben, this was a great deep-dive! I've had the privilege of writing plenty low-level OS code in assembly in my career, and I can assure your viewers that all of these tricks to make the code small and efficient are still in widespread use :-)
Jason Thorpe
2023-07-03 23:31:15 +0000 UTCI think at this point we need to somehow get Steve and Ben together on a call so Ben can do a quick code review and tell Steve he needs to fix the errors before the PR can be approved π Great video! I'm so glad we got this kind of content.
ast_rsk
2023-07-03 18:08:48 +0000 UTCOn looking further, I think the manual is wrong here... after 0.A, XAML is 0x0A (and XAMH is still 0x00). If "R" jumps to that address (0x000A), it will try to execute the final byte of the entered program, which is 0x00, which is the BRK command. That should run the IRQ handler at 0xFFFE, which is... 0x0000 in Woz's ROM. So if I'm right, it's just a coincidence that R runs the program correctly in the previous video (which is also the example from the manual).
Jason Clark
2023-07-02 23:41:01 +0000 UTCAfter watching this again, I'm confused about one point. In the prior video, you demoed how Examining a range of addresses (e.g., 0.9) leaves the first address (0x0000) as the "active" address, so that running with "R" will run from that address. However, the Block Examine code increments XAML in order to print out consecutive memory locations, which means it's pointing at the wrong location when you "R"un the code, which executes "JMP(XAML)". What am I missing?
Jason Clark
2023-07-02 23:28:26 +0000 UTCpotato potatoe ;-)
Lukasz Pacholik
2023-07-02 23:10:54 +0000 UTCWhen examining 0200.027F, I see that all the bytes do not have the high bit set, and are just ASCII codes. Is this because the breadboard computer uses plain ASCII? Would running the original code would have produced 'B0 B2 B0 B0 AE B0 B2 B7' instead?
Mathijs
2023-07-01 21:11:04 +0000 UTCWozinak -> Wozniak
Jesse Thilo
2023-07-01 12:53:53 +0000 UTCA thorough explanation - thank you. I'm sure a lot of ppl would be looking forward to you writing a more complete multi-function monitor that's not constrained to a 256-byte "block".
Don Prefontaine
2023-07-01 12:21:09 +0000 UTCI'm rewriting WozMon for the Atari 8-Bit line for the ABBUC software contest over the last few months and therefore am really happy to see your videos about it here! Will link to them in my documentation. I stumbled across the BVC NOTSTOR comment as well, but couldn't believe that Woz made an error here, so thanks for confirming my assumption :)
Frederik Holst
2023-07-01 12:16:40 +0000 UTCIf all of my Patreon support went away and I had to replace it with ad revenue, I'd need to make videos that consistently get 2-3M views in the first month. This is not the kind of video that will get 3M views, but I'm glad I got to make it anyway. And that's directly because of support on Patreon.
Ben Eater
2023-07-01 02:43:41 +0000 UTCFooled by the comment! You're right, it looks like another typo in the comments. But Jason is right, it's functionally the same.
Ben Eater
2023-07-01 02:35:21 +0000 UTCGood point, looks like another error in the comments. For the purposes of the code, 74 and 7B are functionally equivalent, as both have bit 6 = 1, and so the BIT command will set V.
Jason Clark
2023-07-01 02:06:55 +0000 UTCAlso agree with your disagreement :>) - not sure what winning YouTube means, but Patreons and subscribers win for sure.
Jack Rubin
2023-07-01 01:55:12 +0000 UTCSuper entertaining and educational! Thanks for the deep dive. :-)
Andres Plaza
2023-07-01 01:46:36 +0000 UTCHow does ASL of BA equal 7B? seems to me it should be 74.
Andy Buck
2023-07-01 01:20:41 +0000 UTCGreat video! In an effort to learn both the 6502 and Swift, my current project is implementing a 6502 emulator in Swift, and I'm using Wozmon as my first "full program" test case. I think your prior video had a lot to do with my decision. Watching you demo and explain Wozmon has already helped me identify a few issues with my implementation of the 6502 and my "Apple1PIA" BusDevice that emulates the Apple I keyboard & screen interface. A couple of notes: * around 23:51, you say that BIT "sets the Minus flag", and write an M on the card. The 6502 docs call this the Negative flag, represented by N. Not a big deal (and perhaps more intuitive, given the BPL, BMI instructions) * In the same section, you mention that the comment seems to be wrong, and you couldn't find this referenced online. Earlier today, I found a similar issue in the ECHO section at the end of Wozmon, where both the code comment and the explanation of the hardward in the text disagree with the code, and I couldn't find any reference to it online. I posted that to the Retrocomputing StackExchange: https://retrocomputing.stackexchange.com/questions/27265/error-in-apple-i-manuals-wozmon-listing-or-hardware-description
Jason Clark
2023-07-01 00:57:31 +0000 UTCI disagree. This is EXACTLY the content that will win YouTube!
bencodestx
2023-06-30 23:31:27 +0000 UTC
