Generate the 64 GCR values used by Apple DOS 3.3 and ProDos
for i in range(128):
b = f"{i:07b}" # All these tests apply to the low 7 bits
if '000' in b: continue # 3 consecutive zeros disallowed
if b.count('00') > 1: continue # only one pair of consecutive 0s allowed
if not '11' in b: continue # one pair of consecutive 1s required
print(hex(i | 128), bin(i | 128)) # top bit is always a 1
An interesting finding about Apple DOS 3.3 diskettes
I can't be the first to have noticed this, but if somebody wrote about it years ago there's no way a search engine today will turn it up since they over-value recency and pay little attention to little sites with niche information. So here's my write-up. It feels a little inconclusive, but so be it. I imagine that these disks were mastered in an unsual way, rather than by duplicating a disk written on an Apple II using normal Apple II RWTS code.
I've been working on an open source tool to convert .a2r to .woz. While doing so, I got pretty low level with the Apple flux encoding.
This encoding varied over the years, and of course what really matters is what would work on real machines in the wild.
But one central claim about Apple floppy formats is that early DOS before 3.3 used the repeated bit sequence "1111 1111 0" to allow the floppy interace's state machine to synchronize with the stream of bits on the floppy. Starting with DOS 3.3 and continuing with ProDOS, and ProDOS, the bit sequence "1111 1111 00" is used instead. The newer sequence (also called FF40, because its hex value is FF and it takes 10 bits or 40 microseconds to transmit; the old code is also called FF36) is longer, but fewer repetitions are required to guarantee synchronization.
So I was surprised to see the following when I peered into "DOS 3.3 System Master [1983] - Disk 1, Side A.a2r" from CowgodA2R on archive.org (as are all the a2r files I inspected for this blog post):
111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 111111110 FF36 11111111 FF 11010101 D5 10101010 AA 10010110 96 11111111 FF 11111110 FE 10101010 AA 10101010 AA 10101111 AF 10101010 AA 11111010 FA 11111110 FE 11011110 DE 10101010 AA
That's right, the FF36 sequence appears in this DOS 3.3 floppy from 1983. Essentially the same sequence is seen on ProDOS User's Disk - Disk 1, Side A.a2r.
An even odder sequence is seen on the 1980 edition of DOS 3.3:
1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 111111110 FF36 1111111100 FF40 11111111 FF 11111111 FF 11001111 CF 11110011 F3 11111100 FC 11111111 FF 11010101 D5 10101010 AA 10010110 96 11111111 FF 11111110 FE 10101010 AA 10101010 AA 10101011 AB 10101110 AE 11111110 FE 11111010 FA 11011110 DE 10101010 AA
The CF/F3/FC/FF sequence would be an FF40 sequence if the earlier FF bytes hadn't already left you synchronized, but .. there's absolutely no reason for it!
Finally, a disk from the "non-originals" section, which I assume means it's cracked software, has the sequence I expect (more or less) based on reading documentation from the time as well as modern emulator source code:
1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 1111111100 FF40 11111111 FF 11010101 D5 10101010 AA 10010110 96 11111111 FF 11111110 FE 10101010 AA 10101010 AA 10101010 AA 10101011 AB 11111111 FF 11111111 FF 11111111 FF 11111111 FF 11101000 E8 10111111 BF 11111100 FC
An interesting Disk ][ Interface Card Fault
tl;dr: I recently got into 8-bit Apple retrocomputing due to my work with Adafruit. I had a Disk II setup that couldn't write to any floppy. I fixed it by replacing a 74LS05 IC on the Disk II Interface Card.
Stuff I did in 2021...
Here are some of the things that came up in my inventory of last year's deeds:
- 25 posts on this blog
- 21 posts on the Adafruit blog
- 11 guides on the Adafruit learn system
- 2301 contributions on github
- 6 3D printed designs shared with the world
- 3 circuit sculptures
- 1 PCB design (& got much more comfortable with kicad)
- Got comfortable using FreeCAD
- Fave Projects: CircuitPython WWVB Clock & WWVB Observatory
- 6+ inter-state trips
Where should CircuitPython go in 2022?
Looking back at 2021
Some of the goals I proposed for 2021 have been accomplished, yay!
- Support at least one new microcontroller family (I may have known the RP2040 was coming when I said that)
- Partially addressed the problem of the time required to build CircuitPython core with changes to GitHub CI
- circup and bundlefly greatly ease the problem of gathering items from the bundles for a particular application
- In the Adafruit Learn System, we got much more systematic about how programs and their assets are organized
- The bluetooth workflow is in the wild and people are using it, such as with the PyLeap app for Apple phones.
At a personal level, I accomplished one of the goals I listed, which was to improve my skills at 3D modeling and in particular I have become reasonably competent at part design using FreeCAD.
Some items we made progress on, but there's always room for further improvement:
- Continue to help people grow into the roles of reviewer and contributor
- JP's CircuitPython Parsec and Scott and FoamyGuy's livestreams are great, but I'm sure there's room for more "how to" video content around CircuitPython.
Some big pieces of progress we made that I didn't even anticipate (but wish I had) were
- Getting up to date with MicroPython, and then continuing to merge in their new releases
- Releasing asyncio support for CircuitPython
- Adding typing stubs to the core and types to many libraries in the bundle to work better in advanced IDEs
Looking forward to 2022
I'm not a big picture visionary about where the project should head. Lots of times it's modest or even invisible changes that give me the most satisfaction.
I look forward to implementing new drivers for hardware I wouldn't have dreamed of working on (reading from old DOS floppy drives, perfect example), adapting old algorithms into CircuitPython (let's do more image processing or maybe start doing audio processing), and finding small efficiency improvements and firmware size savings when they're needed.
I want to both learn more about and improve asyncio. A personal project goal would be to have an interactive display that continues to update & respond to input even while doing wifi requests. I don't think this works right now (I could be wrong), and I don't know how much is needed in order to make it work.
I look forward to hearing all about the projects you (yes, you) are planning to do in 2022; let's figure out if CircuitPython is the right coding language for those projects, and if it's not let's figure out if there are sensible additions that will make it work.
The Most Surprising Optimization I've Seen Lately
It seems that under some conditions, gcc (such as gcc 10.1 targeting amd64 linux with -O optimization) compiles the following bad C++ code into non-leaking code. In fact, the generated code is equivalent to return 3; without any call to an allocating function:
class T {
public:
T(int i) : i(i) {}
int i;
};
int should_leak_memory() { return (new T(3))->i;}
80columns
I've gathered up my set of improvements to mist64's 80columns program for Commdore 64 into the main branch of my fork.
Estimating WWVB Signal Health
Over in CWWVB, I've added a health estimate of the WWVB signal.
I define the health of the signal over one symbol as the number of samples that differ from the ideal for that symbol. The health of the signal is simply the health over a larger number of symbols. In the case of CWWVB, I've chosen to track 60 symbols (one minute) by default.
The signal health percentage is just the long-term health number divided by the total number of samples.
I passed a substantial number of logs from the WWVB Observatory to the standalone CWWVB decoder program, printing the health of each successfully decoded minute (the minutes were not checked for correctness against the known time, just for passing the basic tests: fixed mark and 0 symbols present, no BCD decode errors). Only 0.03% of minutes with health below 85% successfully decoded. I ended up picking 97% as the health cut-off number for CWWVB.
Use printf() in Arduino programs
CWWVB: Putting what I've learned about WWVB to use in a new decoder
WWVB Observatory
My experience adding type annotations to a 2.5k-line Python library
Using Adafruit Macropad as LinuxCNC Control Pendant
OBS "insert current timestamp" hotkey
Collatz-bolge: A non-universal language, unless the Collatz Conjecture is false
Hybrid Keyboard Descriptor supports Boot & NKRO
All older entries
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