The sibling comment explains why we prefer to use the lower bits as a tag (these are guaranteed to be zero if the value is a pointer on a 64-bit system).
Another reason why we wouldn’t want to use the top bit is that, as the parent comment suggested, the tagged pointer representation of a fixnum integer isn’t a pointer at all but is instead twice the number it represents. Generally speaking, we represent integers in twos-complement representation which uses that top bit to determine if the value is positive or negative.
Sadly, only the text editor is on public source control at the moment (https://github.com/jpsheehan/ocaml-edit). The game project and operating system are ongoing until I get bored of them and move on to something else. Most of my obsessions over the last few years have been bus projects. My Tetris clone is another example (C, SDL): https://github.com/jpsheehan/tetris
This is essentially how I work on hobby projects these days. My bus commute is about 45 minutes each way and I find this to be just enough time to get work done. I also try not to work on these projects outside of my commuting hours; this gives me time to mull problems over rather than jumping headlong into writing code.
The lack of internet on the bus has not really been a problem since I plan ahead and make sure any dependencies I need are already downloaded.
I use an old (2010 era) Toshiba netbook which is small enough that I'm not causing problems for my seat neighbours. It's also only got a dual-core 1GHz processor which kind of forces me to find performant solutions to problems.
Much like the author I've also been thinking about how I can make my setup more portable. I've been considering purchasing AR glasses and using my Charachorder2 so I don't even need to get the netbook out of my bag. At this stage I can't justify the cost of a pair of AR glasses though.
Some recent projects my commute has given me the time to work on:
I'm fairly certain the tone of the HTMX post (https://htmx.org/essays/future/) was positive about its future of becoming "stable" (as opposed to "stale").
Another reason why we wouldn’t want to use the top bit is that, as the parent comment suggested, the tagged pointer representation of a fixnum integer isn’t a pointer at all but is instead twice the number it represents. Generally speaking, we represent integers in twos-complement representation which uses that top bit to determine if the value is positive or negative.