> which had 32kB of RAM, a huge chunk of which was required for the screenPeople assume 10KB, with the BBC Micro¹² version's version using a mix of modes 4 & 5 (320×256×1bbp and 160×256×2bbp respectively), but as well as the split screen trickery they tweaked the graphics hardware in more detail than that to create custom modes. There were only 248 scanlines and either 256 or 128 pixels per line, so the display took 7.75KB. As well as leaving more memory for the rest of the code, this reduced the amount being drawn, so helped keep the draw speed acceptable.
This left about 24KB⁴ for the rest of the code and its working data. Still a very impressive feat!
> - managed to have eight galaxies with 256 distinct planets each.
IIRC it was originally going to be larger than that, they toned it down a bit but not due to technical limitations (the galaxy generator code would not have needed to be any larger).
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[1] The Electron lacked support that allowed the split screen to work (one of the chips removed meant the required timing signal wasn't available), so it was all monochrome though still with the custom resolution and taking 7.75KB.
[2] The version enhanced for the Master series use a mix of mode 1 and mode 2 to enable more colours, 2bbp for the view of space and 4bbp³ for the dashboard. This still used the custom resolution, so took 15.5KB rather than 20KB. This was achieved by using the shadow RAM bank to host the screen display (some programs instead used this feature to implement double-buffering for smoother screen drawing), allowing the screen to take twice as much space while also freeing up memory to hold things that would have been loaded from disc when needed⁵ in the Micro version.
[3] Actually effectively 3bbp as the BBC hardware did 8 colours and flashing variants instead of 16 colours (which most other systems used to provide two shades of 8 colours, using the 4th bit as an intensity flag).
[4] less a few chunks here & there needed by code in system ROMs when doing things like reading extra data from the floppy
[5] There were two chunks of code, one for in-flight and one for while docked, that were swapped in as you entered or left a space station. On the Micro these were pulled from disk each time, on the Master they could both be in RAM. If playing from cassette you had a cut-down version of the game that didn't need this split.