at the time Z80 was popular ('87 ish) there were few/no tools outside of Hisoft Devpac (http://www.secarica.ro/images/zx/genp351.gif) neither was there any internet / forums etc. to learn how others were solving things - like that bit rotation example

when I found out people were moving the stack pointer register to the screen memory base address and using stack pushes (instead of mov) to move data - to save a clock cycle or two each byte move - it blew my mind - hacks like that would be common knowledge in minutes now

> when I found out people were moving the stack pointer register to the screen memory base address and using stack pushes (instead of mov) to move data - to save a clock cycle or two each byte move ...

So the user might occasionally see interrupt stack frame on screen?

memory is hazy, but using SP to move the memory was fast enough you could blit most (not all) of a screen in one refresh cycle - which is, I think, why games like Uridium (https://www.youtube.com/watch?v=TvULd4zHz8Y) were running the main gameplay in a smaller area

also, on that Uridium vid - notice how they are using the standard font but spacing it out vertically (so they could save memory)

Also no level destruction scene. Intra-level minigame was also completely missing.

Leaving those out must have saved some memory too.

There were lots of magazines and books though - eg the works of R.A.Penfold.