The Sinclair ZX 81 was a tiny machine with a membrane Qwerty keyboard. I developed an adventure game in basic in 1980. We had a memory extension of 16 KB (it was hyper high at the time) but that had a tendency to eject itself when the computer heated, it was taped but the tape did not resist heat ... So we stuck with a big dico.

The memory of this machine was so weak at the base, that during backup and loading, the OS used the screen memory as transfer, so we saw cabalistic signs fill the screen.

At the time, be aware that computers were not powerful enough to work in graphics mode all the time. The default mode was a text mode where each byte of the video card represented a character.
One could go into pseudo graphic mode by reprogramming via the video card the appearance of these graphics (Commodore VIc 20). So, it made it possible to do small games fast enough.

The first to use the graphic mode, by default and more text mode were the Thomson Mo5, this was only one of their many lamentable mistakes ...