I know you can use different parts of the 8 KB ROM in Sinclair ZX81 as character ROM by changing the I register, but it seems one cannot change the I register to point to RAM [and have character mode]. I think the problem is that the RAM then gets overwritten, but I might be wrong [I was wrong]. I know you can get some crude high resolution graphics by using different parts of the ROM, and that you can modify the RAM-pack to get true HR graphics. But the most obvious way to get user defined characters i.e. by having the character pictures in RAM seems to be impossible, because otherwise it would have been used much back in the day. What prevents the official Sinclair 16 K RAM pack from being used as the source for the characters that are used for the display?
I have 56 K SPECIAL RAMPACK by Audio Computers. Can some part of that RAM be used to store User-Defined Graphics (UDG) characters without modifying the hardware?