Defending against page-cache attacks

I had a wobbly RAM pack with an extra flaw: the PSU on my ZX81 was underspec so it didn't generate quite enough power to power the RAM and screen at once. The video signal generation was the first thing to go: you got waves of sync problems like a bad VHS video player working their way over the screen. But it didn't take long for eight-year-old me to figure out that the RAM wasn't holding its content either...

(Obviously I couldn't fix it. An eight year old with terrible coordination go messing in a power supply?! HELL NO.)