THE NONVOLATILE MEMORY BANK
After constructing the two 16K banks described last month, and experimenting with them, I felt the need for at least some permanent (nonvolatile). Loading a full 64K of memory from tape can be tedious, even with SDS or ZXLR8. So I decided to make one of the four 16K banks a nonvolatile block of memory.
There are four types of nonvolatile memory (not including magnetic media — discs, tapes, and bubbles). The first, and most permanent, is ROM (like that 8K IC inside the ZX81 containing the SINCLAIR operating system). The second type is EPROM which has a window to allow it to be erased by exposure to UV light. The EPROM IC is almost always programmed out of the circuit in which it is to be used. The third is E2PROM — electrically erasable and therefore no window; much easier to erase and program in situ. It does suffer from a limited write-erase cycle compared to RAM — you can rewrite into a location only up to about 10000 times although 1 million write erase cycle IC’s will be available soon. Data retention is of the order of 18 years. The fourth and final type of nonvolatile memory is CMOS static RAM with a back-up power supply. The extremely low (less than 1 microamp at 2.2v) allows the use of small nickel-cadmium batteries or lithium cells.