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ram rom selector

These are example circuits targeted to 8-bits homebrew computers. They are simple circuits to select between ROM and RAM memories when they share partial block of address space, i.e. if you have a big ROM chip, let's say 32K but you won't use it every single cell of it. So you can set a ROM and a RAM chip on the same address space and set your computer to select the proper one.
Let me explain. Let's say that you have 1x 32K ROM chip and 2x 32K RAM chips and you're using a Z80. The Z80 needs ROM on the first page of the address space because at reset it points to $0000 address cell. But your firmware only occupies 16/18 KB of space. So you can build this RAM/ROM selector using simple chips from the 74xx series and let the computer switches between the 2 kinds of memory. In the first one, for example, the CPU reads from ROM chip up to address $4FFF (20,479), then reads/writes from/RAM. In the second one, the CPU addresses the ROM for the first 24K of memory then switches to RAM