The Serial Peripheral Interface SPI or four-wire serial bus is easy to use. The CDP1802 (MC) is the SPI master, the EEPROM is the slave. There are many different EEPROM types and sizes available e.g. AT25M02 (2Mbit, 256 KiB, $3), 25LC1024 (128 KiB, $2), or 25LC512 (64 KiB, $1.50). All available in DIL8 packages. 64/128/256 KiB seems very small for today's standards where storage is quantified in GiB, but I think it's more than enough for a small Forth system, the size is similar to early floppy disks. If you want more memory there is 16 MiB serial EEPROM W25Q128J from WINBOND, please note that is a 3.3 V device!
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The Serial Peripheral Interface SPI or four-wire serial bus is easy to use. The CDP1802 (MC) is the SPI master, the EEPROM is the slave. There are many different EEPROM types and sizes available e.g. AT25M02 (2Mbit, 256 KiB, $3), 25LC1024 (128 KiB, $2), or 25LC512 (64 KiB, $1.50). All available in DIL8 packages. 64/128/256 KiB seems very small for today's standards where storage is quantified in GiB, but I think it's more than enough for a small Forth system, the size is similar to early floppy disks. If you want more memory there are 16 MiB serial Flash chips e.g. from WINBOND W25Q128J or IS25LP128F, please note they are 3.3 V devices and in SMT!