One of the fundamental items for a microprocessor/microcontroller system is the ability to have read/write memory storage.  After carefully reading many of the datasheets for original 4-bit systems, it was apparently that many used either a latch or register to control the memory address access to the memory. In my design I've used two 74HC173D which are 4-bit wide. One is used for the upper 4-bits of the memory address, and another for the lower 4-bits. The memory itself is a 32K x 8 parallel SRAM chip (IS62C256AL). This simple module is the basis of the read/write memory for these experiments.