Zenith Z-100 Reference Manual page 84

The ninth memory address bit, MA8, connects to pin 1 on each of the RAM
sockets. Since pin I has no internal connection on 6665 or 4164 ICs, MA8 is
only used when 256 kilobit chips are installed.
The heart of the memory card is the memory itself. The supporting circuitry serves
to time and control access to a given memory address, direct the transfer of data,
enable the parity generation, and check drcuitiy on the card containing the memory
block being accessed.
When a spedfic memory location is accessed, every chip in the bank in which
that address resides is affected. For instance, if access to the contents of an address
in bank 3 was desired, one bit from each of eight chips would contribute either
a logical I or a logical 0 to form the 8-bit data byte. If the address was to be
read from, the ninth chip would contribute a parity bit as an error check of that
location. The parity bit is the least-significant bit (LSB) in the byte.
To understand how memory is accessed, you must first understand how the mem-
ory chip itself is structured.
Each dynamic RAM IC is capable of storing 65,536 (64K) I-bit words. In order
to address 64 kilobits with only eight address lines, it is necessary to multiplex
the row address and column address strobe signals.
First, the row address signal is applied. This allows the RAM to latch the eight
least-signiTicant address bits off the system bus. Then, a few hundred nanoseconds
later, the column address signal is applied and the eight most-significant bits (MSB)
are retrieved to obtain the full 16-bit (or 18-bit for 256K chips) address word.
The low order bits also indude A16, and the high order bits, A17, if 256K chips
are installed.
From a functional standpoint, each memory chip can be considered a 256 by
256 bit matrix. The address byte is gated by the row address signal selecting
the row and the column address signal selecting the column. During a read cyde,
when all the bits from the same rows and columns of the eight data chips in
a bank are combined, one data byte is formed.
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System Memofy