Memory Cycle
Main memory is a read/write memory that allows
data to be retrieved (read) and stored (written) in what
is known as the memory cycle. The memory cycle
includes reading the data out of memory and/or writing
the data into memory, either by a read/write operation
or by separate read and write operations. The memory
cycle is based on fixed (constant) time periods for
reading and/or writing data from and into memory. As
soon as read and/or write operations are initiated,
almost simultaneously, address translation occurs,
then the read and/or write cycle or cycles begin.
MEMORY ADDRESS TRANSLATION.— One
of the most important processes that must occur before
a read or write cycle begins is the memory address
translation. Look at memory as a sequence of memory
locations starting at address 0 and extending to the
maximum memory address available to the requester
(CPU or IO(C)). Receiving or sending mail uses a
similar concept. Before mail can be received or sent,
there must bean address on the envelope. Memory uses
the same principle.
Memory logic identifies the
memory address where a memory word is to be read
from or written into in memory. A memory address
can be anyone of the entire range of memory addresses
(0 to maximum). To identify the desired memory
address, the memory logic uses a register designated as
the address register and/or translators or decoders. The
memory logic receives the logical address from the
CPU or I/O and temporarily stores it in the address
register, and then converts it to a physical address that
can be read from or written into.
Memory Address Register and/or Translator
(Decoder).— The address register and/or translator
identifies the exact location from which to read the bits
or write them. The contents of the address register or
translator identifies the memory address. The memory
logic is designed to make its selection based on the type
of memory it uses.
It can be designed to identify a
memory address of a single memory pcb or it can be
designed to identify an address located in one of four or
more memory modules.
Memory Address Word.— As stated in the
architecture of memory, the word contained in the
memory address can be one or more bits, most
computers have words with at least 8 bits and some up
to 128 bits in length. Variations of reading and/or
writing from and to memory can include the upper or
lower half of the word, or any other variation within the
design of a given computer. Variations are based on the
instruction types and the program. Also, if a computer
is identified as an 8-bit computer and a 16-bit word is
required for a read or write operation, then two
consecutive memory addresses would have to be used
to complete the operation. There are many other
variations; the instruction repertoire set of your
computer and the technical manual will provide details
of your computer’s memory operations and limitations.
Here are two examples of memory address
translation. For the first example, use figure 6-6 as a
reference; it shows a 4-bit memory address. The
memory address register or translator contains 168 as
follows:
Figure 6-6.—4-bit address.
6-6
