Hardware Reference
In-Depth Information
Type
: several types of semiconductor memories have been historically defined,
the most representative ones being: random-access memories (RAMs), read-only
memories (ROMs), and content-addressable memories (CAMs).
RAMs are memories whose cells can be randomly accessed to perform write
and/or read operations, while ROMs are memories whose cells can be read in-
definitely but written just a limited number of times.
Read-only memories can be further characterized according to the number
of possible write operations and to the way in which these can be performed.
ROMs usually identify memory devices that can be written by the manufacturer,
only once. Programmable ROMs (PROMs) can be programmed by the user just
once, while Erasable PROMs (EPROMs) can be programmed by the user several
times (<10
2
), resorting to ultraviolet light. Electrically EPROMs (EEPROMs)
can be reprogrammed several times (<10
5
) by the user using high voltage sig-
nals. Finally, Flash memories can be easily written several times (<10
5
) without
the need of high voltage signals, even if they can only written page-by-page, and
not on a word-by-word basis.
CAMs represent a completely different type of memory devices that differs
from RAMs and ROMs in the way data are accessed. While both in RAMs and
ROMs the user supplies a memory address and the device either returns the data
stored at that address (read operation) or modifies the content of the addressed lo-
cation (write operation), CAMs are designed in such a way that the user supplies
a data word and the device searches for the data in the memory to check whether
it is stored or not. If the data word is found, the CAM returns a list of one or more
storage addresses where the word was found and, in some implementations, the
data word and/or additional associated information items, as well.
Cell structure
: identifies a set of characteristics of each storing element (cell) of
the memory. Typical values for this dimension, related to RAM memories, allow
us to distinguish between static RAMs (SRAMs) and dynamic RAMs (DRAMs).
In an SRAM a bistable electrical component is used to store a binary value.
When a cell is set into one of its two possible states ('0' or '1'), it stays in it
until the device is powered off. DRAMs cells are instead constructed of sim-
ple capacitive elements that store electrical charges to represent a given logical
level, and are characterized by gradual loss of the stored information, due to a
physical phenomenon known as transistor leakage current (
Roy et al.
2003
)
. As
a consequence, DRAM cells need to be periodically “recharged” (“refreshed”,
“rewritten”) not to loose the stored value. Figure
6.3
shows an example of device
level structural model of: (a) an SRAM cell and (b) a DRAM cell.
Number of access ports
: defines the number of users that can access the memory
concurrently. While a single-port RAM allows one access at a time, multi-port
RAMs, being equipped with several I/O ports, allow multiple read or write oper-
ations to occur at the same time, or nearly at the same time.
Type of users
: identifies the target user of the memory. It is a key point to define
an appropriate memory model. Different users typically resort to different models
