through the sense/digit line generating a magnetic field
that is perpendicular to the transverse field. The
direction of the current pulse generated up and down
this line and the resulting longitudinal field are
determined by whether a zero or a one is to be written.
Frame B of figure 6-23 illustrates the bit steering to
store a zero. For a write operation the films magnetic
vector is first put in the hard direction from the
transverse field. A small longitudinal field is then
applied bypassing a current through the sense/digit line
in the proper direction. This longitudinal field steers
the vector toward the zero state. The word current is
then removed, which further accelerates the
magnetization towards the zero state. Then the digit
current itself is removed leaving the film in the zero
state. Frame C of figure 6-23 shows that when a one is
to be stored, the bit current in the sense/digit line is
reversed indirection from that used to store a zero. The
resulting longitudinal field now steers the vector to the
one state.
READ/WRITE CYCLE CIRCUITS. Three
very important circuits used in mated film memories
during the read/write (restore) cycles are as follows:
. Word current generator The word current
generator produces the current pulse that provides the
switching field for the memory film elements. The
current generator is also used to produce drive pulses
used for strobing during the memory cycle.
l Digit drivers The digit drivers supply pulses
required to write or restore data during the write/restore
portion of all full memory cycles. Input to the drivers
is supplied by the output of the data registers flip-flops.
With these signals, the digit drivers are enabled to
generate drive pulses, which write a logic high or a logic
low in the address bit location in accordance with the
binary (one or zero) contents of the data register. The
binary value determines the direction of the digit current
on the sense/digit line.
l Sense amplifiers similar to core memory, the
sense amplifiers sense the state of the data contained in
the film element for storage in a data register for
transmission of the data word or restoration of a one or
a zero. Selected bits in the address register determine
which group of bits are transmitted as a data word or
changed by a memory write cycle.
SEMICONDUCTOR MEMORY
Semiconductor random access memory, or RAM,
as it is often referred to, is used in all types of computers.
RAM is also called a read/write memory or a
scratch-pad memory. Semiconductor RAM refers to
semiconductor IC memories that can be used in a read
mode as well as a write mode. Semiconductor
memories use either a read cycle or a write cycle
depending on the type of request, independent of each
other. The read cycle is normally a shorter time period
than the write cycle.
Semiconductor memories are normally non-
destructive readout and volatile memories. In a
nondestructive readout memory, the data stored in
memory is not destroyed by the procedure used to read
the data from the memory cells. Volatile memories
require electrical power to maintain storage. If the
power goes away for some reason, the data stored
in the memory cells is lost. For this reason, an
uninterruptable power supply (UPS) and a battery
backup system are used in many semiconductor
memory applications to maintain constant power and
prevent loss of information because of power
fluctuations or failures. This is especially important in
microcomputers where configuration data is
maintained in special devices such as a complementary
metal-oxide semiconductor (CMOS). The battery
backup and a filter capacitor provide the required power
when the microcomputer has been powered down.
Computers that use an UPS system have an established
time in which data will be retained for momentary
power losses.
The term random access memory (RAM) is con-
sistently used for read/write devices. Although RAM
only describes one characteristic of read/write devices,
it is used and understood by most people to mean read/
write devices. RAM means random addresses can be
presented to the memory which means data can be written
and read in any desired order from any location. Note:
The term RAM is not used for read-only memories
(ROM), although a ROM can also be random access.
Lets explore the basic building block of
semiconductor memories: the RAM chip. Then we
discuss the two main types of semiconductor RAM
memories: static RAM (SRAM) and dynamic (DRAM),
and variations of the two. SRAMS are faster but require
more logic than DRAMS; thus they are more expensive
than DRAMS.
The RAM Chip
In semiconductor memories, the basic building
block is the RAM chip (fig. 6-24). This is true whether
the memories are static or dynamic memories and are
pcbs in a memory module or a pcb or pcbs mounted
singularly. The semiconductor RAM itself is made up
6-20