DIGITAL LOGIC DESIGN
(EE-233)
LECTURE 27
1ST – FEBRUARY – 2021 (TUESDAY)
Aftab Alam
Fall 2021 Semester
MICROPORGRAMMED CONTROL
• A control unit with its binary control values
stored as a group of bits (control words) in
memory is called a microprogrammed
control.
• Each word in the control memory contains a
microinstruction that specifies one or more
microoperations for the system.
• A sequence of microinstructions constitutes
a microprogram.
• The microprogram is usually fixed at the
system design time and so is stored in ROM.
MEMORY
• Memory is a major component of Computers and majority other digital systems.
• There are two types of memories:
• ROM – stores the data permanently (a form of PLDs already discussed).
• RAM – stores the data temporarily.
• Static RAM (used as an internal cache and registers)
• Dynamic RAM (used as main memory)
• Memory Read Operation
• copying data from Memory
• Memory Write Operation
• copying data to Memory
•`
SERIAL MEMORY VERSUS RAM
• In serial memory, it takes different lengths of time to access
information, depending on where the desired location is
relative to the current physical position of the disk.
• In RAM, Memory cells can be accessed to transfer information
to or from any desired location, with the access taking the
same time regardless of the location.
MEMORY WORD
• Binary information is stored in memory in groups of bits, each
group of which is called a word.
• A word moves in and out of memory as a unit
• A word may represent
• a number
• a instruction
• one or more alphanumeric characters
• other binary-coded information.
• A group of eight bits is called a byte.
• Word size is normally 8-bits, 16-bits, 32-bits or 64-bit.
MEMORY
• Memory is a major component of Computers and majority other digital systems.
• There are two types of memories:
• ROM – stores the data permanently (a form of PLDs already discussed).
• RAM – stores the data temporarily.
• Static RAM (used as an internal cache and registers)
• Dynamic RAM (used as main memory)
• Memory Read Operation
• copying data from Memory
• Memory Write Operation
• copying data to Memory
•`
SERIAL MEMORY VERSUS RAM
• In serial memory, it takes different lengths of time to access
information, depending on where the desired location is
relative to the current physical position of the disk.
• In RAM, Memory cells can be accessed to transfer information
to or from any desired location, with the access taking the
same time regardless of the location.
MEMORY WORD
• Binary information is stored in memory in groups of bits, each
group of which is called a word.
• A word moves in and out of memory as a unit
• A word may represent
• a number
• a instruction
• one or more alphanumeric characters
• other binary-coded information.
• A group of eight bits is called a byte.
• Word size is normally 8-bits, 16-bits, 32-bits or 64-bit.
RAM BASICS
• The total capacity of a memory unit is stated in Bytes it can store.
• Address lines select the particular location.
• To access words of size 2k, we need k address lines.
• It has n input and n output data lines.
• When Read=1, data word is read from the given address and
when Write=1, input data word is written.
WRITE AND READ OPERATIONS OF RAM
• Write operation is transfer a data word to memory. Steps are
• Apply the binary address of the desired word to the address lines.
• Apply the data bits that must be stored in memory to the data input lines.
• Activate the Write input.
• The memory unit will then take the bits from the data input lines
and store them in the word specified by the address lines.
• Read operation is transfer a data word from the memory. Steps are
• Apply the binary address of the desired word to the address lines.
• Activate the Read input.
• The memory will then take the bits from the word that has been
selected by the address and apply them to the data output lines.
PROCESSOR
AND MEMORY
• Memory READ operation
• Processor places required address on the
address bus.
• Processor sends READ control signal to
the memory.
• Data at that address is placed on the bus
and hence sent to the processor.
• Memory WRITE operation
• Processor places the address on the
address bus.
• Processor places the data on the data bus.
• Processor sends WRITE control signal to
the memory.
• Memory writes that data on its address.
MEMORY WRITE CYCLE
MEMORY READ CYCLE
PROPERTIES OF MEMORY
• IC RAM is volatile and may either be static or dynamic.
• SRAM consists of internal latches that store the binary information.
• DRAM stores the binary information in the form of electric charges
on capacitors which are accessed inside the chip by n-channel MOS
transistors.
• In DRAM, stored charge on the capacitors tends to discharge with
time, and therefore refreshing the DRAM is required.
• Refreshing is done by cycling through the words every few milliseconds,
reading and rewriting them to restore the decaying charge.
PROPERTIES OF MEMORY
• DRAM offers reduced power consumption.
• DRAM has high density compared to SRAM
• larger storage capacity in a single chip.
• SRAM is easier to use.
• SRAM is FAST
• Has shorter read and write cycles.
• No refresh is required for SRAM.
SRAM IC
• Memory consists of RAM chips plus additional logic.
• The internal structure of a RAM chip of m words with n
bits per word consists of an array of mn binary storage
cells and associated circuitry.
• The circuitry is made up of
• Decoders to select the word to be read or written
• Read circuits
• Write circuits
• Output logic
LOGIC MODEL OF THE SRAM CELL
• The RAM cell is the basic binary storage cell used in the RAM
chip, which is typically designed as an electronic circuit rather
than a logic circuit.
• Select enables the cell for reading or writing.
RAM BIT
SLICE MODEL
16-WORD
BY 1-BIT
RAM
DIAGRAM
OF A 16 X 1
RAM USING
A4X4
RAM CELL
ARRAY
DIAGRAM OF
A 8 X 2 RAM
USING A 4 X 4
RAM CELL
ARRAY
A SINGLE RAM IC
ARRAY OF RAM
ICS
ARRAY OF RAM ICS