Archive for April 2012
SECONDARY MEMORY: The magnetic memory is used as secondary and backup memory.
It is employed for bulk storage of programs, data and other information. It has
much larger capacity than main memory. It stores system software, assemblers,
compilers, useful packages, large data files etc. the secondary memory should
not be of volatile nature. It must store information permanently. The magnetic
memories such as hard disks are the most common secondary memories used in
computers.
HARD DISK
§ Hard disks are o n-line storage device. The term on-line means that the
device is permanently connected to the computer system.
§ Hard disk is made of aluminum (or other metal or metal
alloy) with a thin coating of magnetic material (iron oxide) over it.
§ Standard size
of hard disk is 3.5 inch. The 2.5 inch disks are used in portable computers.
Floppy
disks are made of Mylar (a plastic material) coated with magnetic material
(iron oxide or barium ferrite). The disk is not a hard plate, rather it is very
thin piece of flexible plastic, and hence, it is called Floppy disk. Floppy
disks are also called diskette. They are removal disks. The size of the floppy
disk is 3.5 inch diameter. A floppy disk rotates at 360 rpm. Its average access
time is 150-250 ms.
OPTICAL DISKS
Optical
disks are used for backup memory. Information is written to or read from an
optical disk using laser beam. It has very high storing capacity as compared to
magnetic floppy disks. An optical disk is a direct access device. It can be randomly
accessed for music, video, a file or any other information.
There
are following types of optical disks:
(i) CD (Compact disk) or CD-ROM
(ii) CD-RAM (CD Recordable)
(iii) CD-RW (CD-Read/Write)
(iv) DVD (Digital Versatile Disk)-ROM
(v) DVD-R
(vi) DVD-RW
ROM
§ ROM stands for Read only memory.
§ It is nonvolatile memory, i.e., the information stored
in it is not lost even if the power supply goes off.
§ It is used for permanent storage of information.
§ It also possesses random access property.
§ ROMs are much cheaper compared to RAMs when produced
in large volumes.
§ The stored information can only be read from ROMs at
the time of operation. Information cannot be written into a ROM by the
users/programmer. In other words ROMs are not accessible to users.
Mask-programmed ROMs- ROMs in which contents are written at the time of IC
manufacture are called mask-programmed ROM.
User-programmable ROMs- PROM, EPROM, E2PROM or any other kind of
PROM are user-programmable ROMs.
PROM:
§ PROM is a programmable ROM.
§ It contents are decided by user. The user can store
permanent programs, data or any other kind of information in a PROM.
§ PROMs are once programmable.
§ PROMs are more cost effective.
§ Example of PROM is 74S287.
EPROM:
§ An EPROM is an erasable PROM.
§ The stored data in EPROMs can be erased by exposing it
to high intensity short wave ultraviolet light for about 20 minutes.
§ EPROMs are cheap, reliable.
§ EPROMs employ MOS technology.
§ Examples of EPROMs are: Intel’s 87C257.
EEPROM (E2PROM):
§ EEPROM is an electrically erasable PROM.
§ Also known as EAPROM (Electrically Alterable PROM).
§ There is a limit on the number of times the EEPROMs
can be reprogrammed, usually 10,000 times. It not need be removed from the
computer’s board for erasing.
§ Examples of EEPROMs are: Intel 2816, Intel 2816A.
RAM
The read and write memory (R/W) memory
of a computer is called RAM. The users can write information into RAM and read
information from it.
- It possesses random access property. In a random access memory any memory location can be accessed in a random manner without going through any other memory location. The access time is same for each memory location.
- RAM is a volatile memory. The information written into it is retained in it as long as the power supply is on. As soon as the power supply goes off its stored information is lost. The programmer has to reload his programs and data into the RAM when the power supply is resumed.
There are two types of RAM
Static RAM:
§
It is also known as SRAM.
§
Static RAMs retain stored information only as long as the power supply is
on.
§
Static RAMs are costlier.
§
It consumes more power.
§
They do not need refreshing circuitry.
§
They have higher speed than dynamic RAMs.
§
Static RAM being faster is used in cache memory.
§
Static RAM holds information in a flip-flop circuit consisting of two
cross-coupled inverters. Six transistors are needed per memory cell in a static
RAM.
Dynamic RAM
§
It is also known as DRAM.
§
Dynamic RAM loses its stored information in a very short time even though
the power supply is on.
§
Dynamic RAMs have to be refreshed periodically, generally every 2
milliseconds.
§
The dynamic RAMs are cheaper and have high packing density and moderate
speed.
§
They consume less power.
§
They are used where large capacity of memory is needed.
§
Dynamic RAM being cheaper is used for main memory.
§
Dynamic RAMs require less number of transistors per memory cell because
information is stored on stray capacitors. Only one transistor is needed to
form a memory cell of the dynamic RAM.
PRIMARY MEMORY: It stores programs and data which are
currently needed by the CPU. The semiconductor memory is employed as the main
memory of the computer. As the CPU employs semiconductor technology and has
very high speed, its matching memory must be very fast. Only semiconductor
devices can provide the matching speed. Thus the main memory also must employ
semiconductor technology. RAM and ROM IC’s are used as the main memory of the
computer.
CLASSIFICATION OF MEMORY
Status code for Intel 8085
RD (OUTPUT): It is a control signal sent by the microprocessor
to control read operation. The selected memory or I/O device is read when RD is
low.
WR (OUTPUT): It is a control signal issued by the microprocessor
to control write operation. The data on the data bus are written into the selected
memory or I/O device when WR is low.
HOLD (INPUT): When an external device wants to use address
and data bus it sends HOLD signal to the microprocessor. When microprocessor
receives HOLD signal it completes its current instruction at hand, and then
relinquishes control of b-uses to allow external device to use them.
HLDA (OUTPUT): It is a HOLD acknowledgement signal. It
is sent by the microprocessor to the external device to indicate that the HOLD
request has been received. On the completion of data transfer, the external
device removes the HOLD request. The HLDA goes low after the removal of HOLD
request.
INTR (INPUT): It is an interrupt signal of the lowest
priority.
INTA (OUTPUT): It is an interrupt acknowledgement
signal. It is issued by the microprocessor after INTR is received.
RST 5.5, 6.5, 7.5, TRAP (INPUT): These are
interrupts. The TRAP is nonmaskable and has the highest priority. Others are
maskable interrupts. The order of priority is TRAP, RST 7.5, RST 6.5, and RST
5.5.
RESET IN (INPUT): When this signal is applied the CPU is
brought to the reset condition. The contents of the program counter become
zero.
RESET OUT (OUTPUT): This signal indicates that the CPU is
being reset.
READY (INPUT): It is an input signal to the
microprocessor. It is sent by a peripheral to indicate whether it is ready to
tran-sfer data or not. The microprocessor examines READY signal before data are
transferred. If READY is high it shows that peripherals are ready to transfer
data. If READY is low the microprocessor waits till READY becomes high. The
status of READY is examined in the second clock cycle of a machine cycle.
X1 and X2 (INPUT): An external crystal oscillator is
connected through these terminals to supply clock for the microprocess-or. A
crystal oscillator drives an internal circuitry which is within the
microprocessor to produce suitable clock for the operation of the
microprocessor.
CLK (OUTPUT): The clock is also required by some
other ICs
Of the computer, Hence CLK is a clock
output from the micro-processor, which can be utilized for the operation of
other ICs.
SID (INPUT): It is an input line for serial data. The data rece-ived from this are loaded into
the 7th bit of the accumulator when RIM instruction is executed.
SOD (OUTPUT): It is an output line for serial data.
The 7th bit of the accumulator is sent through this line when SIM instruction
is executed.
PIN CONFIGURATION OF INTEL 8085
A8-A15
(OUTPUT): These are address lines. They form the address bus. They carry 8 MSBs of
memory address or 8 bits of I/O address.
AD0-AD7
(INPUT/OUTPUT): These are address/data lines. They form
time-multiplexed address/data bus, i.e., they serve dual purpose. They carry 8
LSBs of the memory address or I/O ad-dress during the first clock cycle of a machine
cycle. Again they are used to carry data during the second and third clock
cycle.
ALE
(OUTPUT): It is an address latch enable signal. It goes high during the first
clock cycle of a machine cycle and enables 8 LSBs of address to get latched
into the on-chip latch of peripherals.
IO/M: It is a status
signal to indicate whether the address sent by the microprocessor is for a
memory or an I/O device. When it is high the address on the address bus is for
I/O device and when it is low the address on the address bus is for the memory.
S0 and S1: These are
status signals issued by the microprocessor to identify the various type of
operation.
Stack
Pointer (SP)
The stack pointer is also a 16-bit register used as a memory
pointer. It points to a memory location in R/W memory, called the stack. The
beginning of the stack is defined by loading 16-bit address in the stack
pointer. This programming model will be used in subsequent tutorials to examine
how these registers are affected after the execution of an instruction.
Program
Counter (PC)
This 16-bit register deals with sequencing the execution of
instructions. This register is a memory pointer. Memory locations have 16-bit
addresses, and that is why this is a 16-bit register. The microprocessor uses
this register to sequence the execution of the instructions. The function of
the program counter is to point to the memory address from which the next byte
is to be fetched. When a byte (machine code) is being fetched, the program
counter is incremented by one to point to the next memory location.
Status Register (or Flag register). A flag register contains a number of flags either to indicate certain conditions arising after arithmetic and logical operations or to control certain operations. The flags which indicate conditions are known as condition flags. The flags which are used to control certain operations are called control flags. A flag is a flip-flop which is set or reset by the processor to indicate certain conditions or set/reset by the programmer to control certain operations.
Intel 8085 contains the following condition flags:
§ Carry flag - it indicates whether there is carry or not after an arithmetic and logical operation.
§ Zero flag - it indicates whether the result of an arithmetic or logical operation is zero or nonzero.
§ Sign flag - it indicates whether the result is positive or negative,
§ Parity flag- it indicates whether the result contains odd number of 1s or even number of 1s.
§ Auxiliary carry (or half carry), that is carry from the 3rd bit to 4th is also indicated.
Accumulator
The accumulator is an 8-bit register that is a part of arithmetic/logic unit (ALU). This register is used to store 8-bit data and to perform arithmetic and logical operations. The result of an operation is stored in the accumulator. The accumulator is also identified as register A.
Registers
§ Six general purpose 8-bit registers: B, C, D, E, H, L
§ They can also be combined as register pairs to Perform 16-bit operations: BC, DE, HL
§ Registers are programmable (data load, move, etc.)
Control Bus
§ Consists of various lines carrying the control signals such as read / write enable, flag bits.
§ The Control Bus carries control signals partly unidirectional, partly bi-directional.
§ Controls signals are things like "read or write".This tells memory that we are either Reading from a location, specified on the address bus, or writing to a location Specified. Various other signals to control and co-ordinate the operation of the system.
Address Bus
§ Consists of 16 address lines: A0 – A15.
§ Operates in unidirectional mode: The address bits are always sent from the MPU to peripheral devices, not reverse.
§ 16 address lines are capable of addressing a total of 216 = 65,536. (64k) memory locations.
§ Address locations: 0000 (hex) – FFFF (hex).
§ Data range: 00 (hex) – FF (hex).
Data Bus
§ Consists of 8 data lines: D0 – D7.
§ Operates in bi-directional mode: The data bits are sent from the microprocessor unit to peripheral devices, as well as from the peripheral devices to the microprocessor unit.
A microprocessor is a programmable digital electronic circuit that incorporates the functions of a central processing unit on a single semiconducting integrated circuit.
Microprocessor (Intel 8085) architecture and its operation
The internal architecture of the 8085 CPU is Capable of performing the following operations:
§ Store 8-bit data (Registers, Accumulator).
§ Perform arithmetic and logic operations (ALU).
§ Test for conditions (IF / THEN).
§ Sequence the execution of instructions.
§ Store temporary data in RAM during execution.
Operation with bus
There are three types of microprocessor bus:
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