MEMORY LOCATIONS AND ADDRESSES
In computing, a memory address is a reference to a specific memory location used at
various levels by software and hardware.
Memory addresses are fixed-length sequences of digits conventionally displayed and
manipulated as unsigned integers.
Memory
Holds both instructions and data.
A machine instruction
Byte 3 bytes
opcode address information
Types of memory addresses
Physical addresses
A digital computer’s main memory consists of many memory locations. Each memory
location has a physical address which is a code. The CPU (or other device) can use the
code to access the corresponding memory location.
In computing, a physical address (also real address, or binary address), is a
memory address that is represented in the form of a binary number on the address bus
circuit in order to enable the data bus to access a particular storage cell of main memory,
or a register of memory mapped I/O device.
Logical address
An address generated by the CPU is a logical address whereas address actually available
on memory unit is a physical address. Logical address is also known a Virtual address
• the address generated by the CPU is called as Logical Address.
INSTRUCTION CYCLE
An instruction cycle, also known as fetch-decode-execute cycle is the basic
operational process of a computer. This process is repeated continuously by CPU
from boot up to shut down of computer.
Following are the steps that occur during an instruction cycle:
1. Fetch the Instruction
The instruction is fetched from memory address that is stored in PC(Program
Counter) and stored in the instruction register IR. At the end of the fetch operation,
PC is incremented by 1 and it then points to the next instruction to be executed.
2. Decode the Instruction
The instruction in the IR is executed by the decoder.
3. Read the Effective Address
If the instruction has an indirect address, the effective address is read from the
memory. Otherwise operands are directly read in case of immediate operand
instruction.
4. Execute the Instruction
The Control Unit passes the information in the form of control signals to the
functional unit of CPU. The result generated is stored in main memory or sent to an
output device.
The cycle is then repeated by fetching the next instruction. Thus in this way the
instruction cycle is repeated continuously.
TIMING AND CONTROL
All sequential circuits in the Basic Computer CPU are controlled by a master clock,
clock pulse are applied to flip-flops and registers. Clock pulse does not change the state
of the register unless a control signal is generated by the control unit.
Control unit design and implementation can be done by two general methods:
Hardwired Control
• Control logic is implemented using gates, flip-flops, decoders and other digital
circuits.
• Optimized to produce fast mode of operation.
• Requires changes in wiring among various components if the design has to be
modified.
Microprogrammed Control
• Control information is stored in control memory
• Control memory is programmed to initiate the required sequence of micro
operations.
• Required change can be done by updating the microprogram in the control memory
Bus Organization
Bus is a subsystem that is used to transfer data and other information between devices.
Means various devices in computer like(Memory, CPU, I/O and Other) are
communicate with each other through buses. In general, a bus is said to be as the
communication pathway connecting two or more devices.
A key characteristics of a bus is that it is a shared transmission medium ,as multiple
devices are attached to a bus.
Data Lines:
Data Lines provide a path for moving data between system modules. It is
bidirectional which means data lines are used to transfer data in both directions.
Address Lines:
Address Lines are collectively called as address bus, On these lines, CPU sends out the
address of memory location on I/O Port that is to be written on or read from. In
short, it is an internal channel from CPU to Memory across which the address of
data(not data) are transmitted.
Control Lines
Control Lines are collectively called as Control Bus. Control Lines are gateway used
to transmit and receives control signals between the microprocessor and various
devices attached to it. In other words, Control Lines are used by CPUs for
Communicating with other devices within the computer.
Types Of Bus
System Bus:
A Bus that connects major computer components (Processor, Memory, I/O)
is called a System Bus.
Peripheral Bus(I/O Bus /External Bus):
Peripheral Bus also known as "I/O Bus". It is data pathway that connects
peripheral devices to the CPU.
PCI(Peripheral Component Interconnect):
PCI Bus connects the CPU and expansion boards such as modem cards ,network cards
and sound cards.
Local Bus:
Local Bus are the traditional I/O(Peripheral) buses such as ISA,MCA, or EISA
Buses.
ISA(Industry Standard Architecture Bus):
The ISA Bus permit bus mastering i.e., it enabled peripheral connected directly to the
bus to communicate directly with other peripherals without going through the
processor
EISA(Extended Industry Standard Architecture): The EISA Bus use connectors
that were same size as the ISA connectors but with 4 rows of contacts instead of 2
for 32 bit addressing.
High Speed Bus:
High Speed Bus are specifically designed to support high capacity I/O devices.
High Speed Bus brings high demand devices into closer integration with the processor.
