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Transcript Chapter01

Operating System
• Exploits hardware resources
– one or more processors
– main memory, disk and other I/O devices
• Provides a set of services to system users
– program development, program execution,
access to I/O devices, controlled access to
files and other resources etc.
Operating Systems:
Internals and Design Principles, 6/E
William Stallings
Chapter 1
Computer System Overview
Given Credits
• Most of the lecture notes are based on the
slides from the Textbook’s companion
• Some of the slides are from Dr. David
Tarnoff in East Tennessee State University
• I have modified them and added new
Computer Components: TopLevel View
Processor Registers
• User-visible registers
– Enable programmer to minimize main
memory references by optimizing register use
• Control and status registers
– Used by processor to control operation of the
– Used by privileged OS routines to control the
execution of programs
Control and Status Registers
• Program counter (PC)
– Contains the address of an instruction to be fetched
• Instruction register (IR)
– Contains the instruction most recently fetched
• Program status word (PSW)
– Condition codes
– Interrupt enable/disable
– Kernel/user mode
Control and Status Registers
• Condition codes or flags
– Bits set by processor hardware as a result of
– Can be accessed by a program but not altered
– Example
• Condition code bit set following the execution of
arithmetic instruction: positive, negative, zero, or
Instruction Execution
• Two steps
– Processor reads (fetches) instructions from
– Processor executes each instruction
Basic Instruction Cycle
Instruction Fetch and Execute
• The processor fetches the instruction from
• Program counter (PC) holds address of
the instruction to be fetched next
• PC is incremented after each fetch
Instruction Register
• Fetched instruction loaded into instruction
• An instruction contains bits that specify the
action the processor is to take
• Categories of actions:
– Processor-memory, processor-I/O, data
processing, control
Characteristics of a Hypothetical
Example of Program Execution
• Interrupt the normal sequencing of the
• Why do we need interrupts
Classes of Interrupts
• Most I/O devices are slower than the
– Without interrupts, processor has to pause to wait
for device
Program Flow of Control
Program Flow of Control
Interrupt Stage
• Processor checks for interrupts
• If interrupt
– Suspend execution of program
– Execute interrupt-handler routine
Transfer of Control via Interrupts
Instruction Cycle with Interrupts
Simple Interrupt Processing
Changes in Memory and
Registers for an Interrupt
Changes in Memory and
Registers for an Interrupt
Multiple Interrupts
• What to do if another interrupt happens
when we are handling one interrupt?
Sequential Interrupt Processing
Nested Interrupt Processing
• Processor has more than one program to
• The sequence the programs are executed
depend on their relative priority and
whether they are waiting for I/O
• After an interrupt handler completes,
control may not return to the program that
was executing at the time of the interrupt
Input/Output Techniques
• Programmed I/O
• Interrupt driven – I/O
• Direct Memory Access (DMA)
• What are they & the ranking of their efficiencies
Input/Output Techniques
• Programmed I/O – poll and response
• Interrupt driven – I/O module calls for CPU
when needed
• Direct Memory Access (DMA) – module
has direct access to specified block of
I/O Module Structure
Programmed I/O –
CPU has direct control over I/O
• Processor requests operation with commands sent
to I/O module
– Control – telling a peripheral what to do
– Test – used to check condition of I/O module or device
– Read – obtains data from peripheral so processor can read
it from the data bus
– Write – sends data using the data bus to the peripheral
• I/O module performs operation
• When completed, I/O module updates its status
• Sensing status – involves polling the I/O module's
status registers
Programmed I/O (continued)
• I/O module does not inform CPU directly
• CPU may wait or do something and come back
• Wastes CPU time because
– CPU acts as a bridge for moving data between I/O
module and main memory, i.e., every piece of data
goes through CPU
– CPU waits for I/O module to complete operation
Interrupt Driven I/O
Overcomes CPU waiting
Requires interrupt service routine
No repeated CPU checking of device
I/O module interrupts when ready
Still requires CPU to go between for
moving data between I/O module and
main memory
Interrupt-Driven I/O
• Consumes a lot of
processor time because
every word read or written
passes through the
Direct Memory Access (DMA)
• Impetus behind DMA – Interrupt driven
and programmed I/O require active CPU
intervention (all data must pass through
• Transfer rate is limited by processor's
ability to service the device
• CPU is tied up managing I/O transfer
DMA (continued)
• Additional Module (hardware) on bus
• DMA controller takes over bus from CPU
for I/O
– Waiting for a time when the processor doesn't
need bus
– Cycle stealing – seizing bus from CPU (more
DMA Operation
• CPU tells DMA controller:
– whether it will be a read or write operation
– the address of device to transfer data from or to
– the starting address of memory block for the
data transfer
– the amount of data to be transferred
• DMA performs transfer while CPU does
other processing
• DMA sends interrupt when completes
Cycle Stealing
DMA controller takes over bus for a cycle
Transfer of one word of data
Not an interrupt to CPU operations
CPU suspended just before it accesses
bus – i.e. before an operand or data fetch
or a data write
• Slows down CPU but not as much as CPU
doing transfer
Direct Memory Access
• Transfers a block of data
directly to or from memory
• An interrupt is sent when
the transfer is complete
• Most efficient
The Memory Hierarchy
Going Down the Hierarchy
Decreasing cost per bit
Increasing capacity
Increasing access time
Decreasing frequency of access to the
memory by the processor
Cache Memory
• Processor speed faster than memory
access speed
• Exploit the principle of locality with a small
fast memory
Cache and Main Memory
Cache Principles
• Contains copy of a portion of main
• Processor first checks cache
• If not found, block of memory read into
• Because of locality of reference, likely
future memory references are in that block
Cache/Main-Memory Structure
Cache Read Operation
Cache Principles
• Cache size
– Small caches have significant impact on
• Block size
– The unit of data exchanged between cache
and main memory
– Larger block size more hits until probability of
using newly fetched data becomes less than
the probability of reusing data that have to be
moved out of cache
Cache Principles
• Mapping function
– Determines which cache location the block
will occupy
– Direct Mapped Cache, Fully Associative
Cache, N-Way Set Associative Cache
• Replacement algorithm
– Chooses which block to replace
– Least-recently-used (LRU) algorithm
Cache Principles
• Write policy
– Dictates when the memory write operation
takes place
– Can occur every time the block is updated
– Can occur when the block is replaced
• Minimize write operations
• Leave main memory in an obsolete state
• 2. [25 pts] This problem concerns the performance of
the cache memory in web applications that play media
files. Consider a video streaming workload that accesses
working sets of size 256KB sequentially with the
following byte-address stream:
0, 2, 4, 6, 8, 10, …
Suppose the computer that processes the above stream
has a 32 KB direct-mapped L1 cache. The cache block
size is 32 bytes.
a) What would be the cache miss rate of the address
stream above? Show all calculations.
Every 16th access would be a miss, hence, miss
ratio is 1/16 = 6.25%
• b) If the cache size were changed to 64 KB, what would
be the change in the miss rate? Justify your answer.
• There would be no change; the miss rate depends
only on the block size.
• c) If the cache organization were changed to two-way set
associative, without changing the block size, would the
cache miss rate change? Justify your answer.
• There would be no change; the data is fetched in
units of blocks from the memory, therefore the miss
rate will be the same.
• d) If the cache block size is changed to 16B would the
miss rate change? If so, what would be the new value?
• Now, every 8th access would be a miss, hence the
rate is 1/8 = 12.5%
• e) Prefetching is a technique that can be used effectively
in streaming applications, such as the one described
above. Describe how prefetching works and how it
impacts the cache miss rate.
• In prefetching, cache lines are brought in
speculatively, in anticipation of future accesses. This
works very well in streaming applications, where
memory accesses are in sequential order, and the
preloading for a new block can be overlapped with
the consumption of the current block. This
effectively reduces the miss rate to zero.