computer organization and system software lecturer: szabolcs mikulas url: textbooks:
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Computer Structure - Top Level Computer Main Memory Input Output Systems Interconnection Peripherals Communication lines Central Processing Unit ComputerTRANSCRIPT
Computer Organization and System Software
• Lecturer: Szabolcs Mikulas• E-mail: [email protected]• URL: http://www.dcs.bbk.ac.uk/~szabolcs/coss.html• Textbooks: J.A. Harris, Operating Systems, Schaum’s Outline Series,
McGraw-Hill, 2002 N. Carter, Computer Architecture, Schaum’s Outline
Series, McGraw-Hill, 2002• See also the URL for recommended readings.
Chapter 1Computer System Overview
Patricia RoyManatee Community College, Venice, FL
©2008, Prentice Hall
With additional inputs from Computer Organization and Architecture, Parts 1 and 2
Operating Systems:Internals and Design Principles, 6/E
William Stallings
Computer Structure - Top Level
Computer
Main Memory
InputOutput
SystemsInterconnection
Peripherals
Communicationlines
CentralProcessing Unit
Computer
The Central Processing Unit - CPU
Computer Arithmeticand Logic Unit
ControlUnit
Internal CPUInterconnection
Registers
CPU
I/O
Memory
SystemBus
CPU
Computer Components - Registers
Control and Status Registers• Used by processor to control the operation of
the processor• Used by privileged operating system (OS)
routines to control the execution of programs• Program counter (PC): Contains the address of
the next instruction to be fetched• Instruction register (IR): Contains the
instruction most recently fetched – currently executed
• Program status word (PSW): Contains status information
User-Visible Registers• May be referenced by machine language,
available to all programs – application programs and system programs
• Data• Address
– Index: Adding an index to a base value to get the effective address
– Segment pointer: When memory is divided into segments, memory is referenced by a segment and an offset inside the segment
– Stack pointer: Points to top of stack
Basic Instruction Cycle
Fetch Cycle• Program Counter (PC) holds address of next
instruction to be fetched• Processor fetches instruction from memory
location pointed to by PC• Increment PC
– Unless told otherwise• Instruction loaded into Instruction Register (IR)• Processor interprets instruction and performs
required actions
Execute Cycle• Data transfer (via the bus)
– Between CPU and main memory– Between CPU and I/O module
• Data processing (by the arithmetic-logic unit ALU)– Some arithmetic or logical operation on data
• Control (by the control unit)– Alteration of sequence of operations, e.g. jump
• Combinations of the above
Characteristics of a Hypothetical Machine
Simple Computation
• How to add the contents (3 and 2) of two memory locations (940 and 941) and store the result at a memory location (941)
1.load data (into accumulator register AC): LOAD 940, AC (3 -> AC)
2.perform addition: ADD 941, AC, AC (2+3 -> AC)3.store result (in memory): STORE AC, 941 (5 ->
941)
Example of Program Execution
CPU Speed• Speed of CPU clocked is measured in
frequency: 1 Hz (hertz) – 1 cycle per second• 1 GHz = 10^3 MHz = 10^6 KHz = 10^9 Hz
(instead of 10^3 one can use 2^10=1052)• Length of a cycle measured in seconds• 1 s = 10^3 milliseconds = 10^6 microseconds =
10^9 nanoseconds• Performing one operation may take longer
than one clock cycle!!! – Accessing memory is slower that pure arithmetic operation (using the registers)
Other Performance Measurements
• MIPS: million instruction per secondNB: The same computation may take different
numbers of instructions on different machines, see RISC v CISC
• CPI/IPC: cycles per instruction/instructions per cycle
• Benchmark suites
Connecting
• All the units must be connected• Different type of connection for different type
of unit– Memory– Input/Output– CPU
Physical Realization of Bus Architecture
Computer Modules
Memory Connection
• Receives and sends data• Receives addresses (of locations)• Receives control signals
– Read– Write– Timing
Input/Output Connection(1)
• Similar to memory from computer’s viewpoint• Output
– Receive data from computer– Send data to peripheral
• Input– Receive data from peripheral– Send data to computer
Input/Output Connection(2)
• Receive control signals from computer• Send control signals to peripherals
– e.g. spin disk• Receive addresses from computer
– e.g. port number to identify peripheral• Send interrupt signals (control)
CPU Connection
• Reads instruction and data• Writes out data (after processing)• Sends control signals to other units• Receives (& acts on) interrupts
Bus Interconnection Scheme
Data Bus
• Carries data– Remember that there is no difference between
“data” and “instruction” at this level• Width (number of lines) is a key determinant
of performance, since this determines how many bits can be transferred in one go (cycle)– 32 to hundreds of bits
Address bus
• Identify the source or destination of data• e.g. CPU needs to read an instruction (data)
from a given location in memory• Bus width determines maximum memory
capacity of system– e.g. 8080 has 16 bit address bus giving
2^16=2^6*2^10=64K addresses
Control Bus
• Memory or I/O read/write signals• Interrupt request/acknowledgment• Clock signals• Bus request/grant signals
Traditional (ISA) (with cache)
The Memory Hierarchy
Memory
• Typical memory hierarchy (numbers shown on the right are a bit out-dated)
Memory as storage
• Limited register size, so code and data has to be stored in (main) memory
• These are fetched by the CPU during the execution of the code
• Also the results of the computations must be stored
• These result in frequent access to (main) memory
Going Down the Hierarchy
• Decreasing cost per bit• Increasing capacity• Increasing access time• Decreasing frequency of access to the
memory by the processor (optimally - requires good design)
Main Memory• Contains data (including instructions) in binary
format: sequences of bits• 1B=1 byte=8 bits=8b• Word – a sequence of bytes, length is system
specific (1, 2, 4, 8, etc. bytes)• Block – a sequence of words, typically in the
magnitude of several kilobytes (KB)• An address - a location in memory. It specifies
(the beginning of) a word or block - depending on the size of data transfer
Performance Balance• Processor (logic) speed increases• Memory capacity increases• Memory speed increases but lags behind
processor speed• Speed is measured in frequency – how many
cycles (execution of instruction or data transfer via the bus) happen in one second
• Typically: one bus cycle takes several clock cycles!!!
Logic and Memory Performance Gap
Cache Memory
• Processor speed faster than memory access speed
• Main memory becomes a bottleneck• Exploit the principle of locality of reference:
During the course of the execution of a program, memory references tend to cluster, e.g. loops, and the same data maybe needed again
• Introduction of small, fast memory - cache
Cache and Main Memory
Cache Principles• Contains copy of a (recently accessed) portion
of main memory• Processor first checks cache• If not found (cache miss), block of memory
read into cache (cache line)• Because of locality of reference, likely future
memory references are in that block• Modern systems have several caches
(instruction, data) on different levels (L1 on chip, L2, etc.)
Cache/Main-Memory Structure
Cache Read Operation
Size• Cache size
– Small caches have significant impact on performance, since accessing cache is faster than accessing main memory
• Block size– The unit of data exchanged between cache and
main memory, typically several KB (kilobytes)
(Re)placement
• Mapping function– Determines which cache location the block will
occupy when loaded into the cache• Replacement algorithm
– Chooses which block to replace– Least-recently-used (LRU) algorithm
Write policy
• Dictates when the memory write operation takes place– Write through: occurs every time the block in the
cahce is updated– Write back: occurs when the block is replaced
• Minimize write operations• Leave main memory in an obsolete state
I/O Devices
• Programs with intensive I/O demands• Large data throughput demands• Processors can handle this, but memory is limited and
slow• Problem moving data • Solutions:
– Caching– Buffering– Higher-speed interconnection buses– More elaborate bus structures– Multiple-processor configurations
Typical I/O Device Data Rates(in bit per second)
Hard disk
Speed
• Seek time– Moving head above the correct track
• (Rotational) latency– Waiting for the correct sector to rotate under
head• Access time = Seek + Latency• Transfer rate, typically in bit per second (bps)
Input/Output Problems
• Wide variety of peripherals– Delivering different amounts of data– At different speeds– In different formats
• All slower than CPU and main memory• Need I/O modules
I/O Steps
• CPU checks I/O module device status• I/O module returns status• If ready, CPU requests data transfer• I/O module gets data from device• I/O module transfers data to CPU• Variations for output, DMA, etc.
Input Output Techniques
• Programmed• Interrupt driven• Direct Memory Access (DMA)
Programmed I/O (1)
• CPU has direct control over I/O– Sensing status– Read/write commands– Transferring data
• CPU waits for I/O module to complete operation
• Wastes CPU time
Programmed I/O (2)
• CPU requests I/O operation• I/O module performs operation• I/O module sets status bits• CPU checks status bits periodically• I/O module does not inform CPU directly• I/O module does not interrupt CPU• CPU may wait or come back later
Programmed I/O (3)
• I/O module performs the action• Sets the appropriate bits in the
I/O status register• CPU checks status bits
periodically• No interrupts occur• Processor checks status until
operation is complete
Program Flow of Control
Interrupt Driven I/O
• Overcomes CPU waiting• No repeated CPU checking of device• I/O module interrupts when ready
Program Flow of Control
Interrupts
• Interrupts the normal sequencing of the processor – suspends current activity and runs special code
• Program generated: result of an instruction, e.g. division by 0, overflow, illegal machine instruction
• Hardware generated: timer, I/O (when finished or error), other errors (e.g. parity check)
Interrupt Stage
• Processor checks for interrupts• If interrupt occurred
– Suspend execution of program– Execute interrupt-handler routine/interrupt
service procedure– Afterwards control may be returned to suspended
program
Transfer of Control via Interrupts
Instruction Cycle with Interrupts
Simple Interrupt Processing
Interrupt Driven I/O (2)
• CPU issues read command• I/O module gets data from peripheral while
CPU does other work• I/O module interrupts CPU• CPU requests data• I/O module transfers data
Interrupt-Driven I/O (3)
• Processor is interrupted when I/O module ready to exchange data
• Processor saves context of program executing and begins executing interrupt-handler
Direct Memory Access
• Interrupt driven and programmed I/O require active CPU intervention– Transfer rate is limited– CPU is tied up
• DMA, an additional module (hardware) on bus• DMA controller takes over from CPU for I/O
DMA Configurations (1)
• Single Bus, Detached DMA controller• Each transfer uses bus twice
– I/O to DMA then DMA to memory• CPU is suspended twice
Typical DMA Module Diagram
DMA Operation
• CPU tells DMA controller:-– Read/Write– Device address– Starting address of memory block for data– Amount of data to be transferred
• CPU carries on with other work• DMA controller deals with transfer• DMA controller sends interrupt when finished
Direct Memory Access
• Transfers a block of data directly to or from memory
• An interrupt is sent when the transfer is complete
• More efficient
DMA Transfer - Cycle Stealing
• DMA controller takes over bus for a cycle• Transfer of one word of data• Not an interrupt
– CPU does not switch context• 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
Improvements in Chip Organization and Architecture
• Increase hardware speed of processor– Fundamentally due to shrinking logic gate size
• More gates, packed more tightly, increasing clock rate• Propagation time for signals reduced
• Increase size and speed of caches– Dedicating part of processor chip
• Cache access times drop significantly
• Change processor organization and architecture– Increase effective speed of execution– Parallelism
Problems with Clock Speed and Logic Density• Power
– Power density increases with density of logic and clock speed
– Dissipating heat
• RC delay– Speed at which electrons flow limited by resistance and
capacitance of metal wires connecting them– Delay increases as RC product increases– Wire interconnects thinner, increasing resistance– Wires closer together, increasing capacitance
• Memory latency– Memory speeds lag processor speeds
• Solution: More emphasis on organizational and architectural approaches
Increased Cache Capacity
• Typically two or three levels of cache between processor and main memory
• Chip density increased– More cache memory on chip - faster cache access
• Pentium chip devoted about 10% of chip area to cache
• Pentium 4 devotes about 50%
More Complex Execution Logic
• Enable parallel execution of instructions• Pipeline works like assembly line
– Different stages of execution of different instructions at same time along pipeline
• Superscalar allows multiple pipelines within single processor– Instructions that do not depend on one another
can be executed in parallel
New Approach – Multiple Cores• Multiple processors on single chip
– Large shared cache• Within a processor, increase in performance
proportional to square root of increase in complexity• If software can use multiple processors, doubling
number of processors almost doubles performance• So, use two simpler processors on the chip rather
than one more complex processor• Example: IBM POWER4
– Two cores based on PowerPC
Intel Microprocessor Performance