Download CPU cont, the Memory Hierarchy, Programs vs. Processes

Foundations of Software Design
Fall 2002
Marti Hearst
Lecture 4: Operating Systems
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Today
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Revisit instructions and the CPU
Measuring CPU Performance
The Memory Hierarchy
Operating Systems
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Programs vs. Processes
Process Scheduling
Deadlock
Memory Management
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Instructions and Their Representation
Images from http://courses.cs.vt.edu/~csonline/MachineArchitecture/Lessons/
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The CPU
Bus: bundles of wires connecting
things.
Instruction Register: holds
instructions.
Program Counter: points to the
current memory location of the
executing program.
Decoder: uses the 3-bit opcode to
control the MUX and the ALU and the
memory buses.
MUX: multiplexer – chose 1 out of N
ALU: adds, subtracts two numbers
Accumulator: stores ALU’s output
RAM: stores the code (instructions)
and the data.
Images from http://courses.cs.vt.edu/~csonline/MachineArchitecture/Lessons/
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Machine code to add two numbers that are
stored in memory (locations 13 and 14) and
then store the result into memory (location 15).
The Program Counter starts at memory
location 0.
Image adapted from http://courses.cs.vt.edu/~csonline/MachineArchitecture/Lessons/
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The Program Counter starts at
memory location 0.
(Notice: the instructions and the
data are both stored in the RAM.)
This instruction moves along the
blue bus into the instruction
register.
The decoder checks the number bit
to see how to handle the opcode.
For each instruction, we have to do
something with the operand.
Images from http://courses.cs.vt.edu/~csonline/MachineArchitecture/Lessons/
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If the opcode number bit = 0, we
want the operand to be used as a
memory address.
So the decoder switches the MUX to
connect the operand to the RAM.
If the opcode bit = 1, the MUX
would connect the operand to the
ALU instead.
The green bus does the addressing
of the RAM.
The blue bus does the reading to
and writing from the RAM.
Images from http://courses.cs.vt.edu/~csonline/MachineArchitecture/Lessons/
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The Decoder tells the ALU this is a
LOAD instruction.
The memory location 13 is
activated, and its value placed on
the blue bus. This goes through
the multiplexer into the ALU. It
also trickles down into the
Accumulator (Load is implemented
as an ADD value to 0).
The last step (always, except for a
jump) is for the Program Counter to
get incremented. Now it points to
memory location 1.
Images from http://courses.cs.vt.edu/~csonline/MachineArchitecture/Lessons/
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Instruction: ADD 14
Similar to Load 13, except the
ALU is given the ADD operation.
The contents of the Accumulator
becomes the lefthand operand,
and the contents of memory
becomes the righthand operand.
Results of the ADD end up in the
Accumulator.
The Program Counter
increments.
Images from http://courses.cs.vt.edu/~csonline/MachineArchitecture/Lessons/
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Instruction: STORE 15
Memory address 15 is activated via
the green bus. (Because the
decoder sets to MUX to connect to
the memory bus.)
The contents of the Accumlator are
written to RAM using the blue bus.
The Program Counter increments.
The next instruction is HALT.
Images from http://courses.cs.vt.edu/~csonline/MachineArchitecture/Lessons/
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There are other nice examples
in the online simulation.
They show the use of explicit
numbers (represented by # signs),
along with memory addresses.
Images from http://courses.cs.vt.edu/~csonline/MachineArchitecture/Lessons/
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CPU Performance
• CPU cycles are not the whole story.
• The number of INSTRUCTIONS per second play
a big role.
– Each operation on the computer requires some number
of instructions
Each instruction takes some number of cycles to
execute.
– So … a CPU with an instruction set and circuit design
that requires fewer cycles to get operations done might
have better performance than a CPU with a faster clock
but more complicated circuitry.
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Measuring CPU Performance
• MIPS:
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millions of instructions per second
“meaningless indicator of performance” (a joke)
Some instructions require more time than others
No standard way to measure MIPS
• FLOPS:
– floating-point operations per second
– Measures the speed of the floating point unit
• A special circuit that does math on real numbers
• So, doesn’t measure performance on non-numeric applications
• Benchmark:
– A test used to compare performance of hardware and/or software.
– Different benchmarks measure different aspects of performance
• SPEC:
– Standard Performance Evaluation Corporation
– A set of standardized tasks (benchmarks) that CPUs are run on in
order to assess their performance.
Adapted form http://webopedia.internet.com/
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CPU Performance
CPU clocks are related to another commonly used
term…
Real Time
– Means regular clock time as opposed to CPU clock time
– Other factors also prevent a program from running in
real time
• CPU scheduling, discussed in today’s lecture
In common usage today,
– Real time means right now, as I’m talking, as opposed to
– Offline, when I have to think on my own, or when we’re
not in the middle of a timed event with others listening.
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The Memory Hierarchy
Image from http://www.howstuffworks.com/computer-memory1.htm
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Types of Computer Memory
• Characteristics
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Volatile vs. Nonvolatile
Solid State vs. Mechanical
Read only vs. Read-write (RW)
Expense
Speed
Storage capacity (a function of size & expense)
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Types of Computer Memory
• Registers (made up of flip-flops-like circuits)
– Volatile, small, expensive, fast, solid state, RW
• DRAM (grid format using capacitance)
– Connected to the rest of the CPU via a memory bus
– Volatile, now large, expensive, fast, solid state, RW
• ROM (grid format using diodes)
– Nonvolatile, Read-only, cheap, fast
• Hard Disk
– Nonvolatile, mechanical, RW, cheap, slow
• Flash Memory (grid format using fancy electronics)
– Nonvolatile, solid state, RW, expensive, fast, small
– Versus hard disk: silent, lighter, faster … but expensive!
• Flash RAM (e.g., car radio presets)
– Like Flash memory, but needs power
• Other external storage: CD-ROM, Floppy, Zip
Adapted from http://www.howstuffworks.com/computer-memory.htm
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Caching
• Caches store items that have
been used recently for faster
access
• Based on the notion of
locality
– Something used recently is
more likely to be used again
soon than other items.
• Can be done using
– Registers
– RAM
– Hard Disk, etc
Image from http://cne.gmu.edu/modules/vm/orange/realprog.html
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Types of Computer Memory
• Cache
– The term refers to how the memory is used
– Can be volatile or on disk
• L1 cache - Memory accesses at full microprocessor speed (10
nanoseconds, 4 kilobytes to 16 kilobytes in size)
• L2 cache - Memory access of type SRAM (around 20 to 30
nanoseconds, 128 kilobytes to 512 kilobytes in size)
• Main memory - Memory access of type RAM (around 60
nanoseconds, 32 megabytes to 128 megabytes in size)
• Hard disk - Mechanical, slow (around 12 milliseconds, 1
gigabyte to 10 gigabytes in size)
• Internet - Incredibly slow (between 1 second and 3 days,
unlimited size)
Adapted from http://www.howstuffworks.com/computer-memory.htm
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Levels of a Memory Hierarchy
http://www.ece.arizona.edu/~ece369/lec10S01/sld047.htm
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Operating Systems
• Main Roles:
– Allow user applications to run on a given machine’s
hardware
– Act as a “traffic cop” for allocating resources.
• Resources
– CPU, I/O devices, Memory, Files, Allocation
• Processes
– versus Programs
– Scheduling
– Synchronization
• Memory Management
• File Management
Image from http://courses.cs.vt.edu/~csonline/OS/Lessons/Introduction/index.html
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Programs vs. Processes
• Program
– The code, both in human and machine-readable form
• Process
– A running program is a process.
– It is allocated a number of resources:
• Registers in the CPU
• Some part of the CPU’s RAM
• I/O devices (the monitor, files in the file system)
• Several copies of the same program can be
running as different processes.
– But data values, current Program Counter location,
etc., will differ among the different processes.
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Programs vs. Processes
• How does this relate to Object Oriented Programming?
• An important difference:
– Class definition vs.
– Object creation / instantiation
• Class definitions are created at compile time.
• Objects don’t really get created until the program is
running as a process.
– The “new” command in java tells the operating system to
allocate some memory and put the new object in that space.
• When the process ends,
– the class definitions do not disappear, but
– the objects disappear.
meaning that the memory allocated to the objects gets
reallocated for some other purpose.
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