1. CE 454 Computer Architecture
Lecture 12
Ahmed Ezzat
The Operating System Machine Level,
Ch-(6.1 – 6.3)

2. Outline Introduction Virtual Memory Virtual I/O Instructions
Virtual Instructions for Parallel Processing
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3. Introduction
OS is a program that adds variety of new instructions and features (system calls) beyond the ISA level
OS Machine level includes most ISA instructions + system calls
OSM level is always interpretive, i.e., reading data from a file is executed similar to a microprogram carrying out an ADD instruction – step-by-step
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4. Virtual Memory
Since the IBM compatible PC 286, virtual memory has been used to make believe that memory space is larger than what it is
Using the external memory device (as disk) to extend the addressable space
Before, programmers tried to shrink programs to fit tiny memories
Code and data size is still a constraint in embedded systems though
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5. Virtual Memory: Implementations of VM
Distinction between virtual and physical memory
Paging
The memory space is a large one-dimensional array, which extends to the disk
Disk broken up into regular sized pages
Pages are brought in memory on demand
And written back when needed
Segmentation
The memory is broken up into variable sized segments
We will not consider segmentation in detail
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6. Virtual Memory: Memory Issues
Idea: Separate concepts of
Addressable space (virtual memory)
And memory locations (physical memory)
Example:
Address Space = 216 = memory cells
Memory Size = 4096 memory cells
How can we fit the
Address Space into
Main Memory?
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7. Virtual Memory: Paging
Break the addressable space (Virtual space) into Pages
Normally Main Memory has thousands of pages
page
1 page = 4096 bytes
New Issue: How to manage addressing?
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8. Virtual Memory: Address Mapping
Mapping Virtual Memory addresses to Main Memory addresses (physical addresses)
page
1 page = 4096 bytes
physical address
used by hardware
virtual address
used by program
4095
8191
4096
Physical / Virtual
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9. Virtual Memory: Paging
4095
8191
4096
virtual
physical
Illusion that Main Memory is
Large
Contiguous
Linear
Size(MM) = Size(2ndry M)
Transparent to Programmer
4095 / 0
page
page
4095 / 0
page
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10. Virtual Memory: Paging Implementation
Virtual Address Space & Physical Address Space
Broken up into equal pages
Page size  Always a power of 2
Common Size:
512 to 64K bytes
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11. Virtual Memory: Paging Implementation
Page Frames
Page Tables
Programs use Virtual Addresses
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12. Virtual Memory: Virtual and physical Memory
Page Frame:
Home of VM pages in MM
Page Table:
Home of mappings for VM pages
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13. Virtual Memory: Memory Mapping
Memory Management Unit (MMU):
Device that performs virtual-to-physical mapping
32-bit VM Address
MMU
15-bit Physical Address
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14. Virtual Memory: Memory Management Unit
32-bit Virtual Address
MMU
Breaks VA into 2 portions
20-bit bit
Virtual page # offset in page
How to determine if page is in MM?
Present/Absent Bit
in Page Table Entry
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15. Virtual Memory: Demand Paging and Working Set Model
Page Fault:
Requested page is not in MM
Demand Paging:
Page is demanded by program from disk
Page is loaded into MM + initialize a PTE
Repeat instruction that caused the page fault
Possible Mapping
of pages
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16. Virtual Memory: Demand Paging and Working Set Model
Most programs do not reference their address space uniformly
Instead they tend to cluster on a small number of pages
Working set is the set of pages that are used actively and heavily
Working set typically varies slowly with time  kept in memory to reduce page faults
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17. Virtual Memory: Page Replacement Policy
Replacement: It is necessary to predict which page would have least impact on the running program
Common Replacement Schemes:
Least Recently Used (LRU)
First-In-First-Out (FIFO)
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18. Virtual Memory: Page Replacement Policies
Least Recently Used (LRU)
Generally works well
TROUBLE:
When the working set is larger than the Main Memory
Working Set = 9 pages
Pages are executed in sequence
(08 (repeat))
THRASHING
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19. Virtual Memory: Page Replacement Policies
First-In-First-Out (FIFO)
Removes Least Recently Loaded page
Does not depend on Use
Determined by number of page faults seen: increment counter on P/F
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20. Virtual Memory: Page Replacement Policies
Upon Replacement
Need to know whether to write data back
Add a Dirty-Bit
Dirty Bit = 0; Page is clean; No writing
Dirty Bit = 1; Page is dirty; Write back
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21. Virtual Memory: Page Size Comments
Smaller page size allows for less wasted space
Benefits:
Less internal fragmentation
Less thrashing: large # of small pages is better than small # of large pages (from a Working Set point of view)
Drawbacks:
page table
storage bandwidth
computer cost
time to load
more time spent at disk
miss rate
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22. Virtual Memory: Segmentation
Rather than having one virtual address space, having multiple can be better. For example, compiler produces multiple tables:
Some grows continuously (e.g., symbol table)
Some grow/shrink (e.g., stack)
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23. Virtual I/O Instructions
ISA-level and Microinstruction-level instructions are quite different
OS Machine-level instruction set includes most of the ISA-level instructions Plus additional important instructions (system calls) Minus few damaging ISA instructions (removed from the ISA-level instructions)
As an example, doing I/O at the ISA-level is tedious and complex. No user cares about the low-level details of why an I/O failed for example
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24. Virtual I/O Instructions: Files
File is an abstraction for presenting virtual I/O
Simple form of a file: a sequence of bytes on an I/O device. If the device is disk, data can be read back. If the device is a printer, data can’t be read back
In the above abstraction, any further structure to the file abstraction is left up to the application
Mainframe file abstraction is more sophisticated: sequence of logical records each with a well defined structure
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25. Virtual I/O Instructions: Implementation Issues
Storage allocation: can be a disk sector, but often a block of consecutive sectors?
Is file stored on disk as consecutive blocks or not?
Programmer view the file as a linear sequence of bytes. The OS view the file as an ordered, not necessarily consecutive, collection of blocks on disk
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26. Virtual I/O Instructions: Implementation Issues
OS needs to keep track of which blocks are available on disk?
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27. Virtual Instructions for Parallel Processing
Parallel processing can be done by:
Write the application as a set of cooperating processes running in parallel on different CPUs
Write the application as a set of processes/threads running on the same CPU
In both cases, certain virtual instructions are needed to synchronize among these processes/threads. One such mechanism is semaphores
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28. END
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