The document discusses memory management in the Amoeba distributed operating system. Key points: - Amoeba uses a simple memory model where processes are divided into segments that are stored contiguously in physical memory, with no paging or swapping. This allows for high performance but limits processes to the available physical memory. - Segments act as addressable blocks of memory that can contain code or data and have capabilities controlling access. Processes map and unmap segments in their virtual address space as needed. - Shared memory communications and a main memory file server are enabled through segments that can be read or written by any process with the right capabilities.

1. MEMORY MANAGEMENT IN
AMOEBA
Presented by:
Mani Dumre
Ramesh Adhikari
Suman Lamsal
29 April 2019
Nepal College of Information & Technology

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AMOEBA (DOS)
● The Amoeba distributed operating system project is a research
effort aimed at understanding how to connect computers
together in a seemless way.
● The researchers were motivated by the declining cost of CPU
chips.
● They saw the challenge of designing and implementing
software to manage the growing availability of computing
power in a convenient way.
● Their basic idea: users should not be aware of the number
or location of processors, file servers, or other
resources. The complete system should appear to be a
single computer.

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MEMORY MANAGEMENT
● Entirely in physical memory .
● Segments contiguous in address space .
● Segments are objects .
● Any process with capability to segment could read/write it
(with proper permissions) .
● Shared memory communications (need not be on same
machine.
● Main memory file server .
● When a process is executing, all its segments are in
memory. There is no swapping or paging at present, thus
Amoeba can only run programs that fit in physical memory

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MEMORY MODEL
● Amoeba has an extremely simple memory model.
● A process can have any number of segments.
● They can be located wherever it wants in the process’s virtual
address space.
● The virtual address spaces in Amoeba are constructed from
segments.
● Segments are not swapped or paged, so a process must be
entirely memory resident to run.
● Although the hardware MMU is used, each segment is stored
contiguously in memory.

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MEMORY MODEL cont...
● This design was done for three reasons: Performance,
Simplicity, Economics.
● Having a process entirely in memory all the time makes RPC
go faster.
● This design has allowed Amoeba to achieve extremely high
transfer rates for large RPC’s
● Not having paging or swapping makes the system considerably
simpler and makes the kernel smaller and more feasible.
● Memory is becoming so cheap that within a few years, all
Amoeba machines will probably have tens of megabytes of it.

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SEGMENTS
● A segment is a contigous block of memory that can contain
code or data.
● Each segment has a capability that permits its holder to perform
operations on int, such as reading and writing.
● A segment is somewhat like an in-core file, with similar
properties.
● Segments cannot be swapped or paged.

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SEGMENTS Cont...
● When a segment is created it is given an initial value. This size
may change during the process execution.
● The segment may also given initial value either from another
segment or from a file.
● Processes have several calls available to them for managing
segments.
● Most important among these is the ability to create, read and
write segments.
● When a segment is created, the caller gets back a capability for
it.
● This capability is used for all other calls involving the segment.

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SEGMENT Cont...
● A main memory file server can be constructed using segments
because of their read and write properties.
● The server creates a segment as large as it can.
● This segment will be used as a simulated disk.
● The server then formats the segment as a file system, putting in
whatever data structures it needs to keep track of the files.
● After that, it is open for business, accepting and processing
requests from clients.

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MAPPED SEGMENTS
● When a process is started, it must have at least one segment.
● Once it is running, a process can create additional segments
and map them into its address space at any unused virtual
address.

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S
D
T
S D
T
Process virtual address
space
Memory segments
A process with three segments mapped into its virtual address space.

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UNMAPPED SEGMENTS
● A process can also unmap segments.
● A process can specify a range of virtual addresses and request
that the range be unmapped.
● When a segment is unmapped a capability is returned, so the
segment may still be accessed, or even be remapped .

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Advantage & Disadvantages
● The primary advantage of this scheme is
simplicity and high performance
● The primary disadvantage is that it is not
possible to run programs larger than physical
memory.

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References
● Distributed Operating Systems – By Andrew S.
Tanenbaum
● Amoeba: An Overview of a Distributed Operating
System. By Eric W. Lund - March 29, 1998
● https://en.wikipedia.org/wiki
Amoeba_(operating_system)
● http://indbooks.in/mirror1/?p=355336

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Thank You!