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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 81
50
INTRODUCTION
CHAP. 1
40 cm in diameter and 5 cm high. But it, too, had a single-level directory initially.
When microcomputers came out, CP/M was initially the dominant operating sys-
tem, and it, too, supported just one directory on the (floppy) disk.
Virtual Memory
Virtual memory (discussed in Chap. 3) gives the ability to run programs larger
than the machine’s physical memory by rapidly moving pieces back and forth be-
tween RAM and disk.
It underwent a similar development, first appearing on
mainframes, then moving to the minis and the micros. Virtual memory also allow-
ed having a program dynamically link in a library at run time instead of having it
compiled in.
MULTICS was the first system to allow this. Eventually, the idea
propagated down the line and is now widely used on most UNIX and Windows
systems.
In all these developments, we see ideas invented in one context and later
thrown out when the context changes (assembly-language programming, monopro-
gramming, single-level directories, etc.) only to reappear in a different context
often a decade later. For this reason in this book we will sometimes look at ideas
and algorithms that may seem dated on today’s gigabyte PCs, but which may soon
come back on embedded computers and smart cards.
1.6 SYSTEM CALLS
We have seen that operating systems have two main functions: providing
abstractions to user programs and managing the computer’s resources. For the most
part, the interaction between user programs and the operating system deals with the
former; for example, creating, writing, reading, and deleting files. The re-
source-management part is largely transparent to the users and done automatically.
Thus, the interface between user programs and the operating system is primarily
about dealing with the abstractions.
To really understand what operating systems
do, we must examine this interface closely. The system calls available in the inter-
face vary from one operating system to another (although the underlying concepts
tend to be similar).
We are thus forced to make a choice between (1) vague generalities (‘‘operat-
ing systems have system calls for reading files’’) and (2) some specific system
(‘‘UNIX has a
read
system call with three parameters: one to specify the file, one
to tell where the data are to be put, and one to tell how many bytes to read’’).
We have chosen the latter approach.
It’s more work that way, but it gives more
insight into what operating systems really do. Although this discussion specifically
refers to POSIX (International Standard 9945-1), hence also to UNIX, System V,
BSD, Linux, MINIX 3, and so on, most other modern operating systems have sys-
tem calls that perform the same functions, even if the details differ. Since the actual
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