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Modern Operating Systems by Herbert Bos and Andrew...
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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 79
is not always crucial because network delays are so great that they tend to domi-
nate. Thus the pendulum has already swung several cycles between direct execu-
tion and interpretation and may yet swing again in the future.
Large Memories
Let us now examine some historical developments in hardware and how they
have affected software repeatedly. The first mainframes had limited memory.
fully loaded IBM 7090 or 7094, which played king of the mountain from late 1959
until 1964, had just over 128 KB of memory.
It was mostly programmed in assem-
bly language and its operating system was written in assembly language to save
precious memory.
As time went on, compilers for languages like FORTRAN and COBOL got
good enough that assembly language was pronounced dead. But when the first
commercial minicomputer (the PDP-1) was released, it had only 4096 18-bit words
of memory, and assembly language made a surprise comeback. Eventually, mini-
computers acquired more memory and high-level languages became prevalent on
When microcomputers hit in the early 1980s, the first ones had 4-KB memo-
ries and assembly-language programming rose from the dead. Embedded com-
puters often used the same CPU chips as the microcomputers (8080s, Z80s, and
later 8086s) and were also programmed in assembler initially. Now their descen-
dants, the personal computers, have lots of memory and are programmed in C,
C++, Java, and other high-level languages. Smart cards are undergoing a similar
development, although beyond a certain size, the smart cards often have a Java
interpreter and execute Java programs interpretively, rather than having Java being
compiled to the smart card’s machine language.
Protection Hardware
Early mainframes, like the IBM 7090/7094, had no protection hardware, so
they just ran one program at a time.
A buggy program could wipe out the operat-
ing system and easily crash the machine.
With the introduction of the IBM 360, a
primitive form of hardware protection became available. These machines could
then hold several programs in memory at the same time and let them take turns
running (multiprogramming).
Monoprogramming was declared obsolete.
At least until the first minicomputer showed up—without protection hard-
ware—so multiprogramming was not possible. Although the PDP-1 and PDP-8
had no protection hardware, eventually the PDP-11 did, and this feature led to mul-
tiprogramming and eventually to UNIX.
When the first microcomputers were built, they used the Intel 8080 CPU chip,
which had no hardware protection, so we were back to monoprogramming—one
program in memory at a time.
It was not until the Intel 80286 chip that protection

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