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From: siva@bally.Bally.COM (Siva Chelliah)
Newsgroups: comp.lang.c
Subject: Re: Unix and C
Message-ID: <403@bally.Bally.COM>
Date: 9 Nov 90 00:38:43 GMT
References: <LDB1R3w163w@railnet.UUCP> <4458@mint39.UUCP> <fraser.658038441@bilby>
Reply-To: siva@bally.UUCP (Siva Chelliah)
Organization: Bally Mfg. - Reno, Nevada
Lines: 62

In article <fraser.658038441@bilby> fraser@bilby.cs.uwa.oz.au (Fraser Wilson) writes:
>In <4458@mint39.UUCP> gunda@motcid.UUCP (Indira Gunda) writes:
>
>>richb@railnet.UUCP (Richard Banks) writes:
>
>>>How can Unix be written in C ? I thought all OS`s had to be written
>>>in assembly language, else they'd have the same interrupts as the
>>>OS you booted your complier language under to write the program ?
>
>>Yes ... Why and how can UNIX be written in C?  I'd also like to know.
>
>Why?  Because it makes it very portable.  Any machine that has a C
>compiler and a few kernal routines (actually a fair number) can run Unix
>fairly easily.  If Unix were written in assembly, it would have to be
>completely rewritten for every new computer it runs on.

This discussion reminds me of something.

Q: How is the first compiler is written ? 
A: In assembly language.

Q: How is the first assembler is written ?
A: In machine language.

Q: If you have just the hardware, how would you feed the machine language
   into the computer ?
A: I thing you should burn the machine language into a chip, that would 
   accept a machine language from the user, and execute it. 


>
>How?  Well, you have a few system calls to assembly routines, and then
>write lots of C to use them.  The C compiler itself is written in C.
>

Assume you have a C compiler ( write it any way you want - assembly, or Pascal
or anything ).  It can be as sloppy as you want.  
Let's call it  sloppy_comp_sloppy.exe

sloppy_comm_sloppy.exe ===> a C compiler which is sloppy(doesn't do any 
                            optimization)

Now write a optimizing C compiler in C call it opti_comp.c

opti_comp.c   ===> a C compiler that will optimize other C programs.

compile this with sloppy_comm_sloppy.exe , and you will get

opti_comp_sloppy.exe  ==> it is a sloppy exe file (will run slow, 
                          but will compile other C programs and produce
                          good exe programs)

Now, compile opti_comp.c using opti_comp_sloppy.exe, and you will get

opti_comp_opti.exe  ===> an optimized exe file which will produce other good
                         exe programs.

So we started with a sloppy C compiler and using that we produced a 
optimizing C compiler ( which itself is optimized).

Siva
siva@bally.bally.com