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From: dave@inset.UUCP (Dave Lukes)
Newsgroups: net.lang.c,net.arch
Subject: Re: pointers to freshly minted functions.
Message-ID: <868@inset.UUCP>
Date: Mon, 17-Mar-86 16:06:11 EST
Article-I.D.: inset.868
Posted: Mon Mar 17 16:06:11 1986
Date-Received: Fri, 21-Mar-86 04:31:07 EST
References: <1700003@umn-cs.UUCP> <1700004@umn-cs.UUCP> <6449@utzoo.UUCP> <204@dg_rtp.UUCP> <2277@phri.UUCP>
Reply-To: dave@inset.UUCP (Dave Lukes)
Organization: The Instruction Set Ltd., London, UK.
Lines: 53
Keywords: tagged architectures Burroughs, B6700, B5700
Xref: watmath net.lang.c:8198 net.arch:2869

In article <2277@phri.UUCP> roy@phri.UUCP (Roy Smith) writes:
>In article <204@dg_rtp.UUCP> throopw@dg_rtp.UUCP writes:
>[In an ongoing discussion about self-modifying code]
>> Consider a C
>> function which creates an instruction stream, and returns a pointer to a
>> function which when invoked will execute this instruction stream. [...]
>> It must *always* be possible to implement this function, otherwise
>> compilers would not be possible, since compilers are simply programs that
>> treat code as data ...

WHAAAATTT???
Uh-uh: you mean LOAD-AND-GO compilers aren't possible:
most compilers generate a file of code which is read into the I space by the OS.
			  ^^^^
>	The Buroughs B5700 had (in addition to the strangest subroutine
>linkage I've ever seen) a tagged architecture.  Each memory word had a
>(3-bit?) tag which defined the value stored there as integer, real,
>pointer, instruction, etc.  This tag was not directly accessible by a
					  ^^^ ^^^^^^^^ ^^^^^^^^^^ ^^ ^
>programmer which made it kind of hard to implement a compiler.  Presumably
 ^^^^^^^^^^

Wrong: there is/was (at least on the B6700) a ``set tag'' instruction
(I don't rememeber what it was called) which overwrote the tags in the
operand.

Anyways: there are plenty other reasons why implementing a compiler on
those beasts is strange: the major one being the total lack of protection.

>(I never actually used a B5700) there was some magic way the OS used to
>convert data into code, but I never ran accross any reference to it.

Again (see above), the code was generated in a file usually, then run by the OS.
>	One of the other oddities about the 5700 was a lack of conventional
>assembler -- you did systems programming in an Algol derivitive.

Sure, it was an Algol derivative (ESPOL: Executive Systems PrOgraming
Language it was called), but totally machine oriented.

BTW: tou HAD to program in a high level language: assembler was forbidden
due to the lack of protection (which was all provided by the compilers ...)
>-- 
>Roy Smith, {allegra,philabs}!phri!roy
>System Administrator, Public Health Research Institute
>455 First Avenue, New York, NY 10016

P.S. BTW: Burroughs called their OS MCP (Master Contol Program) long before
Disney did TRON. Does anyone know if Burroughs sued?
-- 
		Dave Lukes. (...!inset!dave)

All opinions, philosophies, dogmas and idiosyncrasies expressed in this article
INCLUDING THIS DISCLAIMER, are solely those of the author.