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From: robison@uiucdcsb.CS.UIUC.EDU
Newsgroups: net.arch
Subject: Re: pointers to freshly minted func
Message-ID: <5600043@uiucdcsb>
Date: Thu, 13-Mar-86 11:43:00 EST
Article-I.D.: uiucdcsb.5600043
Posted: Thu Mar 13 11:43:00 1986
Date-Received: Sat, 15-Mar-86 03:30:53 EST
References: <2277@phri.UUCP>
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Nf-ID: #R:phri.UUCP:2277:uiucdcsb:5600043:000:1221
Nf-From: uiucdcsb.CS.UIUC.EDU!robison    Mar 13 10:43:00 1986


> 	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
> (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.
> 

Much of the 5700's security was enforced by the compiler.
Therefore ordinary users could not set the tags.  There was a priviliged
instruction which could set the tags.  Thus all compilers had to run
in priviledged mode.

The early versios of the compiler didn't enforce the security rules well
enough.  Specifically, a pointer could be set to point to a variable more
local than itself (i.e. an up-level pointer).  When the variable disappeared, 
accessing the pointer could lead to total system failure.  (In the compiler
manual there was an index entry "total system failure"!  The current compilers
on the system disallow anything that even looks like an attempt to make an
up-level pointer.

- Arch Robison