Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!usc!snorkelwacker!bu.edu!bu-cs!mirror!prism!rob
From: rob@prism.TMC.COM
Newsgroups: comp.arch
Subject: Re: LoadAll?
Message-ID: <208300003@prism>
Date: 2 Jan 90 15:50:00 GMT
References: <597@unmvax.unm.edu>
Lines: 19
Nf-ID: #R:unmvax.unm.edu:-59700:prism:208300003:000:1088
Nf-From: prism.TMC.COM!rob    Jan  2 10:50:00 1990


    Another use for LOADALL is in DOS software that accesses extended
memory. The more standard method of switching to protected mode and then
resetting the CPU to re-enter real mode is too slow for some purposes. 
LOADALL, though hardly elegant, allows access to memory above the 1Mb line 
without having to switch modes. Microsoft's ramdisk and disk-caching
programs use LOADALL for accessing extended memory on a 286. As has been
noted, the 386 and 486 can switch much more easily between real and
protected modes, making LOADALL less useful.

   In protected mode, LOADALL could theoretically be used to get around
the CPU protection, since it allows you to directly set the 'shadow'
segment registers. In practice, the problem of setting up a table at
an absolute address under protected mode, plus the fact that LOADALL is
supposedly a priviledged instruction, would make this difficult. 

   Intel is pretty adamant about avoiding LOADALL, and guards information 
about it closely. I've never seen any 'official' documentation of it,
though such documentation undoubtedly exists.