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From: stephens@latcs1.oz.au (Philip J Stephens)
Newsgroups: comp.sys.apple2
Subject: Re: apple // and multitasking, perfect together! (!!!!!!!!!!)
Message-ID: <7377@latcs1.oz.au>
Date: 7 Mar 90 15:21:04 GMT
References: <14298@phoenix.Princeton.EDU> <17861@boulder.Colorado.EDU>
Organization: Comp Sci, La Trobe Uni, Australia
Lines: 73

In article <17861@boulder.Colorado.EDU>, Christopher Hassell writes:
> 
> 	For timed interrupts:
> 
> 		One can compile around a break-point-table <literally> which
> 		gives positions in code where break-instructions can be quickly
> 		put and where the code can be restored after control has been 
> 		handed to the kernel. <for re-entry>

   An intriguing idea.  This is ridiculously easy to implement: All the OS
needs to do is save the registers (and possibly the interrupt address and
status register from the stack), then restore everything before JMPing back
into the program.  Very good idea.  I had never thought of using BRK
instructions for that purpose.
   For even more simplicity, the compiler really only needs to insert BRK
instructions after every _atomic_ statement (not compound ones) of a high
level language.  This ought to provide reasonable response time, and will
minimize the amount of context saving that is needed to switch processes.
 
> 	For protected modes:
>               [details deleted]
>       For virtual-memory-paging:
>               [details deleted]
>       [other implementation details deleted]

   How about this for an idea: get the compiler to generate code to reside
in a fixed area of RAM (e.g. $800 onwards). 
   All branch, JMP and JSR instructions that fall inside a common page are
normal 6502 code.  All other such instructions become JSR's to the OS followed
by the required address, which the OS can then handle.  
   All data references are left untouched.

   All running processes are copied into a disk file that represents the
programs entire memory "image" at any given time.
   The OS loads up the current "program" page of a process into it's true
address range, along with all of the "data" pages that could possibly be
accessed by the code in the "program" page.  It then lets it run until it
hits a breakpoint, or when a JSR to the OS is executed.
   Thus the OS must scan the program page, looking at all of the data
references to see what range the access could lie in.  For absolute modes,
this is obvious.  For indexed absolute modes, you need to cater for the
entire 256 bytes of offset (thus a maximum of 2 pages of data to load). 
Indirect modes are more difficult: the compiler must generate a JSR to the
OS whenever the base address is modified, so that the OS can load in the
2 pages that are necessary at that time.
   Better still, the compiler could generate a list of "data" pages to be
loaded for each "program" page.  This might speed up the loading of "data"
pages considerably.

   Whenever processes are switched, the OS saves all of the data pages 
currently loaded for that process on it's image file, and then loads up the
approapiate pages for the next process.
  
   This technique gives maximum speed, with minimum code modification.  Only
loading up the required data pages will slow it down, and jumps out of the
current page.  I think you'll find however that in any standard "program"
page, references to data will fall within a reasonable number of "data" pages. 
(Particually since most high-level languages are stack oriented).  Jump's
out of pages would be few and far between.

                    --------------------------------

   So there you have it!  One day ago, I was complaining that it would be
too difficult to implement; today someone comes up with a brainstorm - and
now anything is possible :-) :-) :-)

   Any improvements, or comments on gross mistakes ;-) welcome.

</\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\></\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\>
<  Philip J. Stephens                ><   "Many views yield the truth."        >
<  Hons. student, Computer Science   ><   "Therefore, be not alone."           >
<  La Trobe University, Melbourne    ><          - Prime Song of the viggies   >
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