Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!samsung!uunet!microsoft!kevinro
From: kevinro@microsoft.UUCP (Kevin ROSS)
Newsgroups: comp.os.os2
Subject: Re: overlays in 32-bit OS/2
Keywords: Virtual Memory 80386
Message-ID: <54378@microsoft.UUCP>
Date: 30 Apr 90 20:24:21 GMT
References: <1990Apr27.155908.4260@geac.com> <273@caslon.cs.arizona.edu> <4401@plains.UUCP>
Reply-To: kevinro@microsoft.UUCP (Kevin ROSS)
Organization: Microsoft Corp., Redmond WA
Lines: 67

In article <4401@plains.UUCP> harlow@plains.UUCP (Jay B. Harlow) writes:
>In article <273@caslon.cs.arizona.edu> roussos@cs.arizona.edu (George E. Roussos) writes:
>>In article <1990Apr27.155908.4260@geac.com> mike@geac.com (Mike Sweet) writes:
|||    I'm currently using OS/2 version 1.1 and managing to acieve code
||| overlays with DosCreateCSAlias.
||| [stuff deleted - how will this work on 32 bit OS/2].
|||    Anyone know the answer to this? (i.e. can I read code off the disc
||| into a piece of memory and then jump into it) [in 32 bit OS/2] ?

Why the need for code overlays? If you are creating code on the fly,
then DosCreateCSAlias will work. However, there is no such 32-bit function
under OS/2 v2.0. The 16-bit version of the API will still work under 2.0.

||Someone please correct me if I am wrong, but in theory if a program is 
||running under an OS that takes advantage of the 386 architecture, it 
||does not need an overlay scheme to use code that can't be loaded into 
||memory at run time.  This is thanks to the built in hardware support
||for virtual memory in the 386.  

|Have you used OS/2 & the DosCreateCSAlias system call????.
|
|the DosCreateCSAlias is used to create an excutable alias to a data 
|segment,  For it is bad 'karma' to go executing DATA (good sign of a
|bug in your program).  Now you have this routine that compiles a 
|'language' on the fly (imcremental compiling, speadsheets, graphing...)
|that you have NO IDEA what the functions are going to be until
|the user is running you program (god knows where & why).

Executing a DATA segment isn't just bad Karma, it is not possible. CODE
segments are all executable. You cannot write to them. DATA segments 
are marked as read/write, and cannot be executed. This is enforced by
the chip.

|yes OS/2 2.0 is suppose to use PAGED V.M. as oppose to SEGMENTED V.M. &
|only support a 'flat'? 32bit address space (not sure if seperate I&D),
|if you have seperate I&D, then you need to alias, because a data
|address is different from a inst address in seperate I&D

The I&D space is the same. 

Under v2.0, if you want to have the same functionallity as DosCreateCSAlias,
then you would use the following constructs:
 
...

// Make a variable for the function address

 (void * pfnAddr)();	

// Allocate a region to create the code in.

  DosAllocMem(&(void *)pfnAddr,ulSize,PAG_READ|PAG_WRITE|PAG_COMMIT);

// Call some routine that writes code into your region

  CreateSomeCodeFunction(pfnAddr);

// Setup the memory as executable

  DosSetMem(pfnAddr,ulSize,PAG_EXECUTE);

// Call your function

  pfnAddr();


I have never actually tried this, but it should work.