Path: utzoo!utgpu!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!POSTGRES.BERKELEY.EDU!dillon
From: dillon@POSTGRES.BERKELEY.EDU (Matt Dillon)
Newsgroups: comp.sys.amiga.tech
Subject: Re: ExecBase
Message-ID: <8905242209.AA20811@postgres.Berkeley.EDU>
Date: 24 May 89 22:09:13 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 46

:In article <31472@sri-unix.SRI.COM>, thomas@cbmvax.UUCP (Dave Thomas QA)
:writes:
:> In article <10702@cit-vax.Caltech.Edu> bdiscoe@tybalt.caltech.edu.UUCP
:> (Ben W. Discoe) writes:
:>> 
:>>    I'm trying to find Execbase.  I seem to remember reading somewhere that 
:>> it's always at 0000004 in memory, but it doesn't seem to be there, and
:>> I can't find my reference any more.  Could anyone tell me where it is or
:>> how to find it?

	As people have already noted, the pointer to ExecBase can be
found at location 4.

assembly:
	move.l	4,A6

C:
	SysBase = *(struct ExecBase **)4;

	I strongly suggest though that people who write programs or
assembly routines NOT always get execbase from location 4.  The reason is
simply that from what I can tell location 4 is in CHIP memory and thus
subject to fetch delays when one is in a high bandwidth video mode.

	The proper way to access the execbase is to assign it to a
variable at program startup/init and then reference it from the variable.
For example, one always references 'SysBase' from C... the C startup module
always does a 'move.l 4,_SysBase'.  Since SysBase will be in FAST memory 
if the machine has fast memory, the processor will never freeze trying to
access it no matter what video mode you are in.

	The people at C-A have been moving towards getting as much stuff
out of CHIP memory as possible in hopes of eventually making CHIP memory
DMA-only memory for the custom chips, thus making all programs run much
faster in the higher video modes.  The main problem is that currently the exec
library vectors are created before fast memory is turned on and thus usually
gets put into CHIP memory.... the infamous location $676 (p.s. NEVER EVER
assume ExecBase is at $676!).

	Also:  The 68010 and beyond allow you to change the vector
base and this most certainly will be implemented in a future release so
the interrupt vectors can be moved out of CHIP memory.  This is one reason
why serial at high baud rates currently goes to hell when you are in HAM or
640xNx4 mode.

					-Matt