[ECOS] Question about fixed vector table(ARM).

Gary Thomas gthomas@redhat.com
Thu May 16 04:46:00 GMT 2002


On Thu, 2002-05-16 at 02:55, HuangQiang wrote:
> Thank you very much. I still have one question about the fixed vector table.
> In ARM vectors.s file takes care of the vector table(from 0x00) setup and
> fixed vector table(from 0x20) setup. While branch to the vector it first
> branch to the vector table(from 0x00) and then directed to fixed vector
> table(from 0x20) then to the actual services routine. Why this fixed vector
> table is needed here? Can't we just setup the vector table (from 0x00) and
> branch to the services routine indirection of fixed vector table?
> 

The ARM instruction set doesn't provide a way to jump directly to an 
arbitrary location in a single instruction which is required by the 
interrupt/exception processing.  The instruction used:
        ldr     pc,.reset_vector                // 0x00
is a single instruction which picks up an address and jumps to it. The
contents of ".reset_vector" can be set to whatever is desired and is set
up by eCos applications as they start up.

> Thank you very much.
> 
> best regards!
> 
> james
> 
> -----Original Message-----
> From: jlarmour@cambridge.redhat.com [mailto:jlarmour@cambridge.redhat.com]On
> Behalf Of Jonathan Larmour
> Sent: 14 May 2002 22:24
> To: HuangQiang
> Cc: eCos
> Subject: Re: [ECOS] Does the fixed_vector section have anything to do with
> the virtual vector table?
> 
> 
> HuangQiang wrote:
> >
> > Hi all:
> > Question as subject. Thank you for your reply.
> 
> Not at all. Virtual vectors are "soft" vectors - purely there for things
> that software requires. They're nothing to do with hardware devices. That's
> why, for example, the virtual vector table layout is the same for every
> ecos target.
> 
> Jifl
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