[prev in list] [next in list] [prev in thread] [next in thread] 

List:       uclinux-dev
Subject:    Re: [uClinux-dev] Still: kernel and thumb code
From:       Erwin Authried <eauth () softsys ! co ! at>
Date:       2001-10-31 11:02:31
[Download RAW message or body]

Thierry Durand[SMTP:tdurand@atmel.fr] wrote:
> Adrian,
> 
> I have worked on the kernel in thumb some time ago. My intent was to get
> a full thumb code (kernel + libs + apps). Most of the work was not
> really in the syscalls which can be easily used in thumb since there are
> less than 256 of them and the thumb swi can still be used. The big part
> was the port of many small bits of asm code using arm-only instructions.
> Most of them are macros defined in include/asm.
> Instead of porting each and every bit using arm-only into similar thumb
> code I have created a new swi code (not used by the syscalls) to emulate
> arm instructions in an exception routine. The instruction to be emulated
> was stored in a register prior to the swi ARM_EMULATE call. This was not
> done in the same file as the syscalls to avoid confusions between a
> normal syscall and the arm emulation. It is true that there is a small
> overhead in doing so (more instructions, but they are 16-bit instead of
> 32). An advantage is that it still works in arm.
> 
> I have not tried interworking. My conception is that you are almost
> always in one of the two following situations:
> - your priority is system performance. You use 32-bit memories and code
> everything in arm to take advantage of performance.
> - your priority is system cost. You use 16-bit memories and code
> everything (except startup and exceptions entry) in thumb to take
> advantage of compactness.
> As you can see there is few room for interworking. But I understand that
> specific situations may require it (e.g. processors with a 16-bit bus
> interface, but some amount of internal 32-bit memory).
> A common compromise today is to use 16-bit memories together with arm
> code leading to a dramatic 50% drop in system performance. The existence
> of a thumb linux would be really a great thing.
> 
> Please comment !
> --
> Thierry-
> 
That sounds pretty interesting, especially with Atmel's AT91 that has only 
a 16-bit interface. I have not yet thought about using thumb mode, but I have
experimented with moving parts of the code into the internal 32-bit RAM. If you
do that for the i/o instructions, for example, there is a huge performance win.
It can be done in the same way as the init functions are moved into a special
segment. Is the work that you have done available for review?

-Erwin

This message resent by the uclinux-dev@uclinux.org list server http://www.uClinux.org/

[prev in list] [next in list] [prev in thread] [next in thread]