Re: SVC (SWI) for Thumb

[Matt Thomas <matt%3am-software.com@localhost>]

On Aug 14, 2013, at 3:08 PM, David Laight <david%l8s.co.uk@localhost> wrote:

> On Wed, Aug 14, 2013 at 07:37:44AM -0700, Matt Thomas wrote:
>>>>    movs    r0, #(syscall - 254)/8
>>>>    lsls    r0, r0, #3
>>> 
>>> Shouldn't that be a shift by 8 ?
>> 
>> No.  The add can only take a 3 bit value (hence, the divide by 8)
>> 
>>>>    adds    r0, r0, #((syscall - 254) & 7)
> 
> Hmmm... I was looking at the 0x3xxx instruction, not the 0x1exx one.

Ya.  I changed it to use the 0x3xxx instruction

So now it's either

Less than 255:
        movs    r0, #syscall

armv7:
        movw    r0, #syscall

multiple of 4:
        movs    r0, #(syscall/4)
        lsls    r0, r0, #2

multiple of 2:
        movs    r0, #(syscall/2)
        lsls    r0, r0, #1

everything else:
        movs    r0, #(syscall/256)
        lsls    r0, r0, #8
        adds    r0, r0, #(syscall&255)