Re: RE : [fpc-pascal] Reversing bit-order of byte
On 03-05-12 10:45, Ludo Brands wrote: You might also look into the RBIT instruction which does this in one cpu cycle. See http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0489c/Cihjgdi d.html and applied to the STM32 USART http://forum.micromouseonline.com/viewtopic.php?f=7&t=460 Ludo Hi, Thanks all for your replies. I already found the RBIT-instruction. I also found out I need to study the arm assembly language ;-) But how do I get my variable ? Is that in some register ? Is there any documentation about such things ? Then I can use inline assembly. Thanks, Koenraad Lelong. ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
RE : RE : [fpc-pascal] Reversing bit-order of byte
> Thanks all for your replies. > I already found the RBIT-instruction. I also found out I need > to study > the arm assembly language ;-) > > But how do I get my variable ? Is that in some register ? Is > there any > documentation about such things ? Then I can use inline assembly. > ARM has different ABI's. A pragmatic solution is to create a full pascal (inline if you like) function with the function signature you like and a dummy body, look at the assembler generated (use debug info whe compiling) and then insert your instructions. For example Function ReverseBits(b:byte):byte; Begin result:=b shl 2; End; Look at how result:=b shl 2 is implemented and you'll find immmedialy the registers used. Next you replace result:=b shl 2 with your asm block. Note that RBIT is a thumb 2 instruction. Ludo ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: RE : RE : [fpc-pascal] Reversing bit-order of byte
Hi, On Thu, 2012-05-03 at 12:54 +0200, Ludo Brands wrote: > > Thanks all for your replies. > > I already found the RBIT-instruction. I also found out I need > > to study > > the arm assembly language ;-) > > > > But how do I get my variable ? Is that in some register ? Is > > there any > > documentation about such things ? Then I can use inline assembly. > > > > ARM has different ABI's. A pragmatic solution is to create a full pascal We use the standard ARM EABI as gcc uses; for the purpose of this code, it should be sufficient to know that r0 contains the first parameter value, and function values are returned in r0 as well. Without testing, something like the following should be sufficient: function reverse(b : byte) : byte; assembler; nostackframe; asm rbit r0, r0 // rbit reverses the whole word, so now you have // your value in bits 31-24... so shift right by that amount // should fix this up (bits 23-0 contain junk, we shift that out // anyway) lsr r0, r0, #24 end; However, this may not compile because fpc may not recognize the rbit instruction. Following is a version that encodes the instruction directly, retrieved from disassembling some gcc code: function reverse(b : byte) : byte; assembler; nostackframe; asm .long 0xe6ff0f30 // rbit r0, r0 lsr r0, r0, #24 end; Hope this helps; I could not test due to lack of hardware. It may help to use -Cparmv6 and have your (cross-)assembler configured for armv6 code emission. Thomas ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: RE : RE : [fpc-pascal] Reversing bit-order of byte
On 03-05-12 13:27, Thomas Schatzl wrote: ... function reverse(b : byte) : byte; assembler; nostackframe; asm rbit r0, r0 // rbit reverses the whole word, so now you have // your value in bits 31-24... so shift right by that amount // should fix this up (bits 23-0 contain junk, we shift that out // anyway) lsr r0, r0, #24 end; However, this may not compile because fpc may not recognize the rbit instruction. Following is a version that encodes the instruction directly, retrieved from disassembling some gcc code: function reverse(b : byte) : byte; assembler; nostackframe; asm .long 0xe6ff0f30 // rbit r0, r0 lsr r0, r0, #24 end; Thanks guys, I'll try and test this weekend. Regards, Koenraad Lelong. ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: RE : RE : [fpc-pascal] Reversing bit-order of byte
On 03-05-12 13:27, Thomas Schatzl wrote: ... function reverse(b : byte) : byte; assembler; nostackframe; asm .long 0xe6ff0f30 // rbit r0, r0 lsr r0, r0, #23 end; Hi, I've been looking a bit further at this. I disassembled the code and this is the result : 8000150: e6ff0f30rbitr0, r0 8000154: ea4f 50d0 mov.w r0, r0, lsr #23 8000158: 46f7mov pc, lr Wouldn't it be possible to do rbitr0, r0, lsr #23 When reading the ARM programming manual I believe it should. But do I gain something ? Regards, Koenraad Lelong. ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: RE : RE : [fpc-pascal] Reversing bit-order of byte
Hi,
On Fri, 2012-05-04 at 14:14 +0200, Koenraad Lelong wrote:
> On 03-05-12 13:27, Thomas Schatzl wrote:
> ...
> > function reverse(b : byte) : byte; assembler; nostackframe;
> > asm
> >.long 0xe6ff0f30 // rbit r0, r0
> >lsr r0, r0, #23
> > end;
> >
>
> Hi,
>
> I've been looking a bit further at this.
> I disassembled the code and this is the result :
>
> 8000150: e6ff0f30rbitr0, r0
> 8000154: ea4f 50d0 mov.w r0, r0, lsr #23
The constant is wrong here, it should be #24, not #23 imo... typo.
> 8000158: 46f7mov pc, lr
>
It might be good to compile with -Cparmv6 (or -Cparmv7, do not know what
type the processor you use is), this generates a "better" return
instruction ("bx lr") - and for the rbit instruction you need armv6
already.
> Wouldn't it be possible to do
> rbitr0, r0, lsr #23
> When reading the ARM programming manual I believe it should.
>
No, rbit does not allow the "flexible operand" encoding.
> But do I gain something ?
>
If it were possible, at most a few cpu cycles.
Compared to the other variants (e.g. with the multiplies, except for the
table lookup) you already save a lot of cycles - although I guess they
would be sufficiently fast anyway, considering the typical uart speeds.
Thomas
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Re: RE : RE : [fpc-pascal] Reversing bit-order of byte
On 04-05-12 14:31, Thomas Schatzl wrote:
It might be good to compile with -Cparmv6 (or -Cparmv7, do not know what
type the processor you use is), this generates a "better" return
instruction ("bx lr") - and for the rbit instruction you need armv6
already.
Wouldn't it be possible to do
rbitr0, r0, lsr #23
When reading the ARM programming manual I believe it should.
No, rbit does not allow the "flexible operand" encoding.
But do I gain something ?
If it were possible, at most a few cpu cycles.
Compared to the other variants (e.g. with the multiplies, except for the
table lookup) you already save a lot of cycles - although I guess they
would be sufficiently fast anyway, considering the typical uart speeds.
Thomas
Hi Thomas,
My processor flags : -Wpstm32f103rb -Cparmv7m.
The #23 is no typo, I need to reverse 9 bits. I'm using 9 bit
synchronous transmission. My first usart-tests were at 4Mbps. The person
that made the driver recommends max. 6Mbps.
If I need more spare time I'll have to see what DMA can do.
Thanks for all information.
Regards,
Koenraad Lelong.
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Re: RE : RE : [fpc-pascal] Reversing bit-order of byte
On 04-05-12 14:53, Koenraad Lelong wrote: On 04-05-12 14:31, Thomas Schatzl wrote: ... Compared to the other variants (e.g. with the multiplies, except for the table lookup) you already save a lot of cycles - although I guess they would be sufficiently fast anyway, considering the typical uart speeds. Thomas Hi Thomas, My processor flags : -Wpstm32f103rb -Cparmv7m. The #23 is no typo, I need to reverse 9 bits. I'm using 9 bit synchronous transmission. My first usart-tests were at 4Mbps. The person that made the driver recommends max. 6Mbps. Hi, I tested some things this weekend. Strange things happen ;-) The code does not work. Trying to debug with gdb, around the code I got an error, something like 'recovering from a double fault'. After searching the STM32 forum of ST I found that some toolchains assemble the rbit instruction in a wrong way. Someone made a work-around : .word 0xFxAyFA9y //RBIT Rx,Ry x, y = 0 to F My Freepascal version is this : .word 0xFA90 .word 0xF0A0 Then when I disassemble in gdb, I get rbit r0,r0 Disassembling with objdump gives a strange result. I'm going to try this as soon as I can. Regards, Koenraad Lelong. ___ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/mailman/listinfo/fpc-pascal
Re: RE : RE : [fpc-pascal] Reversing bit-order of byte
On 7 May 2012, at 13:26, fpc-pascal-requ...@lists.freepascal.org wrote:
>
>
> Message: 4
> Date: Mon, 07 May 2012 12:59:58 +0200
> From: Koenraad Lelong
> Subject: Re: RE : RE : [fpc-pascal] Reversing bit-order of byte
> To: fpc-pascal@lists.freepascal.org
> Message-ID: <4fa7ab2e.9050...@de-brouwerij.be>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> On 04-05-12 14:53, Koenraad Lelong wrote:
>> On 04-05-12 14:31, Thomas Schatzl wrote:
> ...
>>>
>>> Compared to the other variants (e.g. with the multiplies, except for the
>>> table lookup) you already save a lot of cycles - although I guess they
>>> would be sufficiently fast anyway, considering the typical uart speeds.
>>>
>>> Thomas
>>
>> Hi Thomas,
>>
>> My processor flags : -Wpstm32f103rb -Cparmv7m.
>> The #23 is no typo, I need to reverse 9 bits. I'm using 9 bit
>> synchronous transmission. My first usart-tests were at 4Mbps. The person
>> that made the driver recommends max. 6Mbps.
>
> Hi,
>
> I tested some things this weekend. Strange things happen ;-)
> The code does not work. Trying to debug with gdb, around the code I got
> an error, something like 'recovering from a double fault'.
>
> After searching the STM32 forum of ST I found that some toolchains
> assemble the rbit instruction in a wrong way. Someone made a work-around :
> .word 0xFxAyFA9y //RBIT Rx,Ry x, y = 0 to F
> My Freepascal version is this :
> .word 0xFA90
> .word 0xF0A0
> Then when I disassemble in gdb, I get rbit r0,r0
> Disassembling with objdump gives a strange result.
>
> I'm going to try this as soon as I can.
>
> Regards,
>
> Koenraad Lelong.
I have beed using this:-
function bitreverse(input:dword):dword;assembler;nostackframe; {single
instruction rbit r0,r0 bit reverses r0}
asm
.long 0xf0a0fa90
end;
quite successfully.
You can then choose which byte(s) you need using normal pascal functions.
regards,
Geoffrey___
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