Re: [ECOS] bit band macro names from Cortex-M3

[Ilija Kocho <ilijak at siva dot com dot mk>]

On 08.12.2011 11:19, Bernard Fouché wrote:
>
> Le 08/12/2011 10:54, Ilija Kocho a écrit :
>> Another way to specify bit-band area is by means of sections. Then you
>> declare ordinary variables with section attribute and ld will allocate
>> just enough memory for bit-band space.
>>
>> Ilija
> I don't understand your comment: in the Cortex-M3, bit band allows
> manipulating a single bit inside a 32 bits value in the peripheral
> area and in some parts of the internal MCU RAM. There is no allocation
> issue, it's just a very handy  memory remapping feature that maps 32
> megs of fake address space for each mega of 'real' addresses
> concerned. The LPC17XX chips have two such areas, one for the upper
> 16/32KB of internal RAM (the GPIO pin block is mapped in the same
> area), one for all MCU peripherals.

Following your reference (0x20000000) I am referring to AHB RAM (actual
base addresses are slightly different).

[snip]

> And AFAIK, the compiler/linker can't be aware of the bit band feature:
> it's a MCU option provided by ARM and the chip designer chooses to
> implement it or not, so the -mcpu=cortex-m3 can't consider it as a
> feature always available for all Cortex-M3 chips. I don't see how one
> could benefit from bit-band without explicitly using some kind of
> macro/function.

Instead of using raw memory it is possible to define a bit-band section
within ahb_sram_bank0 or ahb_sram_bank1 region. Then with section
__attribute__ you can instruct linker to store the variables in that
section. I find it convenient because you don't need to take care for
required space.
On a second thought you don't even need a special section. You can use
the existing ones: .ahb_sram0 or .ahb_sram1. Then for bit-band alias
addresses you can use your macros.

Ilija

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