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Re: Microcontroller software stacks

From Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups comp.lang.c
Subject Re: Microcontroller software stacks
Date 2026-08-14 12:51 -0700
Organization A noiseless patient Spider
Message-ID <865x1c7a63.fsf@linuxsc.com> (permalink)
References (3 earlier) <10vlvie$2ne3j$2@dont-email.me> <10vmh2e$b44$1@reader1.panix.com> <fkCTR.17300$_BG8.7520@fx24.iad> <86h5mv8umk.fsf@linuxsc.com> <oac_R.3743$Zhg2.1242@fx12.iad>

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scott@slp53.sl.home (Scott Lurndal) writes:

> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>
>> scott@slp53.sl.home (Scott Lurndal) writes:
>>
>>> One might also define data structures for control and status
>>> registers using bitfield structs.
>>
>> Yeah.  This kind of application (among others) I consider one of
>> the motivating forces behind bitfields.
>>
>> [Some whitespace trimming done in the excerpt below.]
>>
>>> e.g. for the SATA UAHC_GLB_OOBR register:
>>>
>>> union UAHC_GBL_OOBR {
>>>   uint32_t u;
>>>   struct UAHC_GBL_OOBR_s {
>>> #if __BYTE_ORDER == __BIG_ENDIAN
>>>     uint32_t we     :  1; /**< R/W/H - Write enable. */
>>>     uint32_t cwmin  :  7; /**< R/W/H - COMWAKE minimum value [...] */
>>>     uint32_t cwmax  :  8; /**< R/W/H - COMWAKE maximum value [...] */
>>>     uint32_t cimin  :  8; /**< R/W/H - COMINIT minimum value [...] */
>>>     uint32_t cimax  :  8; /**< R/W/H - COMINIT maximum value [...] */
>>> #else
>>>     uint32_t cimax  :  8;
>>>     uint32_t cimin  :  8;
>>>     uint32_t cwmax  :  8;
>>>     uint32_t cwmin  :  7;
>>>     uint32_t we     :  1;
>>> #endif
>>>   } s;
>>> };
>>
>> To me it seems kind of goofy to use uint32_t for the bitfields type.
>> I would just use unsigned, which is just as sure to work as intended,
>> isn't it?
>
> The SATA hardware register is defined as a 32-bit register in the
> SATA specification.  Therefore we explicitly declare it as such.

I understand the motivation for using uint32_t for the union member
u.  My question is only about the type used for the bitfields.  Do
you know of any platform, or even suspect that there might be a
platform, where using 'unsigned' rather than 'uint32_t' for the type
of the bitfields makes any difference at all?

> There are other hardware registers in our implementation of the SATA
> controller that are defined as 64-bit registers, for those we use
> uint64_t (rather than relying on 'unsigned long' for 64-bit linux
> or 'unsigned long long' for 32-bit OS - and this code was designed
> to be compiled for both 32-bit and 64-bit targets originally).

Sure, for the non-bitfield member.  My question is only about
(unsigned) bitfields all of width 8 or less.

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Thread

Re: Microcontroller software stacks (was Re: this girl calls c ugly) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-06-21 14:13 -0700
  Re: Microcontroller software stacks (was Re: this girl calls c ugly) David Brown <david.brown@hesbynett.no> - 2026-06-22 08:58 +0200
    Re: Microcontroller software stacks (was Re: this girl calls c ugly) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-22 03:35 -0700
      Re: Microcontroller software stacks (was Re: this girl calls c ugly) cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-22 10:50 +0000
      Re: Microcontroller software stacks (was Re: this girl calls c ugly) David Brown <david.brown@hesbynett.no> - 2026-06-22 12:59 +0200
      Storage needed when there are bit-field members Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-06-28 09:42 -0700
        Re: Storage needed when there are bit-field members Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-28 18:06 -0700
          Re: Storage needed when there are bit-field members Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-28 20:20 -0700
            Re: Storage needed when there are bit-field members Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-14 14:56 -0700
              Re: Storage needed when there are bit-field members scott@slp53.sl.home (Scott Lurndal) - 2026-08-14 22:30 +0000
                Re: Storage needed when there are bit-field members Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-15 16:10 -0700
                Re: Storage needed when there are bit-field members scott@slp53.sl.home (Scott Lurndal) - 2026-08-16 15:20 +0000
          Re: Storage needed when there are bit-field members Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-14 14:19 -0700
  Re: Microcontroller software stacks (was Re: this girl calls c ugly) cross@spitfire.i.gajendra.net (Dan Cross) - 2026-06-22 10:45 +0000
    Re: Microcontroller software stacks (was Re: this girl calls c ugly) scott@slp53.sl.home (Scott Lurndal) - 2026-06-22 15:23 +0000
    Re: Microcontroller software stacks (was Re: this girl calls c ugly) Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-14 13:23 -0700
      Re: Microcontroller software stacks (was Re: this girl calls c ugly) cross@spitfire.i.gajendra.net (Dan Cross) - 2026-08-18 20:32 +0000
  Re: Microcontroller software stacks (was Re: this girl calls c ugly) scott@slp53.sl.home (Scott Lurndal) - 2026-06-22 15:04 +0000
    Re: Microcontroller software stacks (was Re: this girl calls c ugly) Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-06-22 13:02 -0700
    Re: Microcontroller software stacks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-14 12:51 -0700
      Re: Microcontroller software stacks Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> - 2026-08-15 05:02 +0800
        Re: Microcontroller software stacks bart <bc@freeuk.com> - 2026-08-15 01:18 +0100
          Re: Microcontroller software stacks Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> - 2026-08-16 13:52 +0800
      Re: Microcontroller software stacks Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-08-14 14:06 -0700
      Re: Microcontroller software stacks scott@slp53.sl.home (Scott Lurndal) - 2026-08-14 21:42 +0000
        Re: Microcontroller software stacks Michael S <already5chosen@yahoo.com> - 2026-08-15 22:13 +0300
          Re: Microcontroller software stacks scott@slp53.sl.home (Scott Lurndal) - 2026-08-15 19:39 +0000
            Re: Microcontroller software stacks Michael S <already5chosen@yahoo.com> - 2026-08-15 23:22 +0300
              Re: Microcontroller software stacks scott@slp53.sl.home (Scott Lurndal) - 2026-08-16 14:38 +0000
                Re: Microcontroller software stacks Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-08-16 16:11 -0700
        Re: Microcontroller software stacks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-15 16:16 -0700
          Re: Microcontroller software stacks cross@spitfire.i.gajendra.net (Dan Cross) - 2026-08-18 20:34 +0000
        Re: Microcontroller software stacks Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-08-15 17:31 -0700

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