[M3devel] Modula-3 bitfields

Fri Aug 24 20:10:35 CEST 2012

The Green Book doesn't actually define alignment rules does it?

Your example as I recall also specified precisely 32 bits.

I can think of lots of ways things can go wrong.

Why would you want to tie the hands of the compiler writer to a particular
layout?  Or clutter the interface with lots of pragmas...

Yeah OK there was a day when that was the way to do things.... I think
that time is past.  The method I suggested could take advantage of
situations where you know precisely how to construct the packed types.
If you wanted.  But it would also be entirely portable and independent
of compilers.  Heck you could generate C code at the same time and have
free cross-language interoperability.  (Or any other sets of languages.)

You could also split bitfields in funny ways... 

Why would you do things any other way, really?  

Here's an example.  A PCI Express DLLP is 32 bits of payload followed by
a 16-bit CRC.  Should I be able to write this as follows:

DLLP = RECORD
  payload : BITS 32 FOR [0..16_ffffffff];
  crc     : BITS 16 FOR [0..16_ffff];
END;

What then is the layout of 

ARRAY [0..1] OF DLLP;

?

    Mika

=?utf-8?Q?Dragi=C5=A1a_Duri=C4=87?= writes:
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>What mental model?
>
>We have network order. Everything network is bit streams, from left to =
>right, most significant bits/bytes. No need for mental model here, any =
>communication oriented ISO, ETSI, ITU standard=E2=80=A6=20
>
>I don't understand why you insist on C syntax here. C has its own =
>problems and reasons why it's at war with most things mental. Please =
>refer to my initial example instead.
>
>--
>Divided by a common language
>
>Dragi=C5=A1a Duri=C4=87
>dragisha at m3w.org
>
>
>
>
>On Aug 24, 2012, at 7:48 PM, Jay K wrote:
>
>> printf("%X %X\n", c.u.s.a, c.u.s.b);
>> What do people expect?
>> Do people really have a mental model as to what the layout rules are =
>for bitfields?
>> Are they obvious enough that every C compiler writer has actually =
>implemented them the same and they become in a sense portable?=20
>
>
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><html><head></head><body style=3D"word-wrap: break-word; =
>-webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">What =
>mental model?<div><br></div><div>We have network order. Everything =
>network is bit streams, from left to right, most significant bits/bytes. =
>No need for mental model here, any communication oriented ISO, ETSI, ITU =
>standard=E2=80=A6 </div><div><br></div><div>I don't understand why =
>you insist on C syntax here. C has its own problems and reasons why it's =
>at war with most things mental. Please refer to my initial example =
>instead.</div><div><br><div apple-content-edited=3D"true">
><span class=3D"Apple-style-span" style=3D"border-collapse: separate; =
>color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; =
>font-variant: normal; font-weight: normal; letter-spacing: normal; =
>line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: =
>0px; text-transform: none; white-space: normal; widows: 2; word-spacing: =
>0px; -webkit-border-horizontal-spacing: 0px; =
>-webkit-border-vertical-spacing: 0px; =
>-webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: =
>auto; -webkit-text-stroke-width: 0px; font-size: medium; "><span =
>class=3D"Apple-style-span" style=3D"border-collapse: separate; color: =
>rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: =
>normal; font-weight: normal; letter-spacing: normal; line-height: =
>normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; =
>text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; =
>-webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: =
>0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: =
>auto; -webkit-text-stroke-width: 0px; font-size: medium; "><div =
>style=3D"word-wrap: break-word; -webkit-nbsp-mode: space; =
>-webkit-line-break: after-white-space; "><div>--</div><div>Divided by a =
>common language</div><div><br></div><div>Dragi=C5=A1a =
>Duri=C4=87</div><div><a =
>href=3D"mailto:dragisha at m3w.org">dragisha at m3w.org</a></div><div><br></div>=
></div></span><br class=3D"Apple-interchange-newline"></span><br =
>class=3D"Apple-interchange-newline">
></div>
><br><div><div>On Aug 24, 2012, at 7:48 PM, Jay K wrote:</div><br =
>class=3D"Apple-interchange-newline"><blockquote type=3D"cite"><span =
>class=3D"Apple-style-span" style=3D"border-collapse: separate; =
>font-family: Helvetica; font-style: normal; font-variant: normal; =
>font-weight: normal; letter-spacing: normal; line-height: normal; =
>orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: =
>none; white-space: normal; widows: 2; word-spacing: 0px; =
>-webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: =
>0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: =
>auto; -webkit-text-stroke-width: 0px; font-size: medium; "><span =
>class=3D"Apple-style-span" style=3D"font-family: Calibri; font-size: =
>16px; ">printf("%X %X\n", c.u.s.a, c.u.s.b);<br>What do people =
>expect?<br>Do people really have a mental model as to what the layout =
>rules are for bitfields?<br>Are they obvious enough that every C =
>compiler writer has actually implemented them the same and they become =
>in a sense portable?<span =
>class=3D"Apple-converted-space"> </span></span></span></blockquote></=
>div><br></div></body></html>=
>
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