[fpc-pascal] 64 bits

[Felipe Monteiro de Carvalho]

Ok, as requested, I´m pasting again the contents of the wiki page. =)


Writing portable code regarding the processor architecture
>From FPCWiki

There are several main issues when writing code which is portable
regarding the processor architecture: endianness and 32 vs. 64 bit
processors.
Table of contents [showhide]
1 Endianness
2 Alignment
3 32 Bit vs. 64 Bit
4 Calling conventions

4.1 x86
4.2 ARM
4.3 68K
[edit]
Endianness

Endianness is the way how values larger than a byte (e.g. 16/32/64-bit
integers) are stored by the processor.

Generally there are two ways:

   1. Store the lowest value on the lowest address; longint(4) encoded
as 04 00 00 00 (little endian
(http://en.wikipedia.org/wiki/Little_endian))
   2. Store the highest value on the lowest address; longint(4)
encoded as 00 00 00 04 (big endian
(http://en.wikipedia.org/wiki/Big_endian))

This is generally a given choice per processor family, but some
families of processors can be either big endian or little endian
depending on the mainboard they are attached to (ARM, PPC).

The best known little endian processor family is x86, the processor
family used in PCs, and its brethren x86-64. Typical big endian
processors are PPC (usually, see above note), and m68k
(http://en.wikipedia.org/wiki/68k), many older systems such as the
PDP-11 (http://en.wikipedia.org/wiki/PDP-11) and HP3000
(http://en.wikipedia.org/wiki/HP3000) minicomputers, and mainframes
such as the IBM 370 (http://en.wikipedia.org/wiki/IBM_370) (Z series).

Since TCP/IP specifies that all protocol header structures that go
over the wire should be big endian, so this notation is sometimes also
refered to as network order.

Endianness is important

   1. when exchanging data between different architectures
   2. when accessing data sometimes as (an array of) a larger type,
like integer, and sometimes as (an array of) a byte.

An example of the latter:

    Type
         Q = RECORD
             case boolean of
                 true: (i:integer);
                 false: (p:array[1..4] of byte)
         END;

    Var x:^Q ;

    begin
         new(x);
         x^.i:=5;
         if x^.p[1]=5 then
             writeln(x^.p[1]); writeln('Your machine is Little Endian')
         else
             if x^.p[4]=5 then
                 writeln(x^.p[1]); Writeln('Your machine is Big Endian')
             else
                 writeln(x^.p[1],' ',x^.p[2],' ',x^.p[3],' ',x^.p[4]);
writeln('Your machine''s endianness is indeterminate; please report
the results to the compiler development team');
         writeln;

        {Make it wait so we can see the results }

         write('Press enter when you finish reading this ');
         readln;
    end.

On little endian machines (PCs), the above code will write 5 (since
longint(5) is stored as 05 00 00 00 in memory), while on big endian
machines (e.g. Powermacs) it will write 0 (since longint(5) is stored
as 00 00 00 05 in memory). (If you get the report your machine is
indeterminate, please report on this wiki page what processor does
this and what you get!)

To determine the endianness of the processor, use the ENDIAN_BIG or
ENDIAN_LITTLE (or FPC_LITTLE_ENDIAN and FPC_BIG_ENDIAN starting from
version 1.9) defines that are defined by freepascal automatically
depending on the processor.
[edit]
Alignment

Some processors will allow improperly aligned data but with reduced
efficiency (IBM 370/zSeries). Some processors generate hardware
processor exceptions when data is badly aligned (e.g. Alpha or ARM).
Sometimes the hardware exceptions are caught and fixed using emulation
by the OS, but this is very slow, and should be avoided. This can also
cause records to have different sizes, so always use
sizeof(recordtype); as size of a record. If you define a packed
record, try to ensure that data is naturally aligned, if possible.
Some processors only have alignment requirements for certain types of
data, like floating point (e.g. older PowerPCs).

To check if the CPU requires proper alignment, check the
FPC_REQUIRES_PROPER_ALIGNMENT (version 1.9 and higher) define. On 32
Bit CPUs this usually means that data up to a size of 4 must be
naturally aligned. If you want to access unaligned data, use the move
procedure to move it to an aligned location before processing it. The
move procedure takes care of unaligned data and handles it properly.

There are multiple strategies for aligning:

    * align every field on a multiple of a certain value (typically a
power of two, 1,2,4,8. 1 is equivalent to "packed")
    * pad before every field such that it is aligned on a multiple of
its size (so a longint on 4, an int64 on 8 bytes etc). This is
typically done by C compiler, which is why FPC calls it {$packrecords
C}.

(For arrays or nested records, the size of their largest sub unit is used)

Mac OS X {$packrecords C} seems to pad the entire record at the end to
make it a certain size. This is still being investigated, and probably
will be fixed in compiler.
[edit]
32 Bit vs. 64 Bit

To achive maximum compatiblity with older code, FPC doesn't change the
size of predefined data types like integer, longint or word when
changing from 32 to 64 Bit. However, the size of a pointer is 8 bytes
on a 64 bit architecture so constructs like longint(pointer(p)) are
doomed to crash on 64 bit architectures. However, to allow you to
write portable code, the FPC system unit introduces the types PtrInt
and PtrUInt which are signed and unsigned integer data types with the
same size as a pointer.

Keep in mind that the size change of the "pointer" type also affects
record sizes. If you allocate records with fixed sizes, and not with
new or with getmem (<x>,sizeof(<x>)), this will have to be fixed.
[edit]
Calling conventions

    * For IBM 370 and zSeries, see the ZSeries page for further discussion.

[edit]
x86
[edit]
ARM
[edit]
68K