Writing portable code regarding the processor architecture/id

Ada beberapa isu utama saat menulis kode yang portabel sehubungan dengan arsitektur prosesor: endianness dan prosesor 32 vs. 64 bit.

Endianness

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

Free Pascal supports processors with two types of endianness:

1. Store the lowest value on the lowest address; longint(4) encoded as 04 00 00 00 (little endian)
2. Store the highest value on the lowest address; longint(4) encoded as 00 00 00 04 (big endian)

side note:

Middle endian, also called mixed endian, processors are rare nowadays. The best known, but now historic, middle endian processor is the PDP-11 from DEC.

Endianness 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, many older systems such as the HP3000 minicomputers, and mainframes such as the 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],' Your machine is Little Endian')

     else

         if x^.p[4]=5 then

             writeln(x^.p[1],' Your machine is Big Endian')

         else

             writeln(x^.p[1],' ',x^.p[2],' ',x^.p[3],' ',x^.p[4],' 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.

Penjajaran

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.

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.

This is in line with most open Unix platforms. In the commercial world, there are some exceptions like Tru64, with is ILP64

Konvensi pemanggilan

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

In general avoid relying on internal knowledge, like if a pass by const is on the stack or by value.