Packed

The reserved word packed tells the compiler to use as little memory as possible for a particular complex data type. Without specifying packed, the compiler may insert extra unused bytes between members in order to align the data on full word boundaries for faster access by the CPU.

program packedDemo(input, output, stderr);

type
    signedQword = record
        value: qword;
        signum: -1..1;
    end;
    
    signedQwordPacked = packed record
        value: qword;
        signum: -1..1;
    end;

begin
    writeLn('      signedQword: ', sizeOf(signedQword), 'B');
    writeLn('signedQwordPacked: ', sizeOf(signedQwordPacked), 'B');
end.

So packed sacrifices some speed while reducing the memory used.

Circumvention

Smart ordering of record elements can mitigate the issue. In the example above signedQword as well signedQwordPacked have signum located at an offset of 8. That is OK. If for any reason value and signum were listed in reverse order, signum would appear in both versions at an offset of 0, but in the packed version value has an offset of 1 Byte. This might cause a potentially slower access to the value field.

See also

• {$align}
• {$pack}
• procedures pack and unpack
• “Packing and unpacking an array” in § “Arrays” of the Reference Guide
• bitpacked

From the Free Pascal wiki, page "Packed", as it stood on 2020-05-16 after a change by Kai Burghardt, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's Licensing page. If you have concerns about this material, message the admin.