Chapter 1

To be executed, an executable has to be loaded. This means that its program and data
areas must be prepared for use. There are two cases here:
� The first case is an executable in ROM (drive z:). ROM-based executables are
executed in-place.
� Executables not in ROM must first be loaded into RAM. This applies to executables on
memory cards (removable media drive d:), or in the system Flash, or RAM, disk (drive
c:).
2.4.2 Loading and Sharing
Executables may contain three types of binary data:
� program code
� read-only static data
� read/write static data.
When a .exe is first loaded into RAM, it has its own areas for code, read-only data and
read/write data. If a second version of the same .exe is launched, the code and static data
are shared, and a new area will be allocated only for the read/write data. Even on first use,
ROM-based
.exes allocate a RAM area only for read/write data – the program code and read-only data
are read directly from ROM.
As is explained later in this chapter, Symbian OS DLLs contain no read/write data and so do
not need to allocate RAM to hold it. When a DLL is first loaded into RAM, it is relocated to a
particular address. If a second thread requires the same DLL, it doesn't have to load it – it
merely attaches the copy already there. The DLL appears at the same address in all threads
that use it. ROM-based DLLs are not actually loaded at all – they are simply used in-place in
ROM.
Symbian OS maintains reference counts so that a RAM-based DLL or.exe is only discarded
when the count drops to zero.
2.4.3 Cutting Down the Size
Symbian OS optimizes the formats used for DLLs in order to make them as compact as
possible in ROM and RAM.
� Most systems supporting DLLs or analogous concepts offer two options for identifying
the entry points in them. You can refer to the entry points either by name or by ordinal
number. Names are potentially long and wasteful of ROM and RAM. So Symbian OS
uses link-by-ordinal exclusively.
� Loading into RAM can involve locating the executable at an address that cannot be
determined until load time: this means that relocation information has to be included in
the executable format. Loading into ROM happens effectively at build time. So Symbian
OS ROM- building tools perform the relocation and strip the DLLs of their relocation
information to make them still smaller.
The Symbian OS link-by-ordinal scheme affects the disciplines used for binary compatibility
(a future release of a DLL must use exactly the same ordinals as the previous release). The
preloading scheme means, among other things, that you can't take an executable out of the
ROM and deliver it in another package for RAM loading. These are largely matters for
Symbian OS OEMs and I shan't be describing them further in this book.
2.4.4 Launching Applications and Servers