Chapter 1

Figure 18.7
If Symbian OS had no FBS, then a client program that wanted to blit a bitmap to the screen
would have two options:
� If the bitmap is already in ROM, the client can pass the address of the bitmap to the
window server, and the window server can use the BITGDI to blit the bitmap from the
ROM to the screen window.
� If the bitmap is created and drawn by the client in its own RAM, the client uses the
BITGDI to draw the bitmap and then sends a message to the window server to draw the
bitmap. The window server must then copy the bitmap from the client into its own
memory and use the BITGDI to blit it to screen. This would involve considerable
interthread data transfer from client to window server and also serious space allocation
issues inside the window server.
To avoid the waste of both space and time involved in this kind of data transfer, the font and
bitmap server mediates access to a shared heap that contains all RAM-based bitmaps. With
the FBS, the following take place:
� The usage patterns for ROM-based bitmaps are essentially unchanged.
� To use a RAM-based bitmap, the client requests the FBS create the bitmap in its
shared heap. The client uses the BITGDI to draw to the bitmap. The client locks the
bitmap before it starts drawing and unlocks it again when it has finished.
� When the client wants the window server to draw the bitmap to the screen, it sends a
request specifying the bitmap's FBS-owned handle. The window server then uses this
handle to identify the address of the bitmap in its address space (which may be different
from the address in the client's). Finally, the window server uses the BITGDI to blit the
bitmap to the screen.
This makes Symbian OS very efficient at handling bitmaps, which in turn contributes
significantly to the speed of its graphics.
There is an interesting interaction between the FBS and the window server's client-side
buffer. Imagine a client specifies the following sequence of events:
� Create a shared, RAM-loaded bitmap
� Use the BITGDI to initialize the bitmap contents