Offscreen memory usage

Hi, list.

I have my own video card with 2D engine on Altera and BrookTree RAMDAC. 
Nowadays
onboard memory is not a problem, and I can install up to 128 MBytes on 
the board, but
is it reasonable?

I wrote a driver for XAA interface and it works fine - my 2D engine 
supports almost all things
from XAA.HOWTO: solid lines, solid fills, screen to screen copying, 
color expansion, aperature
(aka data port) and so on. But I was  puzzled, when saw, that my 
offscreen memory is not
willingly used by X server.

I have an application, that creates a number of 32x32 pixmaps (~1000 
pixmaps) and than copies
them many times per second to main window by XCopyArea. As I can see 
there is nothing in
the offscreen memory and each time I call XCopyArea, X server moves data 
through my slow
PCI bus.

Why when I call XCreatePixmap X server doesn't create it in the 
offscreen memory? Maybe I need
to set something in GC?

As I can see, offscreen memory is used only for BackingStore, 
SaveUnder's and Cached Stipples,
is it complete list? And BTW what about cached fonts?

Maybe I'm loosing something? Maybe there are some another extensions 
required?
Or I need to implement something in the driver?

====================================================
supported extensions:

    BIG-REQUESTS
    DOUBLE-BUFFER
    FontCache
    MIT-SHM
    SHAPE
    SYNC
    X-Resource
    XC-MISC
    XFree86-Bigfont
    XFree86-Misc
    XFree86-VidModeExtension
    XInputExtension
    XKEYBOARD
    XTEST
====================================================
...
(**) OGK(0): 2D acceleration is available
(WW) OGK(0): Virtual screen resolution is used, HW Solid Lines will be 
disabled.
(II) OGK(0): Total scanlines 32767
(II) OGK(0): Offscreen Area 1024-32767
(II) OGK(0): Using 31999 lines for offscreen memory.
(II) OGK(0): Using XFree86 Acceleration Architecture (XAA)
        Screen to screen bit blits
        Solid filled rectangles
        8x8 mono pattern filled rectangles
        Screen to Screen color expansion
        Solid Horizontal and Vertical Lines
        Image Writes
        Offscreen Pixmaps
        Driver provided WritePixmap replacement
        Setting up tile and stipple cache:
                32 128x128 slots
                32 256x256 slots
                16 512x512 slots
(==) OGK(0): Backing store disabled
...

Thanks for answers.