How to do ‘page-flipping’
The steps needed when revising
our ‘wfmodel1.cpp’ prototype to
achieve ‘smooth’ animation
How page-flipping works
The ‘visible’ page
The user sees
this part of
video memory
CRTC START-ADDRESS
The ‘drawing’ page
When the ‘drawing’ page is
ready to be viewed, your
program changes the
CRTC START-ADDRESS
and thus the roles of these
two pages are ‘flipped’
VRAM
Your program
draws to this
part of video
memory
Use a Radeon-specific register
• The CRT Controller’s Start-Address can
be accessed via a 32-bit register (called
CRTC_START) which is part of the ATI
Radeon’s 2D drawing engine -- allowing
the Start-Address to be reprogrammed as
a single atomic operation
#define CRTC_START 0x0224
Installation check
• The software must detect the presence of
a Radeon X300 graphics adapter, and it
must discover which i/o port-address got
assigned to the Radeon 2D engine during
the Operating System’s startup process
#define VENDOR_ID 0x1002
#define DEVICE_ID 0x5B60
// ATI
// X300
PCI Find Device service
init8086();
// initialization for vm8086
vm.regs.eax = 0x1B02; // PCI Find Device
vm.regs.ecx = DEVICE_ID;
// X300
vm.regs.edx = VENDOR_ID; // ATI
vm.regs.esi = 0;
// device index
int86( 0x1A, vm );
// invoke PCI BIOS
if ( vm.regs.eax & 0xFF00 ) exit(1); // failed
PCI Read Configuration Dword
vm.regs.eax = 0x1B0A; // PCI Read Dwd
vm.regs.edi = 0x0014; // Resource #1 Adr
int86( 0x1A, vm );
// invoke PCI BIOS
iobase = vm.regs.ecx & 0xFFFFFFFC;
Global Variables
int
int
int
int
iobase; // value is filled in at runtime
page_size = (2 << 20); // 2-megabytes
drawing_page = 0; // page 0 or page 1
visible_page = 0; // page 0 or page 1
Hardware helper-function
void set_visible_page( void )
{
int start_addr = visible_page * page_size;
outl( CRTC_START, iobase + 0 );
outl( start_addr, iobase + 4 );
}
Pixel-drawing helper-function
void draw_pixel( int x, int y, int color )
{
int offset = drawing_page * page+size;
unsigned char
*dstn = vram + offset;
dstn[ y * vres + x ] = color;
}
The ‘page-flipping’ algorithm
• Main loop:
– Compute the new image
– Set drawing_page to (visible_page + 1)%2
– Draw next screen image onto drawing page
– Set visible_page to equal the drawing_page
In-class exercise
• Create a revision of the ‘wfmodel2’ demo
(named ‘wfmodel3.cpp’) to automatically
spin the wireframe model contiunuously,
until the user hits the <ESCAPE>-key
• You will need to use a signal-handler and
asynchronous keyboard-input notification
(as in our earlier ‘persists.cpp’ demo)