How to do pageflipping

1
How to do page-flipping

• The steps needed when revising our wfmodel1.cpp
  prototype to achieve smooth animation

2
How page-flipping works
The user sees this part of video memory
The visible page
CRTC START-ADDRESS
Your program draws to this part of video
memory
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
3
Use a Radeon-specific register

• The CRT Controllers Start-Address can be
  accessed via a 32-bit register (called
  CRTC_START) which is part of the ATI Radeons 2D
  drawing engine -- allowing the Start-Address to
  be reprogrammed as a single atomic operation
• define CRTC_START 0x0224

4
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 Systems startup
  process
• define VENDOR_ID 0x1002 // ATI
• define DEVICE_ID 0x5B60 // X300

5
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

6
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

7
Global Variables

• int iobase // value is filled in at runtime
• int page_size (2 ltlt 20) // 2-megabytes
• int drawing_page 0 // page 0 or page 1
• int visible_page 0 // page 0 or page 1

8
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 )

9
Pixel-drawing helper-function

• void draw_pixel( int x, int y, int color )
• int offset drawing_page pagesize
• unsigned char dstn vram offset
• dstn y vres x color

10
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

11
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 ltESCAPEgt-key
• You will need to use a signal-handler and
  asynchronous keyboard-input notification (as in
  our earlier persists.cpp demo)