Graphics acceleration

1
Graphics acceleration

• An example of line-drawing by the ATI Radeons 2D
  graphics engine

2
Bresenhams algorithm

• Recall this iterative algorithm for doing a
  scanline conversion for a straight line
• It required five parameters
• The starting endpoint coordinates (X0,Y0)
• The ending endpoint coordinates (X1,Y1)
• The foreground color for the solid-color line
• It begins by initializing a decision-variable
• errorTerm 2deltaY - deltaX

3
Algorithms main loop

• for (int y Y0, x X0 x lt X1 x)
• drawPixel( x, y, color )
• if ( errorTerm gt 0 ) errorTerm 2delY
• else y 1 errorTerm 2(delY delX)

4
How much work for CPU?

• Example To draw the longest visible line (in
  1024x768 graphics mode) will require
  approximately 10,000 CPU instructions
• The loop gets executed once for each of the 1024
  horizontal pixels, and each pass through that
  loop requires about ten CPU operations moves,
  compares, branches, adds and subtracts, plus the
  function-calls

5
Is acceleration possible?

• The IBM 8514/A appeared in late 1980s
• It could do line-drawing (and some other common
  graphics operations) if just a few parameters
  were supplied
• So instead of requiring the CPU to do ten
  thousand operations, the CPU could do maybe ten
  operations, then let the 8514/A graphics engine
  do the rest of the work!

6
8514/A Block Diagram
Graphics processor
RAMDAC
VRAM memory
LUT
DAC
Display Monitor
Display processor
CRT controller
Drawing engine
CPU
ROM
PC Bus Interface
PC Bus
7
ATI improved on IBMs 8514/A

• Various OEM vendors soon introduced their own
  graphics accelerator designs
• Because IBM had not released details of its
  design, others had to create their own
  programming interfaces all are different
• Early PC graphics software was therefore NOT
  portable between hardware platforms

8
How does X300 draw lines?

• To demonstrate the line-drawing ability of our
  classrooms Radeon X300 graphics processors, we
  wrote drawline.cpp demo
• We did not have access to ATIs official Radeon
  programming manual, but we had several such
  manuals from other vendors, and we found clues
  in source-code files for the Linux Radeon
  device-driver

9
Programming concepts

• Our demo-program must first verify that it is
  running on a Radeon-equipped machine
• It must determine how it can communicate with the
  Radeons graphics accelerator
• Normal VGA registers are at standard I/O
  port-addresses, but the graphics engine is
  outside the scope of established standards

10
Peripheral Component Interconnect

• An industry committee (led by Intel) has
  established a standard mechanism that PC
  device-drivers can use to identify the peripheral
  devices that a workstation has, and their
  mechanisms for communication
• To simplify the Pre-Boot Execution code, modern
  PCs provide ROM-BIOS routines that can be called
  to identify peripherals

11
PCI Configuration Space
Each peripheral device has a set of nonvolatile
memory-locations which store information about
that device using a standard layout
PCI CONFIGURATION HEADER
256 bytes
ADDITIONAL PCI CONFIGURATION DATA
1024 bytes
This device-information is accessed via I/O
Port-Addresses 0x3C8-0x3CF
12
PCI Configuration Header
Sixteen longword entries (256 bytes)
DEVICE ID
VENDOR ID
BASE-ADDRESS RESOURCE 0
BASE-ADDRESS RESOURCE 1
BASE-ADDRESS RESOURCE 2
BASE-ADDRESS RESOURCE 3
VENDOR-ID 0x1002 Advanced Technologies,
Incorporated DEVICE-ID 0x5B60 ATI Radeon
X300 graphics processor BASE-ADDRESS for
RESOURCE 1 is the 2D engines I/O port
Our findsvga.cpp utility will show you the PCI
Configuration Space for any peripheral devices of
Class 0x030000 (i.e., VGA-compatible graphics
cards)
13
Interface to PCI BIOS

• Our dosio.c device-driver (and int86.cpp
  companion code) allow us access to BIOS
• The PCI BIOS services are accessible (in the
  Pentiums virtual-8086 mode) using function 0xB1
  of software interrupt 0x1A
• There are several subfunctions you can find
  documentation online for example, Professor
  Ralf Browns Interrupt List

14
return_radeon_port_address()

• Our demo invokes these PCI ROM-BIOS subfunctions
  to discover which I/O Port our Radeons 2D
  graphics engine uses
• Subfunction 1 Detect BIOS presence
• Subfunction 3 Find Device in a Class
• Subfunction A Read Configuration Dword
• Configuration Dword at offset 0x14 holds I/O
  Port-Address for 2D graphics engine

15
The ATI I/O Port Interface
iobase 0 iobase 4
MM_INDEX
MM_DATA
You output a registers index to the iobase
0 address
Then you have read or write access to that
register at the iobase 4 address
16
Many 2D engine registers!

• You can peruse the radeon.h header-file to see
  names and register-index numbers for the Radeon
  2D graphics accelerator
• You could also write a programming loop to input
  the contents from various offsets and thereby get
  some idea of which ones appear to hold live
  values (i.e.,hundreds!)
• Only a small number used in line-drawing

17
Main Line-Drawing registers

• DP_GUI_MASTER_CNTL
• DP_BRUSH_FRGD_COLOR
• DP_BRUSH_BKGD_COLOR
• DP_WRITE_MSK
• DST_LINE_START
• DST_LINE_END

18
Others that affect drawing

• RB2D_DSTCACHE_MODE
• MC_FB_LOCATION
• DEFAULT_PITCH_OFFSET
• DST_PITCH_OFFSET
• SRC_PITCH_OFFSET
• DP_DATATYPE
• DEFAULT_SC_TOP_LEFT
• DEFAULT_SC_BOTTOM_RIGHT

19
CPU/GPU synchronization
Intel Pentium CPU
ATI Radeon GPU
When CPU off-loads the work of drawing lines (and
doing other common Graphical operations) tp the
Graphics Processing Unit, then this frees up the
CPU to execute other instructions but it opens
up the possibility that the CPU will send more
drawing commands to the GPU, even before the GPU
is finished doing earlier commands. Some
mechanism is needed to prevent the GPU from
becoming overwhelmed by work the CPU sends it.
Solution is a FIFO for pending commands, plus a
Status Register
20
Engine has 64 FIFO slots

• Before the CPU initiates a new drawing command,
  it checks to see if there are enough free slots
  in the command FIFO for storing that commands
  parameters
• The CPU can do busy-waiting until the GPU
  reports that enough FIFO slots are ready to
  accept new command-arguments
• An alternative is interrupt-driven drawing

21
Testing drawline.cpp

• We developed our drawline.cpp demo on a Radeon
  7000 graphics card, then tested it on a newer and
  faster Radeon 9250
• Our code worked fine
• Tonight we shall try it on the Radeon X300
• If these various models of the Radeon are fully
  compatible with one another, we can expect our
  demo to work fine on the X300

22
Hardware changes?

• But if any significant differences exist in the
  various Radeon design-generations, then we may
  discover that our drawline fails to perform
  properly on an X300
• We would then have to explore the ways in which
  Radeon designs have changed, and try to devise
  fixes for any flaws that we have found in our
  software application

23
In-class exercises

• Try running the drawline.cpp application on our
  classroom or CS Lab workstation maybe it works
  fine, maybe it doesnt
• Look at the source-code files for the Linux
  open-source ATI Radeon device-driver
• If our drawline work ok, see if you can add
  code that programs the engine to fill rectangles
  or copy screen-areas or, if drawline fails,
  see if you can devise a fix