COPROCESSOR AS 6845 GRAPHIC CARD-VIGNESH.R,

2011212025,

M.E.(E.S.T), Ist YEAR.

WHAT IS CO-PROCESSOR ?

A coprocessor is a computer processor used to supplement the functions of the primary processor (the CPU).

Operations performed by the coprocessor may be floating point arithmetic, graphics, Signal processing, string processing, or encryption.

By offloading processor-intensive tasks from the main processor, coprocessors can accelerate system performance

Coprocessors allow a line of computers to be customized, so that customers who do not need the extra performance need not pay for it.

These processors require the host main processor to fetch the coprocessor instructions and handle all other operations aside from the coprocessor functions

Eg. Motorola 68881/68882 , Intel 80386 , sound blaster X-FI

VIDEO DISPLAY CONTROLLERS VDC - Integrated circuit which is the main component in a

video signal generator a device responsible for the production of

TV video signal in a computing (or) game system.

- can also generate audio signals

- responsible for generating the timing of necessary video signals such as horizontal and vertical synchronization signals and blanking interval.

- it functions as a coprocessor that can manipulate

video RAM contents independently

VIDEO DISPLAY CONTROLLERS VS. VIDEO DISPLAY PROCESSORS

VDC’s could not generate graphics

VDC’s donot have the special hardware accelerators to create 2D & 3D graphics

VDC’s only controls the timing of the Video synchronization signals and the access to the Video RAM but VDP can process the contents of Video RAM.

VIDEO DISPLAY CONTROLLER

Video shifters-get data from CPUand convert to serialbits with sync. signals

CRTC-generates video timings,reads data from RAM andoutput via character generatoror directly

Video interface controller-RAM based character generator sets & video RAM dedicated to colour.

Video coprocessors-own internal CPU dedicated to read & write to their own RAM & converting contents of this video RAM to video signal ; CPU can give only commands

MOTOROLA 6845:

The chip generates the signals necessary to interface with a faster display but does not generate the actual pixels !

contribute cursor and video-blanking information to the pixel video (intensity) signals

to produce correctly timed horizontal and vertical sync and provide the address in memory from which the next pixel or set of pixels should be read

The sync generation includes generation of horizontal and vertical video blanking signals, which are used to condition the external pixel generation circuits

process of reading that value, converting it into pixels, and sending it to a CRT is left to other circuits

6845(CONTD.) systems using the 6845 may have very different numbers

and values of colors, or may not support color at all

internal latch is provided which when triggered will duplicate and retain a copy of the video address so that it can later be read back by the CPU

light pens and light guns which can function by sending a pulse to the 6845 when the electron beam passes, allowing a running program to read back the location that was pointed at

Because all aspects of video timing are programmable, a single machine can switch between NTSC and PAL timings in software

INTERFACING CRT CONTROLLER(6845)

MOTOROLA 6845

GENERATION OF COLOR GRAPHICS

ADDRESS

INTERNALS OF 6845: 18 × 8-bit registers controlling all aspects of video timings Only two addresses are exposed to external components -

one to select which internal register is to be read or written to and another to access that register

The 6845 is intended for character based displays Address composed of two parts – i. 14 bit character address

ii. 5 bit row address

the 6845 can address 214+5 = 512 kb of memory

RA0-RA4 CA0-CA13

INTERNALS(CONTD.) character address increases linearly

When the chip signals horizontal sync it increases the row address

If the row address does not equal the programmatically set number of rows per character then the character address is reset to have the same value as it did at the beginning of the current scan line.

Otherwise the row address is reset to zero

If the character address is used to look up a character reference in RAM and the row address to index a table of character graphics in ROM an ordinary TEXT MODE display is constructed.

INTERNALS(CONTD.) The 6845 reads the start address for its display once per

frame

If the internal timing values on the chip are altered at the correct time it can be made to prepare for a new frame without ending the current one

Thus creating a non-continuous break in generated

addresses midway through the display

This is commonly used by games to provide one moving area of the display (usually the play field) and one static (usually a status display).

Specifications

Pin assignments

Pin Function

1 Ground

2 Ground

3 Red

4 Green

5 Blue

6 Intensity

7 Reserved

8 Horizontal Sync

9 Vertical Sync

Type Digital, TTL

Resolution 640h × 200v, 320h × 200v

H-freq 15.75 kHz

V-freq 60 Hz

Colors 16

Signal

The Color Graphics Adapter uses a standard DE-9 connector

Connector description:

CONFIGURING 6845 Despite varying bit depths in 6845's graphics mode, 6845 processes

colors in its palette in four bits, yielding 24 = 16 different colors The four color bits are arranged according to the RGBI color model:

The lower three bits represent

i.red

ii.green

iii. blue color components

fourth "intensifier" bit increases the brightness of all three color components (red, green, and blue) of the pixels it is set for.

The highest display resolution of any mode was 640×200, and the highest color depth supported was 4-bit (16 colors).

WITH RGBI MONITOR

Full CGA 16-color palette

0 black #000000

8 gray #555555

1 blue #0000AA

9 light blue #5555FF

2 green #00AA00

10 light green #55FF55

3 cyan #00AAAA

11 light cyan #55FFFF

4 red #AA0000

12 light red #FF5555

5 magenta #AA00AA

13 light magenta #FF55FF

6 brown #AA5500

14 yellow #FFFF55

7 light gray #AAAAAA

15 white (high intensity) #FFFFFF

These four bits are passed on unmodified to the DE-9 connector

at the back of the card, leaving all color processing to the RGBI monitor

The monitor would use approximately the following formula to process the digital four-bit color number to analog voltages ranging from 0.0 to 1.0:

red := 2/3×(colorNumber & 4)/4 + 1/3×(colorNumber & 8)/8 green := 2/3×(colorNumber & 2)/2 + 1/3×(colorNumber & 8)/8 blue := 2/3×(colorNumber & 1)/1 + 1/3×(colorNumber & 8)/8 Color 6 is treated differently; when using the formula above, color

6 would become dark yellow, but in order to achieve a more pleasing brown tone, special circuitry in most RGBI monitors, including the “IBM 5153” color display makes an exception for color 6 and changes its hue from dark yellow to brown by halving the analogue green signal's amplitude:

if colorNumber = 6 then green := green / 2 It is this "RGBI with tweaked brown" palette, shown in the

complete palette to the right.

STANDARD TEXT MODES 40×25 characters: (in up to 16 colors)i. Each character is a pattern

of 8×8 dots. The effective screen resolution in this mode is 320×200 pixels though individual pixels cannot be addressed independently

ii. The choice of patterns for any location is thus limited to one of the 256 available characters, the patterns for which are stored in a ROM chip on the card itself

iii.The display font in text mode (the code page 437 character set) is therefore fixed and cannot be changed (although when using the original IBM CGA, it is possible to select one of two different fonts—normal or thin—by changing a jumper. The card has sufficient video RAM for eight different text pages in this mode.

o 80×25 characters (in up to 16 colors)i. Each character is again an 8×8 dot pattern (the same character set is used as for 40×25).

ii.The effective screen resolution of this mode is 640×200 pixels. Again, the pixels cannot be individually addressed. Since there are twice as many characters on the screen in this mode

iii. The card has enough video RAM for just four different text pages

STANDARD GRAPHICS MODE

320×200 pixels: i.as with the 40×25 text modeii. In the graphics mode, however, each pixel can be accessed independentlyiii. The tradeoff is that only four colors can be displayed at a time. Also, only one of the four colors can be freely chosen from the 16 CGA colors—only two official palettes for this mode

# Palette 1

Palette 1 in high intensity

0 default default

1 3 — cyan 11 — light cyan

2 5 — magenta 13 — light magenta

3 7 — white 15 — white (high intensity)

# Palette 0 Palette 0 in

high intensity 0 default default

1 2 — green 10 — light green

2 4 — red 12 — light red

3 6 — brown 14 — yellow

Using palette 0: Using palette 1:

STANDARD GRAPHICS(CONTD.) 640×200 pixels :

i.as with the 80×25 text mode

ii.All pixels can be addressed independently ,by default the colors are black and bright white, but the foreground color can be changed to any other color of the CGA palette

iii.This can be done at runtime without refreshing the screen.

iv.The background color cannot be changed from black on a true IBM CGA card.

v.This mode disables the composite color burst signal by default.

vi.The BIOS does not provide an option to turn the color burst on in 640×200 mode, and the user must write directly to the mode control register to enable it

160×100 16 COLOR MODE

i.Technically, this mode is not a graphics mode, but a tweak of the 80×25 text mode

ii.The character cell height register is changed to display only two lines per character cell instead of the normal eight lines

iii.This quadruples the number of text rows displayed from 25 to 100. These "tightly squeezed" text characters are not full characters. The system only displays their top two lines of pixels (eight each) before moving on to the next row.

Character 221.

221 with blue text and red background color.

221 with red text and blue background color.

Character 222.

SPECIAL EFFECTS ON COMPOSITE COLOR MONITORS

With RGBI monitor COMPOSITE monitor

NTSC composite video, the separation between luminance and chrominance is far from perfect, yielding cross-color artifacts, or color "smearing“ Programmers soon found out that this flaw could be turned into an asset, as distinct patterns of high-resolution dots would "smear" into consistent areas of solid colors, thus allowing the display of completely new colors. Since these new colors are the result of cross-color artifacting, they are often called "artifact colors".

BUGS AND ERRATA The higher bandwidth used by 80-column text mode results in

random short horizontal lines appearing onscreen (known as "snow") if a program writes directly to video memory, as the CPU has priority when accessing it. This can be avoided by only accessing the memory during the period of vertical or horizontal retrace

Another peculiarity of 80-column text mode is that, on composite displays, the picture will be grayscale if the border color is set to black, white, or gray. Setting the border color to brown results in the normal 16 colors being displayed, while other values cause the colors to become tinted.

The video controller 6845's row counter being only seven bits wide, display RAM in graphics modes is laid out in a 2:1 “INTERLACE" pattern(i.e.) first laying out the data for rows 0, 2, 4, etc., then the data for rows 1, 3, 5, etc., adding additional software overhead for display RAM manipulation

They donot support “bit blittters” and “sprites”

INTERLACED AND NON-INTERLACED

Non Interlaced scanning (alternatively referred to as progressive scanning) is a way of displaying, storing, or transmitting moving images in which all the lines of each frame are drawn in sequence

Eg. 1080p at 25 full frames per second Interlaced video is where only the odd lines, then the even

lines of each frame (each image called a video field) are drawn alternately

1080i at 50 half-frames per second=>display at 25Hz

SPRITES AND BIT BLITTERS SPRITES were a method of integrating unrelated bitmaps so

that they appeared to be part of the normal bitmap on a screen.

such as creating an animated character that can be moved on a screen without altering the data defining the overall screen.

Such sprites can be created by either circuitry or software Bit BLIT (which stands for bit-block [image] transfer but is

pronounced bit blit) is a computer graphics operation in which several bitmaps are combined into one

RASTER AND RASTER OR

ACING THE VDC’S Working integrated circuits became more and more complex The simple Video Display Controllers were slowly replaced by

chips that had built-in video processing logic such as Blitters and other logic to manipulate the video RAM contents to do things like drawing lines, filling areas, or drawing fonts

Later chips also got special hardware to draw filled triangles to support 3D images, gained hardware Z-buffers and many other methods to accelerate the drawing of 3D pictures

Current Video generator chips almost always are “Graphics Processing Unit(GPU's)”

Entry-level PCs today commonly have the video display integrated into the motherboard chipset, which "steals" some system RAM for the display

The performance of such a system is not as good as one with dedicated video hardware.

REFERENCES

Http://www.wikipedia.com/Coprocessor.htm Http://www.wikipedia.com/motorola6845.htm Http://www.wikipedia.com/graphics processin

g.htm Http://www.wikipedia.com/Color graphics pro

cessing.htm Embedded systems book by Mathivanan

THANK YOU