15 VDU BasicsComponents of VDUMonitors (Display Screens)

Refresh RateResolutionMultisync MonitorFST MonitorInterlaced and Non-Interlaced MonitorsEnergy Saving MonitorsActive Matrix LCD Display

Graphics CircuitryGraphics Card StandardsModern Graphics Cards

Graphics ProcessorVideo MemoryDigital to Analog Converter (DAC)Display ConnectorComputer (Bus) ConnectorGraphics BIOS

Video Display ModesText or GraphicsColors in Text ModeColors in Graphics Mode

Colors in VGAColors in SVGA

Video PagesThe DOS PerspectiveWriting to Video Memory in Text ModeExercise

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or most computer users, the two most important components of the computer system are the Video Display Unit (VDU) and the keyboard. It has been observed that the

decision of customers, who purchase software, is more often than not strongly influenced by how the screens have been designed in the software. Jazzier the screen, more appeal it carries for the users. Your monitor provides the link between you and your computer. Although you can get rid of your printer, disk drives, etc. you cannot sacrifice the monitor. Without it, you would be operating blind!

F

Components of VDU

The video system consists of two basic components:

(a) A monitor on which we actually see the images either in text or in graphics.

(b) Video or graphics circuitry usually fitted on a card but sometimes found on the motherboard itself.

Let us discuss each of these now.

Monitors (Display Screens)In early days of PC not every monitor could support text, graphics and colors. But that is history now. Today almost all monitors support text and graphics capabilities and produce colors. Today what is of more significance regarding the monitor is its refresh rate and resolution. Let us now examine them.

Refresh Rate

How are the images, either text or graphics, produced on the screen? The microprocessor does not have the ability to send signals necessary to produce the images on the screen. This task is performed by graphics card circuitry.

Chapter 15: VDU Basics 497Either the microprocessor or the graphics processor present in the graphics card writes the information to be displayed on the screen into the video memory. Once written, the graphics card circuitry reads the digital information from video memory, converts it to analog signals and transmits them to the electronic gun inside the monitor. The screen is coated with a material called Phosphor Bronze. When the electron beam strikes the Phosphor-Bronze particles they glow. The phosphor particles have a property that unless the electron beam again strikes them they would vanish. To prevent this the graphics card circuitry repeatedly (80 to 100 times a second) reads information from video memory and transfers it to the screen, making the images displayed on the screen clear and steady. This process is called ‘refreshing the screen’, and the rate at which it is refreshed is called ‘refresh rate’. Low refresh rates cause the screen to flicker contributing to eye-strain. The higher the refresh rate better it is for your eyes.

Resolution

Text or graphics on the screen are built up from tiny dots called picture elements or ‘pixels’. The display resolution is defined by the number of rows (called scan lines) from top to bottom, and number of pixels from left to right on each scan line. In general, higher the resolution, more pleasing is the display. Higher resolution means a sharper, clearer picture, with less pronounced ‘staircase’ effect on lines drawn diagonally and better looking text characters. On the other hand, higher resolution also means more memory requirement for the display.

Frequently, one comes across confusing and conflicting terminologies related with the monitor. This involves interlaced and non-interlaced monitors, multisync monitors, energy saving monitors, active matrix LCD display, etc. Let us now try to de-mystify this jargon.

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Multisync Monitor

Some monitors have a fixed refresh rate, whereas, others may support a range of frequencies. This multiple frequency support provides built-in compatibility with future video standards. A monitor that supports many video standards is called a multiple-frequency monitor. Different vendors call multiple-frequency monitor by different names, including multisync, multifrequency, multiscan and asynchronous monitors.

FST Monitor

The traditional screen is curved, meaning that it bulges outwards from the middle of the screen. This design is consistent with the vast majority of cathode ray tube designs, including your television set. FST stands for flat square tube. The flat screen results in reduced glare and a higher quality, more accurate image. The disadvantage is that the technology required to produce flat-screen displays is more expensive, resulting in higher prices for the monitors.

Interlaced and Non-Interlaced Monitors

In non-interlaced (conventional) monitors the electron beam sweeps the screen in lines from top to bottom one line after the other, completing the screen in one pass. In interlaced monitors too, the electron beam sweeps the screen from top to bottom, but it does so in two passes, sweeping the odd lines first and the even lines second. Non-interlaced monitors offer better and stable displays when compared with interlaced ones, even though they are costlier. Most monitors today are non-interlaced.

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Energy Saving Monitors

Monitor is one of the most power-hungry computer components. Hence, putting off the monitor if it is idle for a specific period of time can save energy. Many PC manufacturers are trying to meet Environmental Protection Agency’s (EPA’s) Energy Store Requirements. Any PC-monitor that consumes less than 70 watts can use the Energy Star logo. There are different energy saving standards available for monitors, By far the most popular amongst these is Video Electronic Standards Associations’ Display Power Management Signal (VESA-DPMS) specifications.

Active Matrix LCD Display

Instead of using the CRT technology, the laptop computers make use of Liquid Crystal Diode (LCD) displays. These displays have low glare flat screens and low power requirements (5 watts versus nearly 60 watts for an ordinary monitor). There are 3 LCD choices—passive-matrix monochrome, passive-matrix color and active-matrix color. The differences between these types are beyond the scope of this book.

Graphics Circuitry

Apart from the monitor another major component of the VDU is the graphics circuitry. It is responsible for converting the digital information that the computer produces into something human beings can see. Most desktops use analog display monitors, hence the graphics card convert digital information to analog information for display on the monitor. On laptops the data remains digital

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because the laptop displays are digital. The graphics circuitry is either placed on a card that is plugged into an expansion slot. Alternately, it can also be built right inside the mother board. The graphics cards have evolved over the years. It might be interesting to follow this evolution.

Graphics Card Standards

The first graphics card, introduced in August of 1981 by IBM, was Monochrome Display Adapter (MDA). The monitors that used these cards were typically text-only, with green, amber or white text on a black background.

Color for IBM-compatible computers appeared on the scene with the 4-color Hercules Graphics Card (HGC), followed by the 8-color Color Graphics Adapter (CGA) and 16-color Enhanced Graphics Adapter (EGA).

When IBM introduced the Video Graphics Array (VGA) in 1987, a new graphics standard came into being. A VGA display could support up to 256 colors (out of a possible 2,62,144 colors) at resolutions up to 720 x 400 (resolution and its impact on the image is discussed in the next section). Perhaps the most interesting difference between VGA and the preceding formats is that VGA was analog, whereas displays had been digital up to that point. Why were displays moved from digital to analog when most other electronic systems like compact disk players, newer VCRs and camcorders use digital picture storage? The answer is color. A digital display generates different colors by using the red, green and blue electron beams. In addition an intensity signal is used to display each color at one of the two intensity levels. This gives rise to a capability to generate 16 colors (24).

An analog display works like digital displays that use RGB electron beams to construct various colors, but each color in the

Chapter 15: VDU Basics 501analog system can be displayed at varying levels of intensity—64 levels, in the case of the VGA. This versatility provides 2,62,144 possible colors (643). In computer graphics the color is often important than the resolution, because the human eye perceives the picture that has more colors as being more realistic. Thus IBM moved display system into analog to enhance the color capabilities.

Over the years, VGA gave way to Super Video Graphics Array (SVGA). SVGA cards were based on VGA, but each card manufacturer added resolutions and increased color depth in different ways. Thus SVGA refers to a group of video cards, all with roughly the same capabilities. It does not refer to a specific card, like the VGA technically does.

In order to create some standard out of the chaos of SVGA, the Video Electronics Standards Association (VESA) introduced a standard programming interface called the VESA BIOS Extension. This provided programmers with one common interface to write for instead of trying to tailor their programs to work with several different SVGA cards. All SVGA cards in use today comply with the VESA standard.

Modern Graphics Cards

The graphics card in a modern PC can be connected either to the PCI slot (Peripheral Component Interconnect expansion slot) or to the AGP (Accelerated Graphics Port) slot. The communication between the PCI slot and the main processor takes place through a PCI bus. All the cards connected to different PCI slots share the PCI bus. Unlike this the AGP bus is not shared and is dedicated to exclusive graphics use. As a result, AGP offers a faster performance as compared to PCI.

The modern PC graphics card consists of six main components:

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(a) Graphics processor (b) Video memory(c) Digital to Analog converter (DAC)(d) Display Connector(e) Bus Connector(f) Graphics BIOS

Let us discuss these components now.

Graphics Processor

Early graphics cards merely used to have video memory and circuitry to convert digital data to analog display. The job of writing digital data into video memory used to be done by the main processor. As a result, the main processor used to spent lot of time in performing complex graphics operations. These cards are hardly used any more.

The advent of graphical OS like Windows as well as advances in Multimedia and 3D graphics has dramatically increased the amount of information needing to be displayed on the monitor. This has made it impractical for the main processor to handle all graphics activities. Hence these activities have been now assigned to the dedicated processor present on the graphics card. They have special command sets for graphics manipulation built right into the chip.

This graphics processor is optimized for performing graphics operations. The CPU sends a set of drawing instructions that are interpreted by the graphics card’s proprietary driver and executed by the card’s on-board graphics processor. This greatly reduces the main processor’s workload. Such a graphics processor is often known as Graphics Accelerator.

Another variety of graphics processor called Coprocessor also exists. In this system the graphic cards driver software sends

Chapter 15: VDU Basics 503graphics-related tasks directly to the Coprocessor, which proceeds to carry out those tasks without any assistance from the main processor. The OS sends everything else to the main processor. Thus both the processors work simultaneously resulting in better performance. These Coprocessors are typically used in higher end graphics cards.

Operations including bitmap transfers and painting, window resizing and repositioning, line drawing, font scaling and polygon drawing can be handled by the graphics processor at far greater speeds than the software running on the system’ main processor. The graphics processor then writes the display data to the video memory (also called frame buffer). As there’s less data to transfer, there’s less congestion on the bus.

Video Memory

The memory that holds the video image is called video memory. It is also referred to as the frame buffer. Anything drawn on the screen is made up of individual dots. These dots are called pixels and each pixel has a color. The memory is used to hold the color of each pixel. Today’s graphics cards typically have 4 MB or 8 MB of video memory. The amount of video memory installed on the graphics card has a direct bearing on the number of colors and the resolution that it can support.

Digital to Analog Converter (DAC)

The information in the video memory frame buffer is an image of what appears on the screen, stored as a digital bitmap. But while the video memory contains digital information its output medium, the monitor, uses analogue signals. The analog signal requires more than just an on or off signal, as it’s used to determine where, when and with what intensity the electron guns inside the monitor should be fired as they scan across and down the front of the

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monitor. This is where the DAC comes in. The DAC takes the input from the video.

Display Connector

Graphics cards use standard connectors. Most cards use the 15-pin female connector. The monitor cable is plugged into this connector.

Computer (Bus) Connector

The graphics card connects to the PCI/AGP slot through this connector. Main processor communicates with the graphics card through this connector. Today most of the graphics cards are connected to the AGP slot.

Graphics BIOS

Graphics cards have a small ROM chip containing basic information that tells the other components of the card how to function in relation to each other. The BIOS also performs diagnostic tests on the card’s memory and input/output (I/O) to ensure that everything is functioning correctly.

Video Display Modes

Just as an artist can choose from a variety of media when creating a picture (oils, etching, watercolors etc.), so a programmer can choose from a variety of modes, or formats. Each mode provides a different combination of display characteristics. These characteristics include:

(a) whether text or graphics is to be displayed

Chapter 15: VDU Basics 505(b) the amount of text to be displayed in one line(c) the resolution(d) the number of possible colors

Each mode requires certain hardware (monitor and display adapter) and programming approaches. Each and every mode will not be supported by a particular combination of monitor and display adapter. There are several modes available. Figure 15.1 lists the popular along with the number of colors and resolution they offer.

Mode Reaolution. Colors Bits/Pixel Remark

5 320 x 200 4 212 640 x 480 16 413 320 x 200 256 8101h 640 x 480 256 8 Palette-index mode103h 800 x 600 256 8 Palette-index mode104h 1024 x 768 16 4 Palette-index mode105h 1024 x 768 256 8 Palette-index mode111h 640 x 480 65536 16 Direct color mode112h 640 x 480 1677716 32 Direct color mode114h 800 x 600 65536 16 Direct color mode115h 800 x 600 1677716 32 Direct color mode117h 1024 x 768 65536 16 Direct color mode118h 1024 x 768 1677716 32 Direct color mode11Ah 1280 x 1024 65536 16 Direct color mode11Bh 1280 x 1024 1677716 32 Direct color mode3 640 x 200 16 2 bytes/char Text mode

Figure 15.1

Text or Graphics

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All modes are fundamentally of two types, text or graphics. Some modes display only text, some support more colors, whereas some are made only for graphics. As seen earlier, the graphics card continuously dumps the contents of the video memory on the screen. The amount of memory required for representing a character on screen in text mode and a pixel in graphics mode varies from mode to mode. Figure 15.1 shows the amount of memory required do display a fundamental element in each mode.

In mode 5 each pixel displayed on the screen occupies two bits in video memory. These two bits can generate four values (00, 01, 10 and 11) and hence a pixel can be drawn in four possible colors. In modes 18 and 19 a pixel can be drawn in 16 and 256 colors respectively. How these colors are generated is discussed in a later section in this chapter.

As seen from Figure 15.1, text mode needs two bytes in video memory to represent one character on screen. Of these two bytes the first byte contains the ASCII value of the character being displayed, whereas the second byte is the attribute byte. The attribute byte controls the color in which the character is being displayed.

How does the character actually get displayed on the screen? The ASCII value present in video memory must be translated into a character and drawn on the screen. A character generator program does this drawing. On older display adapters like MA and CGA, the character generator used to be located in ROM (Read Only Memory). VGA and SVGA do not have a character generator ROM. Instead, character generator data is loaded into display RAM. This feature makes it easy for custom character sets to be loaded. Multiple character sets may reside in RAM simultaneously. A set of BIOS services is available for easy loading of character sets. Each character set can contain 256 characters. Either one or two character sets may be active giving

Chapter 15: VDU Basics 507these adapters the capability to display up to 512 different characters on the screen simultaneously. When two character sets are active, a bit in each character attribute byte selects which character set will be used for that character. Using a ROM-BIOS service we can select the active character set. Each character in the standard character set in VGA is 9 pixels wide and 16 pixels tall. Custom character set can also be loaded using BIOS video services (refer appendix B).

The graphics modes can also display characters, but they are produced quite differently. The graphics modes can only store information bit-by-bit and characters are no exception... they must be drawn one bit at a time. The big advantage of this method is that one can design characters of desired style, shape and size.

Colors in Text Mode

In text mode for each character on screen there are two bytes in video memory, one containing the ASCII value of the character and other containing its color. The color byte contains three components—the foreground color (color of the character itself), the background color (color of the area not covered by the character) and the blinking component of the character. Figure 15.2 shows the breakup of the color byte.

Bits

7 6 5 4 3 2 1 0 Purpose

. . . . . . . 1 Blue component of f/g color

. . . . . . 1 . Green component of f/g color

. . . . . 1 . . Red component of f/g color

. . . . 1 . . . Intensity component of f/g color

. . . 1 . . . . Blue component of b/g color

. . 1 . . . . . Green component of b/g color

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. 1 . . . . . . Red component of b/g color

1 . . . . . . . Blinking component

Figure 15.2

The first four bits can produce 16 different colors, whereas the Red, Green and Blue components of background colors can produce 8 different colors. Figure 15.3 shows, which bit setting, will produce what color.

Color Components

Intensity Red Green Blue

Black 0 0 0 0

Blue 0 0 0 1

Green 0 0 1 0

Cyan 0 0 1 1

Red 0 1 0 0

Magenta 0 1 0 1

Brown 0 1 1 0

White 0 1 1 1

Light Black 1 0 0 0

Light Blue 1 0 0 1

Light Green 1 0 1 0

Light Cyan 1 0 1 1

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Light Red 1 1 0 0

Light Magenta 1 1 0 1

Yellow 1 1 1 0

Intense White 1 1 1 1

Figure 15.3

If the bit settings of the color byte are, say, 00010100, then the character produced would be of red color on a blue background. Similarly, 10001110 would produce a yellow character on a black background, and the character would blink on the screen.

Colors in Graphics Mode

So far we have seen how to set color in text modes. Setting color in graphics modes is quite different. In the graphics mode each pixel on the screen has a color associated with it. There are important differences here as compared to setting color is text mode. First, the pixels cannot blink. Second, each pixel is a discrete dot of color, there is no foreground and background; each pixel is simply one color or another. The number of colors that each mode can support and the way each graphics card generates these colors is drastically different. Here we would examine the color generation technique used by VGA.

Colors in VGA

VGA’s video memory is organized in four planes—red, green, blue and intensity. Each plane provides one bit of data for each pixel. Thus any pixel is represented by a 4-bit value, each plane contributing one bit of this 4-bit value. The 4-bit pixel value is from the display memory is used as the address of 1 of the 16 palette registers. For example, a pixel value of 0000 selects the

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palette register 0, a pixel value of 0001 selects register 1, a pixel value of 0010 selects register 2, and so on.

Each palette register is 6 bits long. Once the palette register has been chosen the 6-bit value in it is combined with a 2-bit value from a color select register, resulting into a 8-bit value. This 8-bit value is used as the address of 1 of the 256 DAC (Digital to Analog Converter) registers. Each DAC register contains an 18-bit value that represents the color. The 18-bit value is organized as 6-bit red, green and blue color components. This value is sent to the analog conversion circuitry, which converts it into three proportional analog signals and sends them to the monitor. Since each DAC register is 18-bit long, a pixel can have any of the 2,62,144 values (218).

The translation of a 4-bit color value into a 18-bit DAC register value is shown in Figure 15.4.

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Figure 15.4

0 1 1 1 0 0 0 1

Pixel Value

0 1 1 0

00000001

0010

0110

0111

1111

Palette Register

011100

00000000

00000001

00000010

00000011

01110001

11111100

11111101

11111110

11111111

DAC Registers

18-bit Color Value

001101 001110 101101

Analog

Conversion

Circuit

Analog

Conversion

Circuit

R

G

B

0 1

Color Select Register

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In summary, we cannot speak of ‘color’ at any point in the color translation process until the output stage of DAC. The 4-bit pixel value in memory, the 6-bit value in the palette registers and the 8-bit value sent to the DAC are all addresses, not colors. For example, a pixel with a 4-bit value of 0 is not black; it is address 0 of the palette register. If this palette register contains a value 3Fh then this value is not the white color but a value which would be combined with the 2 bits from the color select register to give a 8-bit value. Suppose the 2-bit value is 0 then the 8-bit value would remain 3Fh. This value is also not the white color. It is the address of the DAC register. It is the value stored in the DAC register 3Fh that represents the color. Suppose the value stored in the DAC register 0x3F is 63 then the pixel would be displayed in color 63. Thus, a pixel value of 0 in display memory can become color 63 when displayed on the screen. In short, it isn’t color until the DAC says it is color.

VGA supports several graphics modes. The two popular ones are—640 x 480, 16-color mode and a 320 x 200, 256-color mode. How come only 16 or 256 colors can be availed at a time from 2,62,144 colors?

In the 256-color mode, the 2-bits from the color select register and 6 bits from the palette register are combined to form a 8-bit value. This 8-bit value can give rise to 256 different combinations. Each combination references a particular DAC register, which holds the actual 18-bit color value. As a result, we can use 256 out of the possible 2,62,144 color values (218).

In the 16-color mode, the color select register bits always have a value 0. Hence, in this mode only the first 64 DAC registers get used. And since there are only 16 palette registers that can be used to refer to these DAC registers, in this mode we can use any 16 out of the 2,62,144 colors.

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Colors in SVGA

There are several modes that are specific to SVGA. These modes can be broadly categorized into:

(a) Palette-indexed modes(b) Direct color modes

In the palette-indexed modes SVGA uses either 4 bits 8 bits to represent a pixel. We would discuss here the case when it uses 8 bits to represent a pixel (the 4-bit model’s discussion would be similar). In the 8-bit model SVGA uses 256 palette registers. Using the 8-bit pixel value it can access 256 palette registers. Each value in the palette register can further access 256 DAC registers. Each DAC register is 24 bits long. Hence, in SVGA we can choose 256 colors simultaneously out of the possible 16,77,716 colors (224). Note that the 8-bit pixel value is merely an index into the palette registers and the palette register value is an index into the DAC registers. The actual color value is the one in the DAC registers.

In direct color modes there are no palette registers or planes. These modes typically use either 16 bits or 32 bits to represent a pixel color. A 16-bit or 32-bit value directly represents the pixel color. In these modes we get either 65536 (216) or 16777216 (224) colors at a time. The first category is often known as a ‘high color’ mode and the second is known as ‘true color’ mode. Why were the palettes abandoned? Two reasons. For one the video memory became cheap and for another the advent of multimedia and 3D graphics demanded more colors to be displayed simultaneously.

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Video Pages

In text mode each character displayed on the screen takes 2 bytes in video memory. As a result a total of 2000 characters (25 x 80) would require 4000 bytes, or roughly 4 KB. As size of video memory is bigger than 4 KB, if only the first 4 KB of video memory is used then the rest would remain unutilized. To avoid this the display memory can be split into several chunks of 4 KB each. These chunks of memory are called video pages. Thus, there are four video pages in mode 3, numbered from 0 to 3. At any given time, contents of one page are displayed on the screen. Information can be written into the displayed page or any of the other pages. Using this technique we can build a screen on an invisible page while another page is being displayed, then switch to the new page when appropriate time comes. Switching screen images this way makes them appear to regenerate instantaneously.

This technique is often used in writing menu-driven programs, where a different menu is written on each page. As a result, to switch over from one menu to another, all that we are required to do is to switch over to an appropriate page, where the menu is already written and ready to be displayed. This is obviously a better procedure than erasing the existing menu and writing a new menu.

In graphics modes too, the display memory is split into video pages. The maximum number of pages permitted depends on the bits required to store information of one pixel and the amount of display memory available.

The DOS Perspective

While working under DOS the processor is made to work in Real mode. In this mode all the CPU registers are reduced to 16-bit. As a result under DOS we cannot access memory blocks bigger than

Chapter 15: VDU Basics 51564 KB (216 bytes). This barrier has been overcome to an extent by using a segment:offset scheme (Refer Appendix G for details of this scheme) that lets you access any byte within 1 MB memory. Even with this scheme from any given address we can access only the next 64 KB chunk.

Due to this limitation, while working under DOS 2 blocks of memory—A block and B block, each of 64 KB—are reserved for access to video memory. In fact, these blocks act as a window to the video memory. Anytime we are to display anything on the screen we write to these blocks, which are mapped into the video memory.

Figure 15.5 shows the exact location of A and B blocks in 1 MB memory map. Note that the first megabyte of memory is divided into 16 blocks of 64 KB each. These blocks are numbered from 0 to 15. As said earlier, blocks A and B map into the video memory. Which out of these two blocks would be used depends on the mode in which you are working (text/graphics).

F

E

D

C

Window to video memory B

Window to video memory A

1MB

640KB

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Figure 15.5

Writing to Video Memory in Text Mode

Windows doesn’t work under text mode so the discussion that follows is specific to MS-DOS. There are 3 ways of displaying characters on the screen.

(a) Using standard library functions(b) Using ROM-BIOS or DOS routines(c) Writing characters directly into video memory

The last option works faster than the other two, because, the standard library functions or ROM-BIOS/DOS routines ultimately write the characters to be displayed into video memory. Obviously, if we are able to write characters directly into video memory we would be able to bypass the standard library functions as well as the ROM-BIOS/DOS routines.

We know that above the 640 KB RAM, there are 2 blocks (block A and Block B) of 64 KB each. While working in text mode all text displayed on the screen is written to the B block starting at address 0xB8000. Each character present on the screen uses 2 bytes in B block. The first byte contains the ASCII value of the character, whereas the next byte contains the color of the character. For example, if ‘A’ is displayed in 0 th row, 0th column on the screen, then address 0xB8000 contains the ASCII value of ‘A’, whereas the immediately adjacent address contains the color of ‘A’.

Chapter 15: VDU Basics 517Thus, if one character occupies two bytes in B block, 80 characters of 0th row will be represented by first 160 bytes from 0xB8000 onwards. Similarly, one screenful of characters would need 4000 bytes (80 x 25 x 2).

Using this concept let us now write a program which fills the entire screen with ‘A’s by writing this character directly into B block of memory.

/* Screenful of 'A's */main( ){

int i ;char far *vidmem = 0xB8000000 ;

for ( i = 0 ; i <= 3999 ; i = i + 2 ) * ( vidmem + i ) = 'A' ;

}

The variable vidmem has been defined as a far pointer since we are trying to access the memory that is beyond the data segment of our program. Data segment is the place where all the variables used in the program reside.

Now suppose we want to change the color of all the characters on the screen to 112, it can be done using the following program.

/* Changing screen attributes */main( ){

int i ;char far *vidmem = 0xB8000000 ;

for ( i = 1 ; i <= 3999 ; i = i + 2 )* ( vidmem + i ) = 112 ;

}

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Now it’s time for something more complicated. The following program draws a filled box on the screen and displays a message in it.

char far *vidmem = 0xB8000000 ; main( ){

int r, c, i ;char message[ ] = "Al Italia!!" ;

for ( r = 5 ; r <= 20 ; r++ ){

for ( c = 5 ; c <= 50 ; c++ )write2vdu ( ' ', 64, r, c ) ;

}

c = 10 ;for ( i = 0 ; i <= 10 ; i++ ){

write2vdu ( message[i], 77, 10, c ) ;c++ ;

}}

write2vdu ( char ch, char color, int row, int col ){

char far *v ;v = vidmem + row * 160 + col * 2 ;*v = ch ;v++ ;*v = color ;

}

In the above example, a separate function write2vdu( ) has been called from the main( ). To it we have sent character to be displayed (ch), its color (color) and the row & column where it is to be displayed. Upon receiving the arguments, the character and its color are stored at appropriate locations in the B block of

Chapter 15: VDU Basics 519memory. Work through the function write2vdu( ) carefully till you have analyzed the whole process.

Exercise[A] State True or False:

(a) High refresh rates cause the screen to flicker contributing to eye-strain.

(b) The microprocessor does not have the ability to send signals necessary to produce the images on the screen.

(c) Text or graphics on the screen are built up from tiny dots called picture elements or ‘pixels’.

(d) The flat screen results in reduced glare and a higher quality, more accurate image.

(e) Interlaced monitors offer better and stable displays as compared with non-interlaced monitors.

(f) The graphics card in a modern PC can be connected either to the PCI slot or to the AGP slot.

(g) In text mode for each character on screen there are two bytes in video memory, one containing the ASCII value of the character and other containing its color.

(h) The amount of memory that is required for representing a character on screen in text mode and a pixel in graphics mode is always same.

(i) VGA’s video memory is organized in four planes and each plane provides one bit of data for each pixel.

(j) In VGA since each DAC register is 18-bit long, a pixel can have any of the 2,62,144 values (218).

(k) In SVGA graphics mode, each DAC register is 24 bits long.(l) While accessing video memory under DOS, A block is used

for the text mode and B block is used for the Graphics mode.

[B] Pick up the correct alternative for each of the following:

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(a) Which of the following would display a message on the screen fastest(1) printf( ) function(2) puts( ) function(3) ROM-BIOS function(4) Directly writing to VDU memory

(b) A multiple-frequency monitor is also called(1) Multisync monitor(2) Multiscan monitor(3) Asynchronous monitor(4) All the above

(c) The monitors that sweeps the screen in lines from top to bottom one line after the other in one pass are known as (1) Non-interlaced monitors(2) Interlaced monitors(3) FST monitors(4) All the above

(d) The first graphics card, introduced in August of 1981 by IBM, was known as (1) Monochrome Display Adapter (MDA)(2) Hercules Graphics Card (HGC)(3) Color Graphics Adapter (CGA)(4) Enhanced Graphics Adapter (EGA)

(e) In the text mode each character displayed on the screen occupies ______ bytes in VDU memory(1) 2(2) 4(3) 6(4) 1

(f) The total number of DAC registers in VGA card are(1) 64

Chapter 15: VDU Basics 521(2) 128(3) 256 (4) 512

[C] Answer the following:

(a) What do you mean by refresh rate? Is it true that higher the refresh rate better is the image on the screen?

(b) What is the purpose of the graphics card? Where is it present in the computer?

(c) Why is it that in text mode there is only one font available, whereas in graphics mode characters can be displayed in a variety of fonts?

(d) What colour byte would you use if a message is to be displayed on the screen with brown background and yellow coloured characters?

(e) When we change over from one mode to another do the screen’s current contents remain intact?

[D] Answer the following:

(a) In the following program the displaymenu( ) function is supposed to write a given menu on a given VDU page by directly writing it in VDU memory. You are required to write the function displaymenu( ). Make it as general as you can.

char *filemenu[ ] = {"Rename file","Copy file","Delete file","Display file"

} ;

522 Let Us C

char *dirmenu[ ] = {"Make directory","Change directory","Remove directory""List directory"

} ;main( ){

int row = 5, col = 20, vdupage, num ;

num = 4 ; /* number of menu items */vdupage = 0 ;displaymenu ( filemenu, row, col, vdupage, num ) ;vdupage = 1 ;displaymenu ( dirmenu, row, col, vdupage, num ) ;

}

(b) Write a general purpose function writestring( ) which will display a message on the screen by writing it directly into VDU memory. The function should be capable of displaying the message in the colour which is sent to it.

(c) Write a program which continuously keeps changing the capital letters present on the screen into small case letters and small case letters present on the screen into capitals. You are not allowed to use printf( ), putchar( ), puts( ) or putch( ).

Chapter 15: VDU Basics 523