Thursday, January 8, 2009

ATX

The ATX (for Advanced Technology Extended) form factor was created by Intel in 1995. It was the first big change in computer case and motherboard design in many years. ATX overtook ATmicroATX, FlexATX and mini-ITX) usually keep the basic rear layout but reduce the size of the board and the number of expansion slot positions. In 2003, Intel announced the BTX standard, intended as a replacement for ATX. As of 2007 the ATX form factor remains the industry standard for do-it-yourselfers; BTX has however made inroads into pre-made systems, being adopted by computer makers like Dell, Gateway, and HP. completely as the default form factor for new systems. ATX addressed many of the AT form factor's annoyances that had frustrated system builders. Other standards for smaller boards (including

The official specifications were released by Intel in 1995, and have been revised numerous times since, the most recent being version 2.2,[1] released in 2004.

A full size ATX board is 305 mm wide by 244 mm deep (12" x 9.6" ). This allows many ATX form factor chassis to accept microATX boards as well.

Power supply

ATX form motherboards became increasingly popular because of their advantages over older AT motherboards.

Main changes from AT design

Power switch

AT-style computer cases had a power button that was directly connected to the system computer power supply (PSU). The general configuration was a double-pole latching mains voltage switch with the four pins connected to wires from a four-core cable. The wires were either soldered to the power button (making it difficult to replace the power supply if it failed) or blade receptacles were used.

Typical ATX power supply
Interior view of an ATX power supply.

An ATX power supply does not directly connect to the system power button, allowing the computer to be turned off via software. However, many ATX power supplies have a manual switch on the back to ensure the computer is truly off and no power is being sent to the components. With this switch on, energy still flows to the components even when the computer appears to be "off." This is known as soft-off or standby and can be used for remote wake up through Wake-on-Ring or Wake-on-LAN, but is generally used to power on the computer through a front switch.

Power connection to the motherboard

The power supply's connection to the motherboard was changed. Older AT power supplies had two similar connectors that could be accidentally switched, usually causing short-circuits and irreversible damage to the motherboard. ATX used one large, keyed connector instead, making a reversed connection very difficult. The new connector also provided a 3.3 volt source, removing the need for motherboards to derive this voltage from one of the other power rails. Some motherboards, particularly late model AT form factor offerings, supported both AT and ATX PSUs.

Because the ATX PSU uses the motherboard's power switch, turning on the power in situations that do not utilize an ATX motherboard is possible by shorting the green wire from the ATX connector to any black wire on the connector (or ground). This allows re-use of an old PC power supply for tasks other than powering a PC, but one must be careful to observe the minimum load requirements of the PSU.

Airflow

ATX was originally designed with the power supply drawing air into the case and exhausting it down onto the motherboard. The plan was to deliver cool air directly to the CPU's and power regulation circuitry's location, which was usually at the top of the motherboard in ATX designs. This was not particularly useful for a variety of reasons. Early ATX systems simply didn't have processors or components with thermal output that required special cooling considerations. Later ATX systems with significantly greater heat output would not be aided in cooling by a power supply, because it would be delivering its often significantly heated exhaust into the case. As a result, the ATX specification was changed to make PSU airflow optional.[1]


ATX power supply revisions

Original ATX

ATX, introduced in late 1995, defined three types of power connectors:

  • 4-pin peripheral connector (commonly but innacurately known as "MOLEX") - transferred directly from AT standard
  • 4-pin floppy connector - transferred directly from AT standard
  • 20-pin main motherboard connector


The power distribution specification defined that most of PSU's power should be provided on 5V and 3.3V rails, because most of motherboard and adjoined electronical components (CPU, RAM, chipset, PCI, AGP and ISA cards) used 5V or 3.3V for power supply. The 12V rail was only used by fans and motors of peripheral devices (HDD, FDD, CD-ROM, etc.).

The original ATX power supply specification remained mostly unrevised until year 2000.


ATX12V 1.x

While designing the Pentium 4 platform in 1999/2000, the standard 20-pin ATX power connector was deemed inadequate to supply increasing electrical load requirements. So, ATX was significantly revised into ATX12V 1.0 standard (that is why ATX12V 1.x is sometimes inaccurately called ATX-P4). ATX12V 1.x was also adopted by Athlon XP and Athlon 64 systems.


ATX12V 1.0

The main changes and additions in ATX12V 1.0 (released in February 2000) were:

  • An extra 4-pin, 12-volt connector. Its purpose is to exclusively power the CPU (in original ATX, the CPU was powered from the 5V rail).
  • A supplemental 6-pin AUX connector providing additional 3.3V and 5V supplies to the motherboard, if it needed it. Although it was present on every ATX12V 1.x PSU (as required per standard), it was rarely implemented on motherboards, because it was not needed. Only some dual processor motherboards used it.
  • Increased the power on the 12V rail (power on 5V and 3.3V rails remained mostly the same).


ATX12V 1.1

This is a minor revision from August 2000. The power on 3.3V rail was slightly increased, among other much lesser changes.


ATX12V 1.2

A relatively minor revision from January 2002. The only significant change was that the -5V rail was not required anymore (it became optional). This voltage was very rarely used, only on some old systems with some ISA add-on cards.


ATX12V 1.3

Introduced in April 2003 (a month after 2.0). Lot of relatively minor changes. Some of them are:

  • Slightly increased the power on 12V rail.
  • Defined minimal required PSU efficiencies for light and normal load.
  • Defined acoustic levels.
  • Introduction of Serial ATA power connector (but defined as optional).


ATX12V 2.x

ATX12V 2.x brought a very significant design change regarding power distribution. When analyzing the then-current PC architectures' power demands, it was determined that it would be much easier (both from economical and engineering perspectives) to power most PC components from 12V rails, instead of from 3.3V and 5V rails.

ATX12V 2.0

The above conclusion was incorporated in ATX12V 2.0 (introduced in February 2003), which defined quite different power distribution from ATX12V 1.x:

  • Most power is now provided on 12V rails. The standard specifies that two independent 12V rails (12V2 for the 4 pin connector and 12V1 for everything else) with independent overcurrent protection are needed to meet the power requirements safely (some very high power PSUs have more than two rails, reccomendations for such large PSUs are not given by the standard).
  • The power on 3.3V and 5.V rails was significantly reduced
  • The main ATX power connector was extended to 24 pins (it is backwards compatible). The extra four pins provide one additional 3.3V, 5V and 12V circuit.
  • The 6-pin AUX connector from ATX12V 1.x was removed because the extra 3.3V and 5V circuits which it provided are now incorporated in the 24-pin main connector.
  • Serial ATA power cable is required.
  • Many other specification changes and additions.
24-pin ATX power supply connector
(20-pin omits the last 4: 11, 12, 23 and 24)
Color Signal Pin Pin Signal Color

+3.3 V 1 13 +3.3 V sense

+3.3 V 2 14 −12 V

Ground 3 15 Ground

+5 V 4 16 Power on

Ground 5 17 Ground

+5 V 6 18 Ground

Ground 7 19 Ground

Power good 8 20 −5 V (optional)

+5 V standby 9 21 +5 V

+12 V 10 22 +5 V

+12 V 11 23 +5 V

+3.3 V 12 24 Ground

ATX12V v2.1

This is a minor revision from March 2005. The -5V rail was completely removed from the specification. The power was slightly increased on all rails.


ATX12V v2.2

  • Main Power Connector changed from 20 pin to 24 pin to support PCI-Express requirements.
  • Minor changes.

ATX power supply derivatives

AMD GES

This is an ATX12V power supply derivative made by AMD to power its Athlon MP (dual processor) platform. It was used only on high-end Athlon MP motherboards. It has a special 8-pin supplemental connector for motherboard, so an AMD GES PSU is required for such motherboards (those motherboards will not work with ATX(12V) PSUs).


EPS12V

Defined in SSI, and used by some (Xeon and Opteron) systems. It has 24 pin main connector, 8 pin secondary connector, optional 4 pin tertiary connector.


Recent specification changes and additions

Because video card power demands have dramatically increased over the 2000s, some high-end graphics cards have power demands that exceed AGP or PCIe slot capabilities. For these cards, supplementary power was delivered through a standard 4-pin peripheral or floppy power connector. Midrange and high-end PCI Express-based video cards manufactured after 2004 typically use a standard 6 or 8-pin PCIe power connector directly from the PSU.

Interchanging old/new systems with old/new PSUs

Although the ATX power supply specifications are all vertically compatible in both ways (both electrically and physically), it is not wise to mix old motherboards/systems with new PSU's, and vice versa.

There are two main reasons for this:

  • The power distribution biases across 3.3V, 5V and 12V rails are very different between older and newer ATX PSU designs, as well as between older and newer PC system designs.
  • Older PSUs may not have connectors which are required for newer PC systems to properly operate.


This is a practical guidance what to mix and what not to mix:


  • Older systems (until Pentium 4 and Athlon XP platforms) were designed to draw most power from 5V and 3.3V rails.
  • Pentium 4 and Athlon XP systems draw much more from 12V rail, instead from 5V and 3.3V rails.
  • Newer systems (Athlon 64, Core Duo etc.) draw most power from 12V rails.


  • Original ATX PSUs have power distribution designed for pre-P4/XP PCs. They lack the supplemental 4-pin 12-volt connector for the CPU, so they simply cannot be used with P4/XP era or newer motherboards (adapters do exist but power drain on the 12V rail must be checked very carefully if using them).
  • ATX12V 1.x PSUs have power distribution designed for P4/XP PCs, but they are also greatly suitable for older PCs, since they give plenty of power (relative to old PCs' needs) both on 12V and on 5V/3.3V. Some of them might not have the -5V rail which is needed for some special add-in ISA cards. It is not recommended to use ATX12V 1.x PSUs on ATX12V 2.x motherboards because those systems require much more power on 12V, and much less on 3.3V/5V than ATX12V PSUs provide.
  • ATX12V 2.x PSUs have power distribution designed for late P4/XP PCs and for Athlon 64 and Core Duo PCs. They can be used with earlier P4/XP PCs, but the power distribution will be significantly suboptimal, so a more powerful ATX12V 2.0 PSU should be used to compensate for that discrepancy. ATX12V 2.x PSUs can also be used with pre-P4/XP systems, but the power distribution will be greatly suboptimal (12V rails will be mostly unused, while the 3.3V/5V rails will be overloaded), so this is not recommended.


Special note: Proprietary brand-name or high-end workstation/server designs do not fit into these guidelines. They usually require an exactly matching power supply unit.

Issues with Dell power supplies

Older Dell computers, particularly those from the Pentium II and III times, are notable for using proprietary power wiring on their power supplies and motherboards. While the motherboard connectors appear to be standard ATX, and will actually fit a standard power supply, they are not compatible. Not only have wires been switched from one location to another, but the number of wires for a given voltage has been changed. Thus, the pins cannot simply be rearranged.[2]

The change affects not only 20-pin ATX connectors, but also auxiliary 6-pin connectors. Modern Dell systems may use standard ATX connectors.[3] Dell PC owners should be careful when attempting to mix non-Dell motherboards and power supplies, as it can cause damage to the power supply or other components. If the power supply color coding on the wiring does not match ATX standards, then it is probably proprietary. Wiring diagrams for Dell systems are usually available on Dell's support page.

[edit] Connectors

ATX I/O plates

On the back of the system, some major changes were made. The AT standard had only a keyboardserial and parallel ports) had to be connected via flying leads to connectors which were mounted either on spaces provided by the case or brackets placed in unused expansion slot positions. ATX allowed each motherboard manufacturer to put these ports in a rectangular area on the back of the system, with an arrangement they could define themselves (though a number of general patterns depending on what ports the motherboard offers have been followed by most manufacturers). Generally the case comes with a snap out panel, also known as an I/O plate, reflecting one of the common arrangements. If necessary, I/O plates can be replaced to suit the arrangement on the motherboard that is being fitted and the I/O plates are usually included when purchasing a motherboard. Panels were also made that allowed fitting an AT motherboard in an ATX case. connector and expansion slots for add-on card backplates. Any other onboard interfaces (such as

ATX also made the PS/2-style mini-DIN keyboard and mouse connectors ubiquitous. AT systems used a 5 pin DIN connector for the keyboard, and were generally used with serial port mice (although PS/2 mouse ports were also found on some systems). Many modern motherboards are phasing out the PS/2-style keyboard and mouse connectors in favor of the modern standard of USB ports. Other legacy connectors that appeared on ATX motherboards but are being phased out include 25-pin parallel ports and 9-pin RS-232 serial ports. In their place are on-board peripheral ports such as Ethernet, Firewire, External SATA, audio ports (analog/S/PDIF), video (D-sub/DVI/HDMI), and extra USB ports.

Variants

There exist several ATX-derived form factors that use the same power supply, mountings and basic back panel arrangement, but set different standards for the size of the board.

ATX Scale Reference
width length color in image
FlexATX 9 inches (228.6 mm) 7.5 inches (190.5 mm)
microATX and EmbATX 9.6 inches (243.8 mm) 9.6 inches (243.8 mm)
Mini ATX 11.2 inches (284 mm) 8.2 inches (208 mm)
Standard ATX 12 inches (304.8 mm) 9.6 inches (243.8 mm)
EATX (extended ATX) 12 inches (304.8 mm) 13 inches (330.2 mm)
WTX (workstation ATX) 14 inches (355.6 mm) 16.75 inches (425.4 mm)

Prototypes

In CeBIT 2008, Foxconn unveiled a Foxconn F1 motherboard prototype, which contains 10-slots, but the board length remains the same as standard ATX motherboard.[2] In January 2008, Lian Li[3] unveiled Armorsuit PC-P80 case with 10 slots designed for the motherboard.

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VGA card information

The VGA board tells the monitor what to expect by the polarity of the horiz. and vert. sync signals. Here's what an NEC MultiSync 2A sets up to. 
Vert Res.      Horiz Freq    H Sync    Vert Freq    V Sync
                          Polarity               Polarity

350 lines      31.5 kHz      pos       70.07 Hz     neg
400 lines      31.5 kHz      neg       70.07 Hz     pos
480 lines      31.5 kHz      neg       59.95 Hz     neg
600 lines      35.2 kHz      pos       56.24 Hz     pos
Your problem is probably one of the following; 1. You are trying a mode your monitor won't support. 2. Your monitor is out of adjustment. 3. Your monitor is faulty. Possibly, the card or monitor is not "playing the rules", eg monitor expects one set of sync rates according to the sync polarities, and the card is sending out another (unlikely but...)

VGA monitor ID signals

Mike Diack asked about VGA monitors.

Mike, I don't know if this is related to your problem or not, but IBM monitors have 3 pins dedicated to a "monitor ID" code, which is available to the VGA (or 8514/A or XGA) card, and also to the software. OS/2 uses it, for example, to automatically install the correct display support. The code: 

PIN 4      PIN 11      PIN 12           Meaning
n/c        n/c         n/c         No monitor attached
n/c        n/c         GND         Mono monitor with no support for 1024x768
n/c        GND         n/c         Color monitor with no support for 1024x768
GND        GND         n/c         Color monitor with support for 1024x768
Maybe your projector is not providing the code to tell the VGA that it is there. If so, you can try modifying the plug.

DISCLAIMER: I know this works for some Sony monitors, which support 1024x768 but don't provide the proper code to the PS/2, so they come up in 640x480. By changing the plug, the system sees the monitor as high-res-capable, and configures itself for 1024x768. Whether grounding pins in your plug will your projector, however, I can't say (although I doubt it).

VGA feature connector

Does anyone know how the VGA video feature connector operates? I would like to know which of the pins are inputs, which are outputs, and which are bidirectional (if any - and how the direction is selected).

I have found a pinout for the connector:

Video Feature Connector Pinouts.

Pin     Name   Function
1       PD0    Dac Pixel data bit 0
2       PD1                   bit 1
3       PD2                   bit 2
4       PD3                       3
5       PD4                       4
6       PD5                       5
7       PD6                       6
8       PD7                       7
9       -      Dac Clock
10      -      Dac Blanking
11      -      Horizontal Sync
12      -      Vertical Sync
13      -      Ground

14      -      Ground
15      -      Ground
16      -      Ground
17      -      Select Internal Video
18      -      Select Internal Sync
19      -      Select Internal Dot Clock
20      -      Not Used
21      -      Ground
22      -      Ground
23      -      Ground
24      -      Ground
25      -      Not Used
26      -      Not Used
And I assume that pins 1 - 12 are outputs, and 17 - 19 are inputs. Is this correct?

The reason is this - I have a Rombo Media Pro+ video digitising card. It chroma keys its output into the vga monitor signal. However, although it is supposed to work with an ET-4000 with Hi-colour RAMDAC, the colours on screen behave as if the top 2 bits of colour information are missing, and red, green, blue signals are swapped around. Rombo has suggested that this may be due to insufficient buffering on the feature connector outputs, and is happy to sell me a buffer device for 50 pounds. I would rather save about 45 pounds, and build my own. I assume it would require (for example) a 74F244 buffer IC (or two).

Can anyone help? Any information on the feature connector would be highly appreciated!

Please could you reply by email :
ee90dhg@brunel.ac.uk

VGA feature connector

component side

pin     function
1       PD0 (DAC pixel data bit 0)
2       PD1 (DAC pixel data bit 1)
3       PD2 (DAC pixel data bit 2)
4       PD3 (DAC pixel data bit 3)
5       PD4 (DAC pixel data bit 4)
6       PD5 (DAC pixel data bit 5)
7       PD6 (DAC pixel data bit 6)
8       PD7 (DAC pixel data bit 7)
9       DAC clock
10      DAC blanking
11      Ext. horizontal sync
12      Ext. vertical sync
13      Ground

back side

pin     function
1       Ground
2       Ground
3       Ground
4       Select Internal Video
5       Select Internal Syncs
6       Select Internal DAC
8..11   Ground

VESA DPMS monitor power management

VESA DPMS is a monitor power managament standard designed for green PC concept. VESA DPMS defines method how a screen saver program can put monitor to power save state when it blanks the screen. The signalling to monitor is handled using normal monitor sync signals: screen saver can turn one of sync signals (or both) off and the monitor knows from this that it must turn to power save mode. 

VESA DPMS power states:

              NORMAL          STANDBY         SUSPENDED       OFF
H-sync          On              Off             On              Off
V-sync          On              On              Off             Off
Power level     100%            80%             <30w>

NUTEK monitor power management

NUTEK is a Swedish standard for monitor power management so that screen saver program can turn monitor to power save mode when computer is not used for a while. NUTEK works using the following pronciple: when the picture signa coming to the monitor has been totally black for lomg enough, the monitor turns to power save mode. When there is other than just black coming to monitor, then the monitor turns back to normal operation.

VESA modes

This list is not complete list of SuperVGA modes standardized by VESA. For complete documentation check VESA VBE standard. 

  --------V-104F02-----------------------------
INT 10 - VESA SuperVGA BIOS - SET SuperVGA VIDEO MODE
      AX = 4F02h
      BX = mode
              bit 15 set means don't clear video memory
Return: AL = 4Fh function supported
      AH = status
          00h successful
          01h failed
SeeAlso: AX=4F01h,AX=4F03h

Values for VESA video mode:
 00h-FFh OEM video modes (see AH=00h)
 100h 640x400x256
 101h 640x480x256
 102h 800x600x16
 103h 800x600x256
 104h 1024x768x16
 105h 1024x768x256
 106h 1280x1024x16
 107h 1280x1024x256
 108h 80x60 text
 109h 132x25 text
 10Ah 132x43 text
 10Bh 132x50 text
 10Ch 132x60 text
---VBE v1.2---
 10Dh 320x200x32K
 10Eh 320x200x64K
 10Fh 320x200x16M
 110h 640x480x32K
 111h 640x480x64K
 112h 640x480x16M
 113h 800x600x32K
 114h 800x600x64K
 115h 800x600x16M
 116h 1024x768x32K
 117h 1024x768x64K
 118h 1024x768x16M
 119h 1280x1024x32K
 11Ah 1280x1024x64K
 11Bh 1280x1024x16M
Index:        video modes

Values for S3 OEM video mode:
 201h 640x480x256
 202h 800x600x16
 203h 800x600x256
 204h 1024x768x16
 205h 1024x768x256
 206h 1280x960x16
 208h 1280x1024x16
 211h 640x480x64K (Diamond Stealth 24X)
 212h 640x480x16M (Diamond Stealth 24X)
 301h 640x480x32K
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Video Graphics Array ( VGA )

The term Video Graphics Array (VGA) refers specifically to the display hardware first introduced with the IBM PS/2 line of computers in 1987[1], but through its widespread adoption has also come to mean either an analog computer display standard, the 15-pin D-subminiature VGA connector, or the 640×480 resolution itself. While this resolution has been superseded in the personal computer[2] market, it is becoming a popular resolution on mobile devices.

VGA was the last graphical standard introduced by IBM that the majority of PC clone manufacturers conformed to, making it today (as of 2008) the lowest common denominator that all PC graphics hardware supports before a device-specific driver is loaded into the computer. For example, the Microsoft Windows splash screen appears while the machine is still operating in VGA mode, which is the reason that this screen always appears in reduced resolution and color depth.

VGA was officially superseded by IBM's XGA standard, but in reality it was superseded by numerous slightly different extensions to VGA made by clone manufacturers that came to be known collectively as "Super VGA".


Technical details

VGA is referred to as an "array" instead of an "adapter" because it was implemented from the start as a single chip, replacing the Motorola 6845 and dozens of discrete logic chips covering a full-length ISA board that the MDA, CGA, and EGA used. This also allowed it to be placed directly on a PC's motherboard with a minimum of difficulty (it only required video memory, timing crystalsRAMDAC), and the first IBM PS/2 models were equipped with VGA on the motherboard. and an external

The VGA specifications are as follows:

The VGA supports both All Points Addressable graphics modes, and alphanumeric text modes. Standard graphics modes are

  • 640×480 in 16 colors
  • 640×350 in 16 colors
  • 320×200 in 16 colors
  • 320×200 in 256 colors (Mode 13h)

As well as the standard modes, VGA can be configured to emulate many of the modes of its predecessors (EGA, CGA, and MDA).

The pinout can be found in the VGA connector page.

Standard text modes

Standard alphanumeric text modes for the VGA use 80×25 or 40×25 text cells. Each cell may choose from one of 16 available colors for its foreground and 8 colors for the background; the 8 background colors allowed are the ones without the high-intensity bit set. Each character may also be made to blink; all that are set to blink will blink in unison. The blinking option for the entire screen can be exchanged for the ability to choose the background color for each cell from among all 16 colors. All of these options are the same as those on the CGA adapter as introduced by IBM.

Like EGA, VGA supports 512 simultaneous characters on screen by disabling one color bit. The glyphs on 80×25 mode are normally made of 9×16 pixels. Users may define their own character set by loading a custom font onto the card. As character data is 8-bit wide, some characters are normally made 9 bit wide by repeating the last vertical line, especially those defining horizontal IBM box drawing characters.[8]

VGA adapters usually support both a monochrome and a color text mode, though the monochrome mode is almost never used. Black and white text on nearly all modern VGA adapters is drawn by using gray colored text on a black background in color mode. VGA monochrome monitors were sold (intended primarily for text applications), but most of them will work at least adequately with a VGA adapter in color mode. Occasionally a faulty connection between a modern monitor and video card will cause the VGA part of the card to detect the monitor as monochrome, and this will cause the BIOS and initial boot sequence to appear in greyscale. Usually once the video card's drivers are loaded (for example by continuing to boot into the operating system) they will override this detection and the monitor will return to color.

In color text mode, each screen character is actually represented by two bytes. The lower, or character byte is the actual character for the current character set, and the higher, or attribute byte is a bit field used to select various video attributes such as color, blinking, character set, and so forth. This byte-pair scheme is among the features that VGA inherited ultimately from CGA.

The VGA color palette

See also the List of monochrome and RGB palettes article — 18-bit RGB section, and the List of 16-bit computer hardware palettes article — MCGA and VGA section.
VGA 256 color palette

The VGA color system is backwards compatible with the EGA and CGA adapters, and adds another level of configuration on top of that. CGA was able to display up to 16 colors, and EGA extended this by allowing each of the 16 colors to be chosen from a 64-color palette (these 64 colors are made up of two bits each for red, green and blue: two bits × three channels = six bits = 64 different values). VGA further extends this scheme by increasing the EGA palette from 64 entries to 256 entries. Two more blocks of 64 colors with progressively darker shades were added, along with 8 "blank" entries that were set to black. [9]

In addition to the extended palette, each of the 256 entries could be assigned an arbitrary color value through the VGA DAC. The EGA BIOS only allowed 2 bits per channel to represent each entry, while VGA allowed 6 bits to represent the intensity of each of the three primaries (red, blue and green). This provided a total of 64 different intensity levels for red, green and blue, resulting in 262,144 possible colors, any 256 of which could be assigned to the palette (and in turn out of those 256, any 16 of them could be displayed in CGA video modes).

This method allowed new VGA colors to be used in EGA and CGA graphics modes, providing one remembered how the different palette systems are laid together. To set the text color to very dark red in text mode, for instance, it will need to be set to one of the CGA colors (for example, the default color, #7: light grey.) This color then maps to one in the EGA palette — in the case of CGA color 7, it maps to EGA palette entry 42. The VGA DAC must then be configured to change color 42 to dark red, and then immediately anything displayed on the screen in light-grey (CGA color 7) will become dark red. This feature was often used in 256-color VGA DOS games when they first loaded, by smoothly fading out the text screen to black.

While CGA and EGA-compatible modes only allowed 16 colors to be displayed at any one time, other VGA modes, such as the widely used mode 13h, allowed all 256 palette entries to be displayed on the screen at the same time, and so in these modes any 256 colors could be shown out of the 262,144 colors available.
Examples of VGA images in 640x480x16 and 320x200x256 modes. Dithering is used to overcome the formats' color limitations.

Addressing details

The video memory of the VGA is mapped to the PC's memory via a window in the range between segments 0xA0000 and 0xC0000 in the PC's real mode address space (A000:0000 and C000:0000 in segment:offset notation). Typically these starting segments are:

  • 0xA0000 for EGA/VGA graphics modes (64 KB)
  • 0xB0000 for monochrome text mode (32 KB)
  • 0xB8000 for color text mode and CGA-compatible graphics modes (32 KB)

Due to the use of different address mappings for different modes, it is possible to have a Monochrome Display Adapter and a color adapter such as the VGA, EGA, or CGA installed in the same machine. At the beginning of the 1980s, this was typically used to display Lotus 1-2-3Turbo Debugger, D86 (by Alan J. Cox) and Microsoft's CodeView could work in a dual monitor setup. Either Turbo Debugger or CodeView could be used to debug Windows. There were also DOS device drivers such as ox.sys, which implemented a serial interface simulation on the MDA display and, for example, allowed the user to receive crash messages from debugging versions of Windows without using an actual serial terminal. It is also possible to use the "MODE MONO" command at the DOS prompt to redirect the output to the monochrome display. When a Monochrome Display Adapter was not present it was possible to use the 0xB000 - 0xB7FF address space as additional memory for other programs (for example by adding the line "DEVICE=EMM386.EXE I=B000-B7FF" into config.sys, this memory would be made available to programs that can be "loaded high" - loaded into high memory.) spreadsheets in high-resolution text on a MDA display and associated graphics on a low-resolution CGA display simultaneously. Many programmers also used such a setup with the monochrome card displaying debugging information while a program ran in graphics mode on the other card. Several debuggers, like Borland's

Programming tricks

An undocumented but popular technique nicknamed Mode X (first coined by Michael Abrash) or "tweaked VGA" was used to make programming techniques and graphics resolutions available that were not otherwise possible in the standard Mode 13h. This was done by "unchaining" the 256 KB VGA memory into four separate "planes", which would make all of VGA's 256 KB of RAM available in 256-color modes. There was a trade-off for extra complexity and performance loss in some types of graphics operations, but this was mitigated by other operations becoming faster in certain situations:

  • Single-color polygon filling could be accelerated due to the ability to set four pixels with a single write to the hardware.
  • The video adapter could assist in copying video RAM regions, which was sometimes faster than doing this with the relatively slow CPU-to-VGA interface.
  • Several higher-resolution display modes were possible: at 16 colors, 704×528, 736×552, 768×576, and even 800×600 were possible. Software such as Xlib (a VGA graphics library for C in the early 1990s) and ColoRIX (a 256-color graphics program), also supported tweaked 256-color modes using many combinations of columns of 256, 320, and 360 pixels, and rows of 200, 240, 256, 400, and 480 lines (the upper limit being 640×400 which used 250 KB of VGA's 256 KB video ram). However, 320×240 was the best known and most-frequently used since it was a typical 4:3 aspect ratio resolution with square pixels.
  • The use of multiple video pages in hardware allowed the programmer to perform double buffering or triple buffering, which, while available in VGA's 320×200 16-color mode, was not possible using stock Mode 13h.

Sometimes the monitor refresh rate had to be reduced to accommodate these modes, increasing eye strain. They were also incompatible with some older monitors, producing display problems such as picture detail disappearing into overscan, flickering, vertical roll, and lack of horizontal syncFractint - a famous fractal generator. depending on the mode being attempted. Because of this, most VGA tweaks used in commercial products were limited to "monitor-safe" combinations, such as 320×240 (square pixels, three video pages), 320×400 (double resolution, two video pages), and 360×480 (highest resolution compatible with standard VGA monitors, one video page). Currently, the highest known tweaked VGA resolution is 400×600×256 (400×600 pixel × 256 colors). It is used in

Comparison chart

Name x
(width) 		y
(height) 		Pixels
(×1 Million) 		Aspect
Ratio		 Percentage of difference in pixels Widescreen
version		 Typical screen
sizes
VGA SVGA XGA XGA+ SXGA SXGA+ UXGA QXGA
VGA 640 480 0.31 1.33 0% −36% −61% −69% −77% −79% −84% −90% WVGA
SVGA 800 600 0.48 1.33 56% 0% −39% −52% −63% −67% −75% −85% WSVGA
XGA 1024 768 0.79 1.33 156% 64% 0% −21% −40% −47% −59% −75% WXGA 15"/ 38cm
XGA+ 1152 864 1.00 1.33 224% 107% 27% 0% −24% −32% −48% −68% WXGA+ 17"/ 43cm
SXGA 1280 1024 1.31 1.25 327% 173% 67% 32% 0% −11% −32% −58% WSXGA 17–19"/ 43–48cm
SXGA+ 1400 1050 1.47 1.33 379% 206% 87% 48% 12% 0% −23% −53% WSXGA+
UXGA 1600 1200 1.92 1.33 525% 300% 144% 93% 46% 31% 0% −39% WUXGA 20"/ 51cm
QXGA 2048 1536 3.15 1.33 924% 555% 300% 216% 140% 114% 64% 0% WQXGA 30"/ 76cm
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CD ROM

Compact Disc

CD-ROM discs are identical in appearance to audio CDs, and data are stored and retrieved in a very similar manner (only differing from audio CDs in the standards used to store the data). Discs are made from a 1.2 mm thick disc of polycarbonate plastic, with a thin layer of aluminiumMini CD standard with an 80 mm diameter, as well as numerous non-standard sizes and shapes (e.g. business card-sized media) are also available. Data is stored on the disc as a series of microscopic indentations. A laser is shown onto the reflective surface of the disc to read the pattern of pits and lands ("pits", with the gaps between them referred to as "lands"). Because the depth of the pits is approximately one-quarter to one-sixth of the wavelength of the laser light used to read the disc, the reflected beam's phase is shifted in relation to the incoming beam, causing destructive interference and reducing the reflected beam's intensity. This pattern of changing intensity of the reflected beam is converted into binary data. to make a reflective surface. The most common size of CD-ROM disc is 120 mm in diameter, though the smaller

Standard

There are several formats used for data stored on compact discs, known collectively as the Rainbow Books, or CD's. These include the original Red Book standards for CD audio, White BookYellow Book CD-ROM. The ECMA-130 standard, which gives a thorough description of the physics and physical layer of the CD-ROM, inclusive of Cross-interleaved Reed-Solomon codingEight-to-Fourteen Modulation, can be downloaded from [1]. and (CIRC) and

ISO 9660 defines the standard file system of a CD-ROM, although it is due to be replaced by ISO 13490. UDF format is used on user-writeable CD-R and CD-RW discs that are intended to be extended or overwritten. The bootable CD specification, to make a CD emulate a hard disk or floppy, is called El Torito. Apparently named this because its design originated in an El Torito restaurant in Irvine, California.

CD-ROM format

A CD-ROM sector contains 2352 bytes, divided into 98 24-byte frames. The CD-ROM is, in essence, a data disk, which cannot rely on error concealment or interpolation, and therefore requires a higher reliability of the retrieved data. In order to achieve improved error correction and detection, a CD-ROM has a third layer of Reed-Solomon error correction.[2] A Mode-1 CD-ROM, which has the full three layers of error correction data, contains a net 2048 bytes of the available 2352 per sector. In a Mode-2 CD-ROM, which is mostly used for video files, there are 2336 user-available bytes per sector. The net byte rate of a Mode-1 CD-ROM, based on comparison to CDDA audio standards, is 44.1k/s×4B×2048/2352 = 153.6 kB/s. The playing time is 74 minutes, or 4440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 MB.

A 1x speed CD drive reads 75 consecutive sectors per second.

CD sector contents

  • A standard 74 min CD contains 333,000 blocks or sectors.
  • Each sector is 2352 bytes, and contains 2048 bytes of PC (MODE1) Data, 2336 bytes of PSX/VCD (MODE2) Data, or 2352 bytes of AUDIO.
  • The difference between sector size and data content are the Headers info and the Error Correction Codes, that are big for Data (high precision required), small for VCD (standard for video) and none for audio.
  • If extracting the disc in RAW format (standard for creating images) always extract 2352 bytes per sector, not 2048/2336/2352 bytes depending on data type (basically, extracting the whole sector). This fact has two main consequences:
    • Recording data CDs at very high speed (40x) can be done without losing information. However, as audio CDs do not contain a third layer of error correction codes, recording these at high speed may result in more unrecoverable errors or 'clicks' in the audio.
    • On a 74 minute CD, one can fit larger images using RAW mode, up to 333,000 × 2352 = 783,216,000 bytes (747~ MB). This is the upper limit for RAW images created on a 74 min or 650~ MB Red Book CD. The 14.8% increase is due to the discarding of error correction data
    • The sync pattern for Mode 1 CDs is 0xff00ffffffffffffffff00ff[citation needed]
  • Please note that an image size is always a multiple of 2352 bytes (the size of a block) when extracting in RAW mode.[3]
Layout Type ← 2,352 bytes block →
CD Digital Audio: 2,352 bytes of Digital Audio
CD-ROM (MODE1): 12 4 2,048 bytes of user data 4 8 276
CD-ROM (MODE2): 12 4 2,336 bytes of user data
Legend (bytes)
12 sync
4 sector ID

data
4 error detection
8 blank/null
276 error correction


Manufacture

CD manufacturing

Pre-pressed CD-ROMs are mass-produced by a process of stamping where a glass master disc is created and used to make "stampers", which are in turn used to manufacture multiple copies of the final disc with the pits already present. Recordable (CD-R) and rewritable (CD-RW) discs are manufactured by a similar method, but the data are recorded on them by a laser changing the properties of a dye or phase change material in a process that is often referred to as "burning".

Capacity

The CD-ROM can easily contain all the encyclopaedia's words and images, plus audio & video clips

CD-ROM capacities are normally expressed with binary prefixes, subtracting the space used for error correction data. A standard 120 mm, "700 MB" CD-ROM can hold about 847 MB of data, or 737 MB (703 MiB) with error correction. In comparison, a single-layer DVD-ROM can hold 4.7 GB of error-protected data, more than 6 CD-ROMs.

Capacities of Compact Disc types
Type Sectors Data max size Audio max size Time
(MB) (MB) (min)
8 cm 94,500 193.536 222.264 21

283,500 580.608 666.792 63
650 MB 333,000 681.984 783.216 74
700 MB 360,000 737.280 846.720 80
800 MB 405,000 829.440 952.560 90
900 MB 445,500 912.384 1,047.816 99
Note: Megabyte (MB) and minute (min) values are exact.

CD-ROM drives

Further information: Optical disc drive
Old 4x CD-ROM Drive

CD-ROM discs are read using CD-ROM drives, which are now almost universal on personal computers. A CD-ROM drive may be connected to the computer via an IDE (ATA), SCSI, S-ATA, Firewire, or USB interface or a proprietary interface, such as the Panasonic CD interface. Virtually all modern CD-ROM drives can also play audio CDs as well as Video CDs and other data standards when used in conjunction with the right software.

Laser and Optics

CD-ROM drives employ a near-infrared 780 nm laser diode. The laser beam is directed onto the disc via an opto-electronic tracking module, which then detects whether the beam has been reflected or scattered.

Transfer rates

The rate at which CD-ROM drives can transfer data from the disc is gauged by a speed factor relative to music CDs: 1x or 1-speed which gives a data transfer rate of 150 kilobytes per secondmegabytes per second. Above 12x speed, vibration and heat can become a problem. CD-ROM drives above this speed tackle the problem in several ways. Constant angular velocitySamsung Electronics introduced the SCR-3230, a 32x CD-ROM drive which uses a ball bearing system to balance the spinning disc in the drive to reduce vibration and noise. As of 2004, the fastest transfer rate commonly available is about 52x or 10,350 rpm and 7.62 megabytes per second, though this is only when reading information from the outer parts of a disc. Future speed increases based simply upon spinning the disc faster are particularly limited by the strength of polycarbonate plastic used in CD manufacturing, though improvements can still be obtained by the use of multiple laser pickups as demonstrated by the Kenwood TrueX 72x which uses seven laser beams and a rotation speed of approximately 10x. in the most common data format. By increasing the speed at which the disc is spun, data can be transferred at greater rates. For example, a CD-ROM drive that can read at 8x speed spins the disc at up to 4000 rpm (compared to the 500 rpm maximum for 1x speed), giving a transfer rate of 1.2 (CAV) drives spin the disc at a constant rate, leading to faster data transfer when reading from the outer parts of the disc, but slower towards the centre. 20x was thought to be the maximum speed due to mechanical constraints until

CD-Recordable drives are often sold with three different speed ratings, one speed for write-once operations, one for re-write operations, and one for read-only operations. The speeds are typically listed in that order; ie a 12x/10x/32x CD drive can, CPU and media permitting, write to CD-R discs at 12x speed (1.80 MB/s), write to CD-RW discs at 10x speed (1.50 MB/s), and read from CD discs at 32x speed (4.80 MB/s).

The 1x speed rating for CD-ROM (150 kB/s) is different than 1x speed rating for audio CD (172.3 kB/s) and is not to be confused with the 1x speed rating for DVDs (1.32 MB/s).

A view of a CD-ROM drive's disassembled laser system.
The movement of the laser enables reading at any position of the CD.
The laser system of a CD Drive.

Common transfer speeds:

Data Transfer Speeds
Transfer Speed KiB/s Mb/s
1x 150 1.2288
2x 300 2.4576
4x 600 4.9152
8x 1200 9.8304
10x 1500 12.2880
12x 1800 14.7456
20x 3000 24.5760
32x 4800 39.3216
36x 5400 44.2368
40x 6000 49.1520
48x 7200 58.9824
50x 7500 61.4400
52x 7800 63.8976

Copyright issues

CD/DVD copy protection

There has been a move by the recording industry to make audio CDs (CDDAs, Red Book CDs) unplayable on computer CD-ROM drives, to prevent the copying of music. This is done by intentionally introducing errors onto the disc that the embedded circuits on most stand-alone audio players can automatically compensate for, but which may confuse CD-ROM drives. Consumer rights advocates are as of October 2001 pushing to require warning labels on compact discs that do not conform to the official Compact Disc Digital Audio standard (often called the Red Book) to inform consumers of which discs do not permit full fair use of their content.

In 2005, Sony BMG Music Entertainment was criticised when a copy protection mechanism known as Extended Copy Protection (XCP) used on some of their audio CDs automatically and surreptitiously installed copy-prevention software on computers (see 2005 Sony BMG CD copy protection scandal). Such discs are not legally allowed to be called CDs or Compact Discs because they break the Red Book standard governing CDs, and Amazon.com for example describes them as "copy protected discs" rather than "compact discs" or "CDs".

Software distributors, and in particular distributors of computer games, often make use of various copy protection schemes to prevent software running from any media besides the original CD-ROMs. This differs somewhat from audio CD protection in that it is usually implemented in both the media and the software itself. The CD-ROM itself may contain "weak" sectors to make copying the disc more difficult, and additional data that may be difficult or impossible to copy to a CD-R or disc image, but which the software checks for each time it is run to ensure an original disc and not an unauthorized copy is present in the computer's CD-ROM drive.

Manufacturers of CD writers (CD-R or CD-RW) are encouraged by the music industry to ensure that every drive they produce has a unique identifier, which will be encoded by the drive on every disc that it records: the RID or Recorder Identification Code.[4] This is a counterpart to the SID—the Source Identification Code, an eight character code beginning with "IFPI" that is usually stamped on discs produced by CD recording

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