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The '''IBM Color Graphics Adapter''' ('''CGA''') is the original 8-bit ISA colour graphics card IBM released alongside the [[IBM PC (5150)]] in 1981. It uses a [[Motorola 6845]] CRT controller, carries 16 KB of dedicated video RAM, and drives a TTL RGBI monitor (the [[IBM 5153]]) or a composite TV through an NTSC encoder.
{{Infobox computer peripheral
| name          = IBM Color Graphics Adapter
| image        = [[File:IBM CGA card.jpg|260px]]
| caption      = An IBM Color Graphics Adapter (CGA) card
| manufacturer  = IBM
| type          = 8-bit ISA video display adapter
| release_date  = 1981 (with the original IBM PC)
| interface    = 8-bit ISA
| compatible    = [[IBM PC (5150)]], [[IBM PC XT (5160)]], [[IBM PC AT (5170)]], PC-compatible clones
| connectivity  = DB-9 TTL RGBI (drives the [[IBM 5153]]); composite NTSC RCA; light-pen header
}}
 
The '''IBM Color Graphics Adapter''' ('''CGA''') is the original 8-bit ISA colour graphics card IBM released alongside the [[IBM PC (5150)]] in 1981. It uses a Motorola MC6845 CRT controller, carries 16 KB of dedicated video RAM, and drives a TTL RGBI monitor (the [[IBM 5153]]) or a composite TV through an NTSC encoder.


== Video Modes ==
== Video Modes ==


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<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:100%; text-align:center;"
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''CGA video modes'''
|+'''CGA video modes'''
! Mode !! Resolution !! Colours !! Notes
! Mode !! Resolution !! Colours !! Notes
|-
|-
| 0 || 40 × 25 text || 16 fg / 8 bg || Composite or RGBI
| 0 || 40 × 25 text || 16 fg / 8 bg || Colour burst on
|-
|-
| 1 || 40 × 25 text || 16 fg / 8 bg || Colour burst suppressed
| 1 || 40 × 25 text || 16 fg / 8 bg || Colour burst suppressed
|-
|-
| 2 || 80 × 25 text || 16 fg / 8 bg || 8 × 8 character cell
| 2 || 80 × 25 text || 16 fg / 8 bg ||
|-
|-
| 3 || 80 × 25 text || 16 fg / 8 bg || Colour burst suppressed
| 3 || 80 × 25 text || 16 fg / 8 bg || Colour burst suppressed
|-
|-
| 4 || 320 × 200 graphics || 4 from two fixed palettes || Palette 0: cyan/magenta/white; Palette 1: green/red/yellow
| 4 || 320 × 200 graphics || 4 from fixed palettes || Palette 0: cyan/magenta/white; Palette 1: green/red/yellow
|-
|-
| 5 || 320 × 200 graphics || 4 (greyscale on composite) || Composite "alternate" palette
| 5 || 320 × 200 graphics || 4 (greyscale on composite) || Composite "alternate" palette
|-
|-
| 6 || 640 × 200 graphics || 2 (background + foreground) || Highest resolution
| 6 || 640 × 200 graphics || 2 (background + foreground) || Highest standard resolution
|}
|}


Several undocumented modes can be coaxed out of the 6845 (160 × 100 16-colour text-block "mode 7", composite NTSC artefact colour, "8088 MPH"-style techniques), but they are not part of the BIOS interface.
Several undocumented modes can be coaxed out of the 6845 (160 × 100 16-colour text-block mode, composite NTSC artefact colour, and various demoscene techniques), but they are not part of the BIOS interface.


== Hardware ==
== Hardware ==


* '''6845 CRTC''' — clock and sync generation, addressing.
* '''MC6845 CRTC''' — clock and sync generation, addressing.
* '''16 KB video RAM''' (8 × 4116 DRAM on early boards, later consolidated).
* '''16 KB video RAM.'''
* '''TTL RGBI output''' on DB-9 (drives the [[IBM 5153]]).
* '''TTL RGBI output''' on DB-9 (drives the [[IBM 5153]]).
* '''Composite NTSC output''' on RCA jack (drives any NTSC TV).
* '''Composite NTSC output''' on RCA jack.
* '''Light-pen connector''' (rarely used).
* '''Light-pen connector.'''


== Common Faults ==
== Common Faults ==


* '''Snow''' — the famous CGA "snow" effect is not a fault but a design quirk: the video RAM is single-ported and CPU writes during the active raster cause briefly-visible noise. Disable by waiting for the horizontal retrace (most CGA-aware games already do this).
* '''Snow''' — the CGA "snow" effect is not a fault but a design quirk: the video RAM is single-ported and CPU writes during the active raster cause briefly-visible noise. Most CGA-aware games wait for the horizontal retrace to avoid it.
* '''No video, beep code 1 long + 2 short''' — the CGA card is dead or unseated. Reseat first.
* '''No video, beep code 1 long + 2 short''' — reseat the CGA card first.
* '''Missing one colour channel''' — bad output buffer at the DB-9, or cracked solder joint on the connector.
* '''Missing one colour channel''' — bad output buffer at the DB-9, or cracked solder joint on the connector.
* '''Smoke from a discoloured tantalum''' — replace the failed tantalum (typically a 22 µF / 16 V) and any series resistor that may have died with it. See [[IBM PC (5150) Capacitor Replacement Guide#ISA Card Capacitors|ISA card capacitor positions]].
* '''Smoke from a discoloured tantalum''' — replace the failed tantalum and any series resistor that may have died with it.
 
== References ==
 
* [https://en.wikipedia.org/wiki/Color_Graphics_Adapter Color Graphics Adapter — Wikipedia].


== Related Pages ==
== Related Pages ==


* [[IBM PC (5150)]]
* [[IBM PC (5150)]]
* [[IBM 5153]] — the matching colour monitor
* [[IBM 5153]]
* [[IBM Monochrome Display Adapter]] — the monochrome alternative
* [[IBM Monochrome Display Adapter]]
* [[IBM PC (5150) Capacitor Replacement Guide]]
* [[IBM PC (5150) Capacitor Replacement Guide]]


[[Category:IBM]]
[[Category:IBM]]

Latest revision as of 10:28, 21 May 2026

IBM Color Graphics Adapter
An IBM Color Graphics Adapter (CGA) card
Manufacturer IBM
Type 8-bit ISA video display adapter
Release date 1981 (with the original IBM PC)
Interface 8-bit ISA
Compatible IBM PC (5150), IBM PC XT (5160), IBM PC AT (5170), PC-compatible clones
Connectivity DB-9 TTL RGBI (drives the IBM 5153); composite NTSC RCA; light-pen header

The IBM Color Graphics Adapter (CGA) is the original 8-bit ISA colour graphics card IBM released alongside the IBM PC (5150) in 1981. It uses a Motorola MC6845 CRT controller, carries 16 KB of dedicated video RAM, and drives a TTL RGBI monitor (the IBM 5153) or a composite TV through an NTSC encoder.

Video Modes

[edit | edit source]
CGA video modes
Mode Resolution Colours Notes
0 40 × 25 text 16 fg / 8 bg Colour burst on
1 40 × 25 text 16 fg / 8 bg Colour burst suppressed
2 80 × 25 text 16 fg / 8 bg
3 80 × 25 text 16 fg / 8 bg Colour burst suppressed
4 320 × 200 graphics 4 from fixed palettes Palette 0: cyan/magenta/white; Palette 1: green/red/yellow
5 320 × 200 graphics 4 (greyscale on composite) Composite "alternate" palette
6 640 × 200 graphics 2 (background + foreground) Highest standard resolution

Several undocumented modes can be coaxed out of the 6845 (160 × 100 16-colour text-block mode, composite NTSC artefact colour, and various demoscene techniques), but they are not part of the BIOS interface.

Hardware

[edit | edit source]
  • MC6845 CRTC — clock and sync generation, addressing.
  • 16 KB video RAM.
  • TTL RGBI output on DB-9 (drives the IBM 5153).
  • Composite NTSC output on RCA jack.
  • Light-pen connector.

Common Faults

[edit | edit source]
  • Snow — the CGA "snow" effect is not a fault but a design quirk: the video RAM is single-ported and CPU writes during the active raster cause briefly-visible noise. Most CGA-aware games wait for the horizontal retrace to avoid it.
  • No video, beep code 1 long + 2 short — reseat the CGA card first.
  • Missing one colour channel — bad output buffer at the DB-9, or cracked solder joint on the connector.
  • Smoke from a discoloured tantalum — replace the failed tantalum and any series resistor that may have died with it.

References

[edit | edit source]
[edit | edit source]
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