Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!usc!ucsd!sdcsvax!ucsdhub!hp-sdd!ncr-sd!ncrlnk!ncrwic!encad!enprt!gharring
From: gharring@enprt.Wichita.NCR.COM (Gary Harrington)
Newsgroups: comp.sys.ibm.pc
Subject: Re: HELP!  Screen memory organisation.
Summary: CGA Graphics memory
Keywords: CGA Graphics
Message-ID: <494@enprt.Wichita.NCR.COM>
Date: 28 Jun 89 21:00:58 GMT
References: <278@galadriel.UUCP>
Reply-To: gharring@enprt.UUCP (Gary Harrington)
Organization: NCR Corporation, Wichita KS
Lines: 44

In article <278@galadriel.UUCP> stevep@galadriel.UUCP (Steve Paine) writes:
>I need help with the PC's screen memory...must use standard graphics...

A very quick view of CGA graphics memory:

----------------------------------------------------------------------------
|
|  CGA Graphics, 320 x 200 pixels, 4 colors, memory organization:
|  =================================================
|  |  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
|  =================================================
|   \         /                         \         /
|    \       /                           \       /
|    pixel 0,0                           pixel 3,0
|
| address 0B8000H - 0B9F3FH:  even scan lines (0,2,4,...198)
| address 0BA000H - 0BBF3FH:  odd  scan lines (1,3,5,...199)
|
|Of each bit pair, the most significant bit controls the red gun, the other
|controls the green gun.
|
|This gives:
|
|       Palette 0                           Palette 1
|    ---------------                     ---------------
|    MSB  LSB  Color                     MSB  LSB  Color
|    ---  ---  -----                     ---  ---  -----
|     0    1   Green                      0    1   Cyan
|     1    0   Red                        1    0   Magenta
|     1    1   Brown                      1    1   White
|
+---------------------------------------------------------------------------
|
|  CGA Graphics, 640 x 200 pixels, black & white, memory organization:
|  =================================================
|  |  7  |  6  |  5  |  4  |  3  |  2  |  1  |  0  |
|  =================================================
|     |                                         |
|     |                                         |
|  pixel 0,0                                 pixel 7,0
|
| addresses are the same as for 320 x 200 above.
|
---------------------------------------------------------------------------