Online HelpPC 2.10 Quick Reference Utility Copyright 1991 David Jurgens
6845 - Motorola CRT Controller
3B0-3BB Monochrome Monitor Adapter
3D0-3DC Color Graphics Adapter (mapped similarly)
3B0/3D0 port address decodes to 3B4/3D4
3B1/3D1 port address decodes to 3B5/3D5
3B2/3D2 port address decodes to 3B4/3D4
3B3/3D3 port address decodes to 3B5/3D5
3B4/3D4 6845 index register, selects which register [0-11h]
is to be accessed through port 3B5/3D5
3B5/3D5 6845 data register [0-11h] selected by port 3B4/3D4,
registers 0C-0F may be read. If a read occurs
without the adapter installed, FFh is returned.
3B6/3D6 port address decodes to 3B4/3D4
3B7/3D7 port address decodes to 3B5/3D5
3B8/3D8 6845 mode control register
3B9/3D9 color select register on color adapter
3BA/3DA status register (read only)
3BB/3DB light pen strobe reset
3DC preset light pen latch
3DF CRT/CPU page register (PCjr only)
Registers: Accessed through ports 3B5 & 3D5 VALID VALUES
MONO CO40 CO80 GRPH
00 - Horiz. total characters 61 38 71 38
01 - Horiz. displayed characters per line 50 28 50 28
02 - Horiz. synch position 52 2D 5A 2D
03 - Horiz. synch width in characters 0F 0A 0A 0A
04 - Vert. total lines 19 1F 1F 7F
05 - Vert. total adjust (scan lines) 06 06 06 06
06 - Vert. displayed rows 19 19 19 64
07 - Vert. synch position (character rows) 19 1C 1C 70
08 - Interlace mode 02 02 02 02
09 - Maximum scan line address 0D 07 07 01
0A - Cursor start (scan line) 0B 06 06 06
0B - Cursor end (scan line) 0C 07 07 07
0C - Start address (MSB) 00 00 00 00
0D - Start address (LSB) 00 00 00 00
0E - Cursor address (MSB) (read/write) 00 -- -- --
0F - Cursor address (LSB) (read/write) 00 -- -- --
10 - Light pen (MSB) (read only) -- -- -- --
11 - Light pen (LSB) (read only) -- -- -- --
- Registers 00-0D are write only, registers 0E-0F are read/write and
registers 10-11 are read only
- Cursor address is calculated with using the following (row*80)+col
6845 - Port 3B8 (Monochrome)
│7│6│5│4│3│2│1│0│ 3B8 CRT Control Port
│ │ │ │ │ │ │ └──── 1 = 80x25 text
│ │ │ │ │ └─┴───── unused
│ │ │ │ └──────── 1 = enable video signal
│ │ │ └───────── unused
│ │ └────────── 1 = blinking on
└─┴─────────── unused
6845 - Port 3D8 (Color)
│7│6│5│4│3│2│1│0│ 3D8 Mode Select Register
│ │ │ │ │ │ │ └──── 1 = 80x25 text, 0 = 40x25 text
│ │ │ │ │ │ └───── 1 = 320x200 graphics, 0 = text
│ │ │ │ │ └────── 1 = B/W, 0 = color
│ │ │ │ └─────── 1 = enable video signal
│ │ │ └──────── 1 = 640x200 B/W graphics
│ │ └───────── 1 = blink, 0 = no blink
└─┴────────── unused
6845 - Port 3D9 Color Text Modes
│7│6│5│4│3│2│1│0│ 3D9 Color Select Register (3B9 not used)
│ │ │ │ │ └─┴─┴──── screen/border RGB
│ │ │ │ └───────── select intensity setting
│ │ │ └────────── background intensity
└─┴─┴─────────── unused
6845 - Port 3D9 Color Graphics Modes
│7│6│5│4│3│2│1│0│ 3D9 Color Select Register (3B9 not used)
│ │ │ │ │ └─┴─┴──── RGB for background
│ │ │ │ └───────── intensity
│ │ │ └────────── unused
│ │ └─────────── 1 = palette 1, 0=palette 0 (see below)
└─┴──────────── unused
Palette 0 = green, red, brown
Palette 1 = cyan, magenta, white
6845 - Port 3DA Status Register
│7│6│5│4│3│2│1│0│ 3DA Status Register
│ │ │ │ │ │ │ └──── 1 = display enable, RAM access is OK
│ │ │ │ │ │ └───── 1 = light pen trigger set
│ │ │ │ │ └────── 0 = light pen on, 1 = light pen off
│ │ │ │ └─────── 1 = vertical retrace, RAM access OK for next 1.25ms
└─┴─┴─┴──────── unused
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