VT8601 Apollo ProMedia
Revision 1.3 September 8, 1999
-71
VGA Registers
7HFKQRORJLHV ,QF
:H&
:H&RRQQHFW
QQHFW
VGA Sequencer Registers (SR)
Port 3C4 – VGA Sequencer Index ...................................RW
7-0
Sequencer Index
Only the lower 3 bits are implemented in a standard
VGA to point to Sequencer registers 0-4. However,
all 8 bits are implemented here to allow for extended
registers up to index FF.
Port 3C5 Index 0 – Sequencer Reset................................RW
7-2
Reserved
........................................ always reads 0
1
Synchronous Reset
0
Asynchronous Reset
Port 3C5 Index 1 – Sequencer Clocking Mode ...............RW
7-6
Reserved
........................................ always reads 0
5
Screen Off
4
Shift 4
3
Dot Clock
2
Shift Load
1
Reserved
........................................ always reads 0
0
8/9 Dot Clocks
Port 3C5 Index 2 – Sequencer Map Mask ......................RW
7-4
Reserved
........................................ always reads 0
3
Enable Map 3
2
Enable Map 2
1
Enable Map 1
0
Enable Map 0
Port 3C5 Index 3 – Sequencer Character Map Select....RW
7-6
Reserved
........................................ always reads 0
5
Character Map Select A
4
Character Map Select B
3-2
Character Map Select A
1-0
Character Map Select B
Port 3C5 Index 4 – Sequencer Memory Mode................RW
7-4
Reserved
........................................ always reads 0
3Chain 4
2
Odd / Even
1
Extended Memory
0
Reserved
........................................ always reads 0
VGA RAMDAC Registers
Port 3C6 – VGA RAMDAC Pixel Mask......................... RW
7-0
Palette Address Mask
Port 3C6 – VGA RAMDAC Command .......................... RW
This register is a non-standard VGA register (“extension
register”) located at the same port address as the VGA
RAMDAC Pixel Mask register.
In order to maintain
compatibility with standard VGA operations, access to this
register is restricted:
access is enabled by performing four
successive accesses to the Pixel Mask register at 3C6 (i.e.,
read 3C6 four times).
7-4
Color Mode Select
0000 Pseudo-Color Mode............................... default
0001 Hi-Color Mode (15-bit direct interface)
0010 Muxed Pseudo-Color Mode (16-bit pixel bus)
0011 XGA Color Mode (16-bit direct interface)
01xx -reserved-
10xx -reserved-
1100 -reserved-
1101 True Color Mode (24-bit direct interface)
111x -reserved-
3
Reserved
........................................ always reads 0
2
DAC Disable
0
DAC On (if SR20[0] = 1) ...................... default
1DAC Off
1
Reserved
........................................ always reads 0
0
RAMDAC Enable
0
Disable (Bypass) RAMDAC.................. default
1
Enable RAMDAC
Port 3C7 – VGA RAMDAC Read Index ........................ WO
Port 3C8 – VGA RAMDAC Write Index ....................... WO
Port 3C8 – VGA RAMDAC Index Readback ................. RO
7-0
RAMDAC Index
Port 3C9 Index 0-FF – RAMDAC Color Palette ........... RW
7-0
RAMDAC Color Data
There are 768 data entries in the palette consisting of 256
three-byte entries. R, G, and B 8-bit values are accessed on
successive
operations
to
this
port
with
the
index
autoincremented after every 3 accesses.
Refer to a VGA
programmers guide for further information.
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