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Currently the XGA exists only in Microchannel versions.
All register accesses happen through a block of 16 registers starting
at a adapter dependant register (called xga in the following).
The register xga+0Ah works as an indexed register ie. xga+0Ah is the
index register and xga+0Bh is the data register.
xga+0: Operating Mode
bit 0 Set if VGA, clear if dual monitor.
2 Set if in an extended mode.
xga+1: Video Memory Aperture Control
0 disables 64k aperture
xga+4: Interrupt Enable
0 disables interrupts.
xga+5: Interrupt Status
bit 0 Start of blanking
writing 0FFh clears interrupts.
xga+6: Virtual Memory Control
xga+8: Video Memory Aperture Index.
bit 0-7 64k bank number.
xga+9: Memory Access Mode
set to 2 in 16 color modes, 3 in 256 color modes and
4 in 65536 color modes.
xga+Ah index 10h: Horizontal Total low.
bit 0-7 lower 8 bits of the total number of character clocks
in a scanline.
xga+Ah index 11h: Horizontal Total high.
bit 0-7 upper 8 bits of the total number of character clocks
in a scanline.
xga+Ah index 12h: Horizontal Displayed low.
bit 0-7 lower 8 bits of the number of displayed character clocks
in a scanline.
xga+Ah index 13h: Horizontal Displayed high.
bit 0-7 upper 8 bits of the number of displayed character clocks
in a scanline.
xga+Ah index 14h: Horizontal Blanking Start low.
bit 0-7 lower 8 bits of the count at which blanking starts.
xga+Ah index 15h: Horizontal Blanking Start high.
bit 0-7 upper 8 bits of the count at which blanking starts.
xga+Ah index 16h: Horizontal Blanking End low.
bit 0-7 lower 8 bits of the count at which blanking ends.
xga+Ah index 17h: Horizontal Blanking End high.
bit 0-7 upper 8 bits of the count at which blanking ends.
xga+Ah index 18h: Horizontal Sync Start low.
bit 0-7 lower 8 bits of the Horizontal Sync Start.
xga+Ah index 19h: Horizontal Sync Start high.
bit 0-7 upper 8 bits of the Horizontal Sync Start.
xga+Ah index 1Ah: Horizontal Sync End low.
bit 0-7 lower 8 bits of the Horizontal Sync End.
xga+Ah index 1Bh: Horizontal Sync End high.
bit 0-7 upper 8 bits of the Horizontal Sync End.
xga+Ah index 1Ch: Horizontal Sync Position.
bit 0-7 lower 8 bits of the Horizontal sync position.
xga+Ah index 1Eh: Horizontal Sync Position.
bit 0-7 upper 8 bits of the Horizontal sync position.
xga+Ah index 20h: Vertical Total low.
bit 0-7 lower 8 bits of the total number of scanlines.
xga+Ah index 21h: Vertical Total high.
bit 0-7 upper 8 bits of the total number of scanlines.
xga+Ah index 22h: Vertical Displayed End low.
bit 0-7 lower 8 bits of the number of displayed scanlines.
xga+Ah index 23h: Vertical Displayed End high.
bit 0-7 upper 8 bits of the number of displayed scanlines.
xga+Ah index 24h: Vertical Blanking Start low.
bit 0-7 lower 8 bits of the Vertical Blanking Start.
xga+Ah index 25h: Vertical Blanking Start high.
bit 0-7 upper 8 bits of the Vertical Blanking Start.
xga+Ah index 26h: Vertical Blanking End low.
bit 0-7 lower 8 bits of the Vertical Blanking End.
xga+Ah index 27h: Vertical Blanking End high.
bit 0-7 upper 8 bits of the Vertical Blanking End.
xga+Ah index 28h: Vertical Sync Start low.
bit 0-7 lower 8 bits of the Vertical Sync Start.
xga+Ah index 29h: Vertical Sync Start high.
bit 0-7 upper 8 bits of the Vertical Sync Start.
xga+Ah index 2Ah: Vertical Sync End.
bit 0-7 .
xga+Ah index 2Ch: Vertical Line Compare low.
bit 0-7 lower 8 bits of the Line Compare register.
xga+Ah index 2Dh: Vertical Line Compare high.
bit 0-7 upper 8 bits of the Line Compare register.
xga+Ah index 36h: Sprite Control.
xga+Ah index 40h: Start Address low.
bit 0-7 bit 0-7 of the display start address.
xga+Ah index 41h: Start Address middle.
bit 0-7 bit 8-15 of the display Start Address.
xga+Ah index 42h: Start Address high.
bit 0-7 bit 16-23 of the display Start Address.
xga+Ah index 43h: Pixel Map Width low.
bit 0-7 lower 8 bits of the Pixel Map Width.
xga+Ah index 44h: Pixel Map Width high.
bit 0-7 upper 8 bits of the Pixel Map Width.
xga+Ah index 50h: Display Mode 1.
7=800x600, Fh=1024x768 and C7h=640x480.
xga+Ah index 51h: Display Mode 2.
2=16 color mode, 3=256 color mode and 4=64k color mode.
xga+Ah index 52h: Monitor type.
bit 2 Clear if capable of 1024x768.
xga+Ah index 54h: Clock Select.
0=640x480, 1=800x600, Dh=1024x768 and 4=VGA.
xga+Ah index 55h: Border Color.
xga+Ah index 60h: Palette Address Index low.
bit 0-7 lower 8 bits of the Palette Address Index.
xga+Ah index 61h: Palette Address Index high.
bit 0-7 upper 8 bits of the Palette Address Index.
xga+Ah index 62h: Sprite Pre low.
bit 0-7 lower 8 bits of the Sprite Pre.
xga+Ah index 63h: Sprite Pre high.
bit 0-7 upper 8 bits of the Sprite Pre.
xga+Ah index 64h: Palette Access.
0 disables display, FFh enables display.
xga+Ah index 65h: Palette Data Port.
bit 0-7 Palette data is read and written to this port.
Each read or write of the register will increment the
palette address, first through the Red, Green Blue
cycle, and then increment the Palette Address Index.
xga+Ah index 66h: Palette Mode.
xga+Ah index 70h: External Clock.
Video Modes:
640x480 256 colors
640x480 65536 colors
800x600 16 colors
800x600 256 colors
800x600 65536 colors
1024x768 16 colors
1024x768 256 colors
All modes use a linear address mode, where bits 16-23 of the address
are in the Video Aperture Index register (xga+8), and the lower 16 bits
are the offset from 0A000:0.
In 16 color modes two pixels are stored in each byte. Even pixels are in
bits 0-3 and odd pixels in bits 4-7.
Address of pixel = (row *(pixels per row)+coloumn) /2.
In 256 color modes each pixel occupies a byte. The pixels are addressed
in linear fashion. Address of pixel = row *(pixels per row) + coloumn.
In 65536 color modes each pixel occupies two bytes.
Address of pixel = (row *(pixels per row)+coloumn)*2.