3C0h: Attribute Controller: Address register
bit 0-4 Address of data register to write to port 3C0h
or read from port 3C1h (Reads only on VGA).
5 If set screen output is enabled and the palette can not be modified,
if clear screen output is disabled and the palette can be modified.
Port 3C0h is special in that it is both address and data-write
register. Data reads happen from port 3C1h. An internal
flip-flop remembers whether it is currently acting as
address or data register.
Accesses to the attribute controller must be seperated by
at least 250ns.
Reading port 3dAh will reset the flip-flop to address mode.
3C0h index 0-Fh (r/W): Attribute: Palette
bit 0 (EGA) Primary Blue
1 (EGA) Primary Green
2 (EGA) Primary Red
3 (EGA) Secondary Blue
4 (EGA) Secondary Green
5 (EGA) Secondary Red
0-5 (VGA) Index into the 256 color DAC table.
May be modified by 3C0h index 10h and 14h.
3C0h index 10h (r/W): Attribute: Mode Control Register
bit 0 Graphics mode if set, Alphanumeric mode else.
1 Monochrome mode if set, color mode else.
2 9-bit wide caharcters if set.
The 9th bit of characters C0h-DFh will be the same as
the 8th bit. Otherwise it will be the background color.
3 If set Attribute bit 7 is blinking, else high intensity.
5 (VGA Only) If set the PEL panning register (3C0h index 13h)
is temporarily set to 0 from when the line
compare causes a wrap around until the next
vertical retrace when the register is automatically
reloaded with the old value, else the PEL
panning register ignores line compares.
6 (VGA Only) If set pixels are 8 bits wide.
Used in 256 color modes.
7 (VGA Only) If set bit 4-5 of the index into the DAC table
are taken from port 3C0h index 14h bit 0-1,
else the bits in the palette register are used.
3C0h index 11h (r/W): Attribute: Overscan Color Register.
bit 0-5 Color of screen border. Color is defined as in the
palette regsiters.
Note: The EGA requires the Overscan color to be 0 in high resolution
modes.
3C0h index 12h (r/W): Attribute: Color Plane Enable Register
bit 0 Bit plane 0 is enabled if set.
1 Bit plane 1 is enabled if set.
2 Bit plane 2 is enabled if set.
3 Bit plane 3 is enabled if set.
4-5 Video Status MUX. Diagnostics use only.
Two attribute bits appear on bits 4 and 5 of the Input
Status Register 1 (3dAh).
Value EGA VGA
0 Red/Blue Bit 2/Bit 0
1 Blue'/Green Bit 5/Bit 4
2 Red'/Green' Bit 3/Bit 1
3 Bit 7/Bit 6
3C0h index 13h (r/W): Attribute: Horizontal PEL Panning Register
bit 0-3 Indicates number of pixels to shift the display left
Value 9bit textmode 256color mode Other modes
0 1 0 0
1 2 n/a 1
2 3 1 2
3 4 n/a 3
4 5 2 4
5 6 n/a 5
6 7 3 6
7 8 n/a 7
8 0 n/a n/a
3C0h index 14h (r/W): Attribute: Color Select Register (VGA Only)
bit 0-1 If 3C0h index 10h bit 7 is set these 2 bits are used
as bits 4-5 of the index into the DAC table.
2-3 These 2 bits are used as bit 6-7 of the index into the
DAC table.
Note: this register does not affect 256 color modes.
3C2h (R): Input Status #0 Register
bit 4 Status of the switch selected by the Miscellaneous Output
Register 3C2h bit 2-3. Switch high if set.
5 (EGA Only) Pin 19 of the Feature Connector (FEAT0)
is high if set
6 (EGA Only) Pin 17 of the Feature Connector (FEAT1)
is high if set
7 (EGA Only ??) If set IRQ 2 has happened due to Vertical
Retrace. Should be cleared by IRQ 2 interrupt routine
by clearing port 3d4h index 11h bit 4.
3C2h (W): Miscellaneous Output Register
bit 0 If set Color Emulation. Base Address=3Dxh
else Mono Emulation. Base Address=3Bxh.
1 Enable CPU Access to video memory if set
2-3 Clock Select
0: 14MHz(EGA) 25MHz(VGA)
1: 16MHz(EGA) 28MHz(VGA)
2: External(EGA) Reserved(VGA)
4 (EGA Only) Disable internal video drivers if set
5 When in Odd/Even modes Select High 64k bank if set
6 Horizontal Sync Polarity. Negative if set
7 Vertical Sync Polarity. Negative if set
Bit 6-7 indicates the number of lines on the display:
0=200(EGA) Reserved(VGA)
1= 400(VGA)
2=350(EGA) 350(VGA)
3= 480(VGA).
Note: Set to all zero on a hardware reset.
Note: On the VGA this register can be read from port 3CCh.
3C3h (W): Video Subsystem Enable Register
bit 0 Enables the VGA display if set
3C4h index 0 (r/W): Sequencer: Reset
bit 0 (EGA) Asynchronous Reset if clear
0 (VGA) Synchronous Reset just as bit 1
1 Synchronous Reset if clear
3C4h index 1 (r/W): Sequencer: Clocking Mode
bit 0 If set character clocks are 8 dots wide, else 9.
1 (EGA Only) If set the CRTC uses 2/5 of the clock cycles, else 4/5.
2 If set loads video serializers every other character
clock cycle, else every one.
3 If set the Dot Clock is Master Clock/2, else same as
Master Clock (See 3C2h bit 2-3). (Doubles pixels).
4 (VGA Only) If set loads video serializers every fourth
character clock cycle, else every one.
5 (VGA Only) if set turns off screen and gives all memory
cycles to the CPU interface.
3C4h index 2 (r/W): Sequencer: Map Mask Register
bit 0 Enable writes to plane 0 if set
1 Enable writes to plane 1 if set
2 Enable writes to plane 2 if set
3 Enable writes to plane 3 if set
3C4h index 3 (r/W): Sequencer: Character Map Select Register
bit 0-1 (EGA) Selects EGA Character Map (0..3) if bit 3 of
the character attribute is clear.
2-3 (EGA) Selects EGA Character Map (0..3) if bit 3 of
the character attribute is set.
0,1,4 (VGA) Selects VGA Character Map (0..7) if bit 3 of
the character attribute is clear.
2,3,5 (VGA) Selects VGA Character Map (0..7) if bit 3 of
the character attribute is set.
Character Maps are placed at:
Map no. (EGA/VGA) Map no. (VGA)
0 0k 4 8k
1 16k 5 24k
2 32k 6 40k
3 48k 7 56k
3C4h index 4 (r/W): Sequencer: Memory Mode Register
bit 0 Set if in an alphanumeric mode, clear in graphics modes.
1 Set if more than 64kbytes on the adapter.
2 Enables Odd/Even addressing mode if set.
Odd/Even mode places all odd bytes in plane 1&3, and
all even bytes in plane 0&2.
3 (VGA Only) If set address bit 0-1 selects video memory
planes (256 color mode), rather than the
Map Mask and Read Map Select Registers.
3C6h (R/W): PEL Mask (VGA Only)
bit 0-7 Reserved for internal use.
Should always be set to FFh.
3C7h (R): DAC State Register (VGA Only)
bit 0-1 0 indicates the DAC is in Read Mode and 3 indicates
write mode.
3C7h (W): PEL Address Read Mode (VGA Only)
bit 0-7 The PEL data register (0..255) to be read from 3C9h.
Note: After reading the 3 bytes at 3C9h this register will
increment, pointing to the next data register.
3C8h (R/W): PEL Address Write Mode (VGA Only)
bit 0-7 The PEL data register (0..255) to be written to 3C9h.
Note: After writing the 3 bytes at 3C9h this register will
increment, pointing to the next data register.
3C9h (R/W): PEL Data Register (VGA Only)
bit 0-5 Color value
Note: Each read or write of this register will cycle through first
the registers for Red, Blue and Green, then increment the
appropriate address register, thus the entire palette can be
loaded by writing 0 to the PEL Address Write Mode register 3C8h
and then writing all 768 bytes of the palette to this register.
3CAh (R): Feature Control Register (VGA Only)
Bit 3 (VGA Only) Vertical Sync Select
If set Vertical Sync to the monitor is the logical OR
of the vertical sync and the vertical display enable.
Note: This register is written to port 3dAh and read from 3CAh.
3CAh (W): Graphics 2 Position (EGA Only)
bit 0-1 Select which bit planes should be controlled by
Graphics Controller #2. Always set to 1.
3CCh (R): Miscellaneous Output Register (VGA Only)
bit 0 If set Color Emulation. Base Address=3Dxh
else Mono Emulation. Base Address=3Bxh.
1 Enable CPU Access to video memory if set
2-3 Clock Select
0= 25MHz, 1= 28MHz, 2= Reserved
5 When in Odd/Even modes Select High 64k bank if set
6 Horizontal Sync Polarity. Negative if set
7 Vertical Sync Polarity. Negative if set
Bit 6-7 indicates the number of lines on the display:
0=Reserved, 1=400, 2=350, 3=480.
Note: This register is written to port 3C2h and read from port 3CCh.
3CCh (W): Graphics 1 Position (EGA Only)
bit 0-1 Select which bit planes should be controlled by
Graphics Controller #1. Always set to 0.
3CEh index 0 (r/W): Graphics: Set/Reset Register
bit 0 If in Write Mode 0 and bit 0 of 3CEh index 1 is set
a write to display memory will set all the bits in
plane 0 of the byte to this bit, if the corresponding
bit is set in the Map Mask Register (3CEh index 8).
1 Same for plane 1 and bit 1 of 3CEh index 1.
2 Same for plane 2 and bit 2 of 3CEh index 1.
3 Same for plane 3 and bit 3 of 3CEh index 1.
3CEh index 1 (r/W): Graphics: Enable Set/Reset Register
bit 0 If set enables Set/reset of plane 0 in Write Mode 0.
1 Same for plane 1.
2 Same for plane 2.
3 Same for plane 3.
3CEh index 2 (r/W): Graphics: Color Compare Register
bit 0-3 In Read Mode 1 each pixel at the address of the byte read
is compared to this color and the corresponding bit in
the output set to 1 if they match, 0 if not.
The Color Dont Care Register (3CEh index 7) can exclude
bitplanes from the comparison.
3CEh index 3 (r/W): Graphics: Data Rotate
bit 0-3 Number of positions to rotate data right before it is
written to display memory.
4-5 In Write Mode 2 this field controls the relation between
the data written from the CPU, the data latched from the
previous read and the data written to display memory:
0: CPU Data is written unmodified
1: CPU data is ANDed with the latched data
2: CPU data is ORed with the latch data.
3: CPU data is XORed with the latched data.
3CEh index 4 (r/W): Graphics: Read Map Select Register
bit 0-1 Number of the plane read Mode 0 will read from.
3CEh index 5 (r/W): Graphics: Mode Register
bit 0-1 Write Mode: Controls how data from the CPU is
transformed before being written to display memory:
0: Mode 0 works as a Read-Modify-Write operation.
First a read access loads the data latches of the EGA/VGA
with the value in video memory at the addressed location.
Then a write access will provide the destination address
and the CPU data byte. The data written is modified by the
function code in the Data Rotate register (3CEh index 3) as
a function of the CPU data and the latches, then data
is rotated as specified by the same register.
1: Mode 1 is used for video to video transfers.
A read access will load the data latches with the contents
of the addressed byte of video memory. A write access will
write the contents of the latches to the addressed byte.
Thus a single MOVSB instruction can copy all pixels in the
source address byte to the destination address.
2: Mode 2 writes a color to all pixels in the addressed byte
of video memory. Bit 0 of the CPU data is written to plane 0
etc. Individual bits can be enabled or disabled through the
Bit Mask register (3CEh index 8).
3: (VGA Only)
2 (EGA Only) Forces all outputs to a high impedance state if set.
3 Read Mode
0: Data is read from one of 4 bit planes depending
on the Read Map Select Register (3CEh index 4).
1: Data returned is a comparison between the 8 pixels
occupying the read byte and the color in the
Color Compare Register (3CEh index 2).
A bit is set if the color of the corresponding
pixel matches the register.
4 Enables Odd/Even mode if set (See 3C4h index 4 bit 2).
5 Enables CGA style 4 color pixels using even/odd bit pairs
if set.
6 (VGA Only) Enables 256 color mode if set.
3CEh index 6 (r/W): Graphics: Miscellaneous Register
bit 0 Indicates Graphics Mode if set, Alphanumeric mode else.
1 Enables Odd/Even mode if set.
2-3 Memory Mapping:
0: use A000h-BFFFh
1: use A000h-AFFFh EGA/VGA Graphics modes
2: use B000h-B7FFh Monochrome modes
3: use B800h-BFFFh CGA modes
3CEh index 7 (r/W): Graphics: Color Dont Care Register
bit 0 Ignore bit plane 0 in Read mode 1 if clear.
1 Ignore bit plane 1 in Read mode 1 if clear.
2 Ignore bit plane 2 in Read mode 1 if clear.
3 Ignore bit plane 3 in Read mode 1 if clear.
3CEh index 8 (r/W): Graphics: Bit Mask Register
bit 0-7 Each bit if set enables writing to the corresponding
bit of a byte in display memory.
3d4h index 0 (r/W): CRTC: Horizontal Total Register
bit 0-7 (EGA) Horizontal Total Character Clocks-2
0-7 (VGA) Horizontal Total Character Clocks-5
3d4h index 1 (r/W): CRTC: Horizontal Display End Register
bit 0-7 Number of Character Clocks Displayed -1
3d4h index 2 (r/W): CRTC: Start Horizontal Blanking Register
bit 0-7 The count at which Horizontal Blanking starts
3d4h index 3 (r/W): CRTC: End Horizontal Blanking Register
bit 0-4 Horizontal Blanking ends when the last 5 (6 for VGA)
bits of the character counter equals this field.
(VGA) The sixth bit is found in port 3d4h index 5 bit 7.
5-6 Number of character clocks to delay start of display
after Horizontal Total has been reached.
7 (VGA Only) Access to Vertical Retrace registers if set
3d4h index 4 (r/W): CRTC: Start Horizontal Retrace Register
bit 0-7 Horizontal Retrace starts when the Character Counter
reaches this value.
3d4h index 5 (r/W): CRTC: End Horizontal Retrace Register
bit 0-4 Horizontal Retrace ends when the last 5 bits of the
character counter equals this value.
5-6 Number of character clocks to delay start of display
after Horizontal Retrace.
7 (EGA) Provides Smooth Scrolling in Odd/Even mode.
When set display starts from an odd byte.
7 (VGA) bit 5 of the End Horizontal Blanking count
(See 3d4h index 3 bit 0-4).
3d4h index 6 (r/W): CRTC: Vertical Total Register
bit 0-7 Lower 8 bits of the Vertical Total
Bit 8 is found in 3d4h index 7 bit 0.
(VGA) Bit 9 is found in 3d4h index 7 bit 5.
Note: For the VGA this value is the number of scan lines in the display -2.
3d4h index 7 (r/W): CRTC: Overflow Register
bit 0 Bit 8 of Vertical Total (3d4h index 6)
1 Bit 8 of Vertical Display End (3d4h index 12h)
2 Bit 8 of Vertical Retrace Start (3d4h index 10h)
3 Bit 8 of Start Vertical Blanking (3d4h index 15h)
4 Bit 8 of Line Compare Register (3d4h index 18h)
5 (VGA) Bit 9 of Vertical Total (3d4h index 6)
6 (VGA) Bit 9 of Vertical Display End (3d4h index 12h)
7 (VGA) Bit 9 of Vertical Retrace Start (3d4h index 10h)
3d4h index 8 (r/W): CRTC: Preset Row Scan Register
bit 0-4 Number of lines we have scrolled down in the first
character row. Provides Smooth Vertical Scrolling.
5-6 (VGA Only) Number of bytes to skip at the start of
scanline. Provides Smooth Horizontal Scrolling
together with the Horizontal Panning Register
(3C0h index 13h).
3d4h index 9 (r/W): CRTC: Maximum Scan Line Register
bit 0-4 Number of scan lines in a character row -1
5 (VGA) Bit 9 of Start Vertical Blanking
6 (VGA) Bit 9 of Line Compare Register
7 (VGA) Doubles each scan line if set.
I.e displays 200 lines on a 400 display.
3d4h index Ah (r/W): CRTC: Cursor Start Register
bit 0-4 First scanline of cursor within character.
5 (VGA) Turns Cursor off if set
3d4h index Bh (r/W): CRTC: Cursor End Register
bit 0-4 Last scanline of cursor within character
5-6 Delay of cursor data in character clocks.
3d4h index Ch (r/W): CRTC: Start Address High Register
bit 0-7 Upper 8 bits of the start address of the display buffer
3d4h index Dh (r/W): CRTC: Start Address Low Register
bit 0-7 Lower 8 bits of the start address of the display buffer
3d4h index Eh (r/W): CRTC: Cursor Location High Register
bit 0-7 Upper 8 bits of the address of the cursor
3d4h index Fh (r/W): CRTC: Cursor Location Low Register
bit 0-7 Lower 8 bits of the address of the cursor
3d4h index 10h (R): CRTC: Light Pen High Register (EGA Only)
bit 0-7 (EGA Only) Upper 8 bits of the address of the
lightpen position.
3d4h index 10h (r/W): CRTC: Vertical Retrace Start Register
bit 0-7 Lower 8 bits of Vertical Retrace Start. Vertical Retrace
starts when the line counter reaches this value.
Bit 8 is found in 3d4h index 7 bit 2.
(VGA Only) Bit 9 is found in 3d4h index 7 bit 7.
3d4h index 11h (R): CRTC: Light Pen Low Register (EGA Only)
bit 0-7 (EGA Only) Lower 8 bits of the address of the
lightpen position.
3d4h index 11h (r/W): CRTC: Vertical Retrace End Register
bit 0-3 Vertical Retrace ends when the last 4 bits of the
line counter equals this value.
4 if clear Clears pending Vertical Interrupts.
5 Vertical Interrupts (IRQ 2) disabled if set.
Can usually be left disabled, but some systems
(including PS/2) require it to be enabled.
6 (VGA Only) If set selects 5 refresh cycles per
scanline rather than 3.
7 (VGA Only) Disables writing to registers 0-7 if set
3d4h index 12h (r/W): CRTC: Vertical Display End Register
bit 0-7 Lower 8 bits of Vertical Display End. The display
ends when the line counter reaches this value.
Bit 8 is found in 3d4h index 7 bit 1.
(VGA Only) Bit 9 is found in 3d4h index 6.
3d4h index 13h (r/W): CRTC: Offset register
bit 0-7 Number of bytes in a scanline / K. Where K is 2 for
byte mode, 4 for word mode and 8 for Double Word mode.
3d4h index 14h (r/W): CRTC: Underline Location Register
bit 0-4 Position of underline within Character cell.
5 (VGA Only) If set memory address is only changed
every fourth character clock.
6 (VGA Only) Double Word mode addressing if set
3d4h index 15h (r/W): CRTC: Start Vertical Blank Register
bit 0-7 Lower 8 bits of Vertical Blank Start. Vertical blanking
starts when the line counter reaches this value.
Bit 8 is found in 3d4h index 7 bit 3.
3d4h index 16h (r/W): CRTC: End Vertical Blank Register
bit 0-4 (EGA) Vertical blanking stops when the lower 5 bits
of the line counter equals this field.
0-6 (VGA) Vertical blanking stops when the lower 7 bits
of the line counter equals this field.
3d4h index 17h (r/W): CRTC: Mode Control Register
bit 0 If clear use CGA compatible memory addressing system
by swapping address bit 0 and 13.
Creates 2 banks for even and odd scan lines.
1 If clear use Hercules compatible memory addressing
system by swapping address bit 1 and 14.
2 If set increase scan line counter only every second line.
3 If set increase memory address counter only every other
character clock.
4 (EGA Only) If set disable the EGA output drivers. This bit
is used for other purposes in some Super VGA chips.
5 When in Word Mode bit 15 is rotated to bit 0 if this bit
is set else bit 13 is rotated into bit 0.
6 If clear system is in word mode. Addresses are rotated
1 position up bringing either bit 13 or 15 into bit 0.
7 Clearing this bit will reset the display system
until the bit is set again.
3d4h index 18h (r/W): CRTC: Line Compare Register
bit 0-7 Lower 8 bits of the Line Compare. When the Line counter
reaches this value, the display address wraps to 0.
Privides Split Screen facilities.
Bit 8 is found in 3d4h index 7 bit 4.
(VGA Only) Bit 9 is found in 3d4h index 9 bit 6.
3dAh (R): Input Status #1 Register
bit 0 Either Vertical or Horizontal Retrace active if set
1 (EGA Only) Light Pen has triggered if set
2 (EGA Only) Light Pen switch is open if set
3 Vertical Retrace in progress if set
4-5 (EGA Only) Shows two of the 6 color outputs,
depending on 3C0h index 12h bit 4-5:
Attr: Bit 4-5: Out bit 4 Out bit 5
0 Blue Red
1 I Blue Green
2 I Red I Green
3dAh (W): Feature Control Register
bit 0 (EGA Only) Output to pin 21 of the Feature Connector.
1 (EGA Only) Output to pin 20 of the Feature Connector.
3 (VGA Only) Vertical Sync Select
If set Vertical Sync to the monitor is the logical OR
of the vertical sync and the vertical display enable.
Note: On the VGA this register can be read from port 3CAh.