VGA Hardware: Difference between revisions
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The VGA is a complex piece of hardware. Even though it's old, many modern graphics cards are compatible with it, including NVidia and ATI cards. This can make writing a VGA driver rather attractive. The amount of compatibility varies however, so never assume a card is compatible without proper hardware detection. ▼
Apart from real machines, several emulators and virtual machines provide VGA emulation: [[Bochs]], [[QEMU]] and [[Microsoft Virtual PC]] to name a few.▼
{{Video warning}}
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▲Apart from real machines, several emulators and virtual machines provide VGA emulation: [[Bochs]], [[QEMU]] and [[Microsoft Virtual PC]] to name a few. After 2011 video card manufacturers begun to drop VGA compatibility in favour of [[GOP]] under [[UEFI]] (VirtualBox UEFI and TianoCore both supports that).
== Overview ==
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=== What's not covered ===
While this page tries to be a complete overview on what the VGA can do, it does not fully cover the whole set of graphics. After all, a video card only turns bytes in
=== Getting started ===
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=== Port 0x3C4, 0x3CE, 0x3D4 ===
These are the most used indexed registers. The index byte is written to the port given, then the data byte can be read/written from port+1. Some programs use a single
Port 0x3D4 has some extra requirements - it requires bit 0 of the '''Miscellaneous Output Register''' to be set before it responds to this address (if cleared, these ports appears at 0x3B4). Also, registers 0-7 of 0x3D4 are write protected by the protect bit (bit 7 of index 0x11)
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The Plane Write Enable register is used to choose the plane to be written, then the memory can be
=== Memory Layout in 256-color graphics modes ===
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Although 32 bytes are reserved for each character, only 16, 14, or 8 of them are commonly used, depending on the character height.
Planes 0 and 1 are accessible from the host by writing to the video memory range. Plane 0 is accessed on even addresses, plane 1 is accessed on odd addresses, with each consecutive
=== Memory Layout in 4-color modes ===
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The read/write logic has several different operation modes. These can be chosen by setting the Graphics Mode register. The VGA has four write modes and 2 read modes, which can be set independently. By default, the VGA operates in read mode 0 and write mode 0 in such a fashion that all written data goes straight to memory, and read data from each plane is ORed together.
Registers involved:
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* The Bit Mask Register is checked, for each set bit the corresponding bit from the ALU is forwarded. If the bit is clear the bit is taken directly from the Latch.
* The Memory Plane Write Enable field is ANDed with the input from the address logic. For each set bit in the result, the corresponding plane is loaded with the result.
==== Write mode 3 ====
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* The computed bit mask is checked, for each set bit the corresponding bit from the set/reset logic is forwarded. If the bit is clear the bit is taken directly from the Latch. The result is sent towards memory.
* Finally, The Memory Plane Write Enable field and the input line from the address logic are ANDed together. The bits that remain set are the planes that are actually written.
''' Todo: more write modes, read modes '''
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==== Timing Model ====
The horizontal timing registers are based on a unit called 'character' (As they match one character in text mode). Each character equals 8 ('''9/8 Dot Mode''' is set) or 9 ('''9/8 Dot Mode''' is clear) pixels. Each scanline contains '''Horizontal Total''' + 5 characters, zero based. '''Horizontal Display End''' tells us the last character that is calculated from memory (i.e. the horizontal resolution in characters minus one). '''Horizontal Blanking Start''' and '''Horizontal Retrace Start''' give us
The vertical timing is similar, apart from the fact that these registers operate on scan lines (pixels) instead of characters. The '''Vertical Retrace End''' and '''Vertical Blank End''' registers work also similar, although they are different sizes. The Retrace End is 4 bits wide (AND with 0xF, period is 1-15 scanlines), The Blank End size is at least 7 bits (some say its 8, some say its 7), so the value is computed by ANDing with 0xFF, with the period ranging from 1-127 scanlines. As with horizontal timing, at least one scan line of overscan must be present to avoid possible artefacts.
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! index
! mode 3h (80x25 text mode)
! mode 12h (640x480 planar 16
! mode 13h (320x200 linear 256-color mode)
! mode X (320x240 planar 256 color mode)
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== See Also ==
=== External Links ===
* http://tinyvga.com/vga-timing — VGA Signal Timing
* http://www.osdever.net/FreeVGA/vga/vga.htm — includes some things not explained here
[[Category:VGA]]
[[Category:Video]]
[[Category:Hardware]]
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