The standard VGA BIOS used interrupt 10h for comminication with the software, with function numbers 00h to 07h. The extended BIOS now uses function 4Fh of the same interrupt. The standard BIOS should do nothing when given a function number on 4Fh. The new BIOS will return status information:
Input: AH = 4Fh
AL = 00h
ES:[DI] = Pointer to buffer
Output: AX = Status
all other registers left untouched
The function fills a 256 byte buffer with data, pointed to by ES:[DI]. The structure of this
buffer is:
TYPE DataBuffer
Signature 4 bytes ;
VersionL byte ; lower Version Number
VersionH byte ; higher Version Number
OEMStringPtr long word ; pointer to description string
CapableOf long word ; 32 flags of graphics card capabilities
VidModePtr long word ; Pointer to list of available modes
TotalMemory word ; Memory available of card
Reserved 236 bytes ; --
END TYPE
VersionL: This holds the least significant digit of the version number
of the BIOS.
VersionH: This holds the most significant digit of the version number
of the BIOS. I.e. if the VersionL contained 2 and VersionH contained 1, then this would
be version 1.2 of the VESA BIOS.
OEMStringPtr: Long pointer (segment: offset) to Origional Equipment Manufacturer string. This points to an ASCIIZ string describing the Graphics Card. For example "Matrox Graphics Inc." ASCIIZ means that the string contains standard ASCII characters, and will have a zero byte to mark the end of the string.
CapableOf: This is a set of 32 bits which indicate whether or not the card if capable of certain functions. So far only the first bit had been defined:
bit 0 = DAC is switchable
0 = DAC is fixed width, with 6-bits per primary color
1 = DAC width is switchable
bits 1-31 = Reserved
VideoModePtr: Long pointer to list of video modes supported by the graphics card.
The Video Mode List is a list of Mode Numbers, each occupying one word, terminated by a
-1 (0FFFFh). To find out exactly what these modes are, their resolutions etc, see
Function 01h.
TotalMemory: This field specifies the amount of memory available on the card in 64Kb chunks.
Input: AH = 4Fh Super VGA support
AL = 01h Return Super VGA mode information
CX = Super VGA video mode
(mode number must be one of those returned by Function 0)
ES:DI = Pointer to 256 byte buffer
Output: AX = Status
(All other registers are preserved)
Again you will need another 256 byte buffer for the function to fill with information:
TYPE ModeInfoBlock
ModeAttributes word ; mode attributes
WinAAttributes byte ; window A attributes
WinBAttributes byte ; window B attributes
WinGranularity word ; window granularity
WinSize word ; window size
WinASegment word ; window A start segment
WinBSegment word ; window B start segment
WinFuncPtr long word ; pointer to window function
BytesPerScanLine word ; bytes per scan line
XResolution word ; horizontal resolution
YResolution word ; vertical resolution
XCharSize byte ; character cell width
YCharSize byte ; character cell height
NumberOfPlanes byte ; number of memory planes
BitsPerPixel byte ; bits per pixel
NumberOfBanks byte ; number of banks
MemoryModel byte ; memory model type
BankSize byte ; bank size in kb
NumberOfImagePages byte ; number of images
Reserved byte ; reserved for page function
RedMaskSize byte ; size of direct color red mask in bits
RedFieldPosition byte ; bit position of LSB of red mask
GreenMaskSize byte ; size of direct color green mask in bits
GreenFieldPosition byte ; bit position of LSB of green mask
BlueMaskSize byte ; size of direct color blue mask in bits
BlueFieldPosition byte ; bit position of LSB of blue mask
RsvdMaskSize byte ; size of direct color reserved mask in bits
DirectColorModeInfo byte ; Direct Color mode attributes
Reserved 216 bytes ; remainder of ModeInfoBlock
END TYPE
ModeAttributes: This word is a set of 16 flags. So far, only the first 5 have been defined.
bit 0 = Mode supported ?
0 = This mode is not supported in hardware
1 = This mode is supported in hardware
bit 1 = 1 (Reserved)
bit 2 = Output functions supported by BIOS
0 = Output functions not supported by BIOS
1 = Output functions supported by BIOS
bit 3 = Monochrome/color mode
0 = Monochrome mode
1 = Color mode
bit 4 = Mode type
0 = Text mode
1 = Graphics mode
bits 5-15 = Reserved
WinAAttributes and WinBAttributes: These two bytes, specify the attributes of two memory access windows (see section on Memory banks & Windowing).
D0 = Is this window supported ?
0 = Window is not supported
1 = Window is supported
D1 = Window readable
0 = Window is not readable
1 = Window is readable
D2 = Window writeable
0 = Window is not writeable
1 = Window is writeable
D3-D7 = Reserved
WinGranularity: This field tells you how much freedom you have to move
the windows around in memory. The value returned here is the smallest boundary (in Kb)
on which you can place a window. You may be able to position a window every 4Kb, or
you may only be able to align them to 64Kb. WinSize: The size of the windows in Kb. (normally 64)
WinASegment & WinBSegment: The segment address of the windows in memory. (normally 0A000h)
WinFuncPtr: Rather than use an interrupt to move the windows around, there is often a procedure available which you can call directly. This is a much faster way to move windows than using int 10. This field points to the procedure in memory. More about how to use that later.
BytesPerScanLine: The number of bytes in each logical scanline. This may be larger than the number of pixels, since the virtual screen may be larger than the visible screen.
XResolution & YResolution: The resolution of the visible part of the screen.
XCharSize & YCharSize: The size of the characters in pixels.
NumberOfPlanes: In some modes, you cannot access all of the bits of a pixel with one memory write, because the memory is split up into planes. In this case, the number of planes will be more than 1. If you find a mode with more than one plane, leave it alone, it's way too scarey. In nice modes, like VGA mode 13h (320x200 x 256), there is only one plane. BitsPerPixel: The number of bits allocated to each pixel. The number of colours available will be 2^BitsPerPixel. So in 256 colour modes, there will be 8 bits, and in 16.7M colour modes, there will be 24 bits.
RedMaskSize . . . BlueFieldPosition: These fields describe the layout of the bits in a 15, 16 or 24 bit colour mode. The MaskSizes specify the number of bits used for each colour, and the FieldPositions specify where those bits should be placed. Lets have an example.
This diagram shows the layout of the 16 bits in a pixel in 65536 colour mode. The values would be set as follows:
RedMaskSize = 5
RedFieldPosition = 11
GreenMaskSize = 6
GreenFieldPosition = 5
BlueMaskSize = 5
BlueFieldPosition = 0
The rest of the information block will be set to zero.
Input: AH = 4Fh Super VGA support
AL = 02h Set Super VGA video mode
BX = Video mode
D0-D14 = Video mode
D15 = Clear memory flag
0 = Clear video memory
1 = Don't clear video memory
Output: AX = Status
(All other registers are preserved)
Input: AH = 4Fh Super VGA support
AL = 03h Return current video mode
Output: AX = Status
BX = Current video mode
(All other registers are preserved)
Input: AH = 4Fh Super VGA support
AL = 05h Super VGA video memory window control
BH = 00h Select Super VGA video memory window
BL = Window number
0 = Window A
1 = Window B
DX = Window position in video memory
(in window granularity units)
Output: AX = Status
Input: AH = 4Fh Super VGA support
AL = 05h Super VGA video memory window control
BH = 01h Return Super VGA video memory window
BL = Window number
0 = Window A
1 = Window B
Output: AX = Status
DX = Window position in video memory
(in window granularity units)
Input: AH = 4Fh Super VGA support
AL = 06h Logical Scan Line Length
BL = 00h Select Scan Line Length
CX = Desired width in pixels
Output: AX = Status
BX = Bytes Per Scan Line
CX = Actual Pixels Per Scan Line
DX = Maximum Number of Scan Lines
Input: AH = 4Fh Super VGA support
AL = 06h Logical Scan Line Length
BL = 01h Return Scan Line Length
Output: AX = Status
BX = Bytes Per Scan Line
CX = Actual Pixels Per Scan Line
DX = Maximum Number of Scan Lines
Imagine the screen as being just the visible part of a much larger picture. The amount of that picture which you can see is defined by the X and Y Resolution of whatever mode you are in. The width of that picture is called the Logical Scanline Length.
The width of the virtual screen can be almost anything you like. From the X resolution of the
screen to the maximum possible due to the amount of memory available on the graphics card.
You can in fact set the width to be less then the X resolution of the screen, but then you get
funny results.
Function 07h lets you the move the visible part of the screen around, allowing you to perform
very smooth scrolling and panning in SVGA.
Input: AH = 4Fh Super VGA support
AL = 07h Display Start Control
BH = 00h Reserved and must be 0
BL = 00h Select Display Start
CX = First Displayed Pixel in Scan Line
DX = First Displayed Scan Line
Output: AX = Status
Input: AH = 4Fh Super VGA support
AL = 07h Display Start Control
BL = 01h Return Display Start
Output: AX = Status
BH = 00h Reserved and will be 0
CX = First Displayed Pixel in Scan Line
DX = First Displayed Scan Line
This function allows you to set the pixel on the virtual screen which will be the top-left pixel
of the physical screen. This effectively gives you the ability to scroll the screen around.Pass the X-Coordinate in the CX register, and the Y-Coordinate in the DX register.
