JPEG Display Process page-5

JPEG Display Process

Extended VGA BIOS

Several new BIOS calls have been defined to support Super VGA modes. For maximum compatibility with the standard VGA BIOS, these calls are grouped under one function number. This number is passed in the AH register to the INT 10h handler. The designated Super VGA extended function number is 4Fh. This function number is presently unused in most, if not all, VGA BIOS implementations. A standard VGA BIOS performs no action when function call 4Fh is made. Super VGA Standard VS900602 defines subfunctions 00h through 07h. Subfunction numbers 08h through 0FFh are reserved for future use.

Status Information

Every function returns status information in the AX register. The format of the status word is as follows:

AL == 4Fh: Function is supported

Al != 4Fh: Function is not supported

AH == 00h: Function call successful

AH == 01h: Function call failed

Software should treat a non-zero value in the AH register as a general failure condition. In later versions of the VESA BIOS Extension new error codes might be defined.

Function 00h - Return Super VGA Information

The purpose of this function is to provide information to the calling program about the general capabilities of the Super VGA environment. The function fills an information block structure at the address specified by the caller. The information block size is 256 bytes.

Input: AH = 4Fh Super VGA support

AL = 00h Return Super VGA information

ES:DI = Pointer to buffer

Output: AX = Status

(All other registers are preserved)

The information block has the following structure:

Description Size

SVGA signature –should be ‘VESA’ 4 bytes

VESA version number 2 bytes

For example , 0x0102 standa for 2 bytes version 1.02

Vendor String – this is a far pointer to vendor supplied 4 bytes

, zero terminated . this string usually it is left to te vendor

to specify the text.

Capabilities – this is currently undefined and is reserved 4 bytes

for possible use in future specifications .

Mode list - this is a far pointer to alist of SVGA video 4 bytes

Modes supported by the adapter .Each entry in tis list is

a word.The end of this list is identified by an 0XFFFF

value .This list can include VESA specified mode numbers

as well as vendor unique modes . The list may include modes

not supported due to monitor or adapter memory limitations.

Total memory - this shows the total amount of vedio 2 bytes

memory that the adaper has . Multiply this word by

64k to get the total memory size,For example , a value

of 8 indicates the card has 1 MB of memory.

Unused -the remainder of the buffer is reserved for future 236 bytes

specifications. Most Vendors return all zeros in this area.

Function 01h - Return Super VGA mode information

This function returns information about a specific Super VGA video mode that was returned by Function 0. The function fills a mode information block structure at the address specified by the caller. The mode information block size is maximum 256 bytes.

Some information provided by this function is implicitly defined by the VESA mode number. However, some Super VGA implementations might support other video modes than those defined by VESA. To provide access to these modes, this function also returns various other information about the mode.

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)

The contents of the mode information:

Description Size

Mode Attributes 2 bytes

Bit 15-5 unused normally set to 0

Bit 4 = 0 Text mode

= 1 Graphics mode

Bit 3 = 0 Monochrome mode

= 1 Color mode

Bit 2 = 0 BIOS output function are not supported for this video mode

= 1 BIOS output function supported

Bit 1 = 0 No extended mode information

= 1 Extended mode information is available

Bit 0 = 0 Mode not supported

= 1 Video mode supported

Window A attributes 1byte

Bit 7-3 Unused ,normally set to 0

Bit 2 = 0 window A not writeable

= 1 window A writeable

Bit 1 = 0 window A is not readable

= 1 window A is readable

Bit 0 = 0 window A is not supported

= 1 window A supported

Window B attributes 1byte

Bit distribution is same as in window A

Window granularity – The smallest boundary in KB that the 2bytes

window can be placed in vdu memory .

Window size The number of KB of memory the window requires 2bytes

Window A segment – The segment in video memory where 2bytes

window A begins

Window B segment – The segment in video memory where 2bytes

window B begins

Far pointer to window scheme function handler 4bytes

Bytes per logical scan line 2bytes

Horizontal resolution 2bytes

Vertical resolution 2bytes

Character Cell Width 1byte

Character Cell Height 1byte

Number of memory planes – This indicates the nuber of color 1byte

Planes required to make up the video memory.

For a standard VGA 16 color mode , there are 4 planes.

Bits per pixel – The number of color/shade bits per screen dot 1byte

For a 256 color mode , there must be 8 bits per pixel.

Number of Banks 1byte

For graphics modes, this byte holds the number of scan line

grouping . Modes 4-6 have two banks . For modes that do

not have scan line bank , the value returned is 1.

Modes 0Dh-13h do not have Scan line Banks .

Memory Organization 1byte

Bank size in kilo bytes – for graphics modes that have

groups of scan lines , the memory size for each bank in 1KB

increments . For modes that do not support banks , this value

is set to zero.

Number of image pages – Total number of pages that the adapter 1byte

can handle at one time for this mode.

RedMaskSize size of direct color red mask in bits 2byte

RedFieldPosition bit position of LSB of red mask 2byte

GreenMaskSize size of direct color green mask in bits 2byte

GreenFieldPosition bit position of LSB of green mask 2byte
BlueMaskSize size of direct color blue mask in bits 2byte

BlueFieldPosition bit position of LSB of blue mask 2byte

RsvdMaskSize size of direct color reserved mask in bits 4bytes

DirectColorModeInfo Direct Color mode attributes 4bytes

Reserved ( unused ) remainder of ModeInfoBlock 216bytes ModeInfoBlock ENDS

The ModeAttributes field describes certain important characteristics of the video mode. Bit D0 specifies whether this mode can be initialized in the present video configuration. This bit can be used to block access to a video mode if it requires a certain monitor type, and that this monitor is presently not connected. Prior to Version 1.2 of the VESA BIOS Extension, it was not required that the BIOS return valid information for the fields after BytesPerScanline. Bit D1 was used to signify if the optional information was present. Version 1.2 of the VBE requires that all fields of the ModeInfoBlock contain valid data, except for the Direct Color fields, which are valid only if MemoryModel field is set to a 6 (Direct Color) or 7 (YUV). Bit D1 is now reserved, and must be set to a 1. Bit D2 indicates whether the BIOS has support for output functions like TTY output, scroll, pixel output, etc. in this mode (it is recommended, but not mandatory, that the BIOS have support for all output functions). If bit D2 is 1 then the BIOS must support all of the standard output functions.

The field is defined as follows:

D0 = Mode supported in hardware

0 = Mode not supported in hardware

1 = Mode supported in hardware

D1 = 1 (Reserved)

D2 = Output functions supported by BIOS

0 = Output functions not supported by BIOS

1 = Output functions supported by BIOS

D3 = Monochrome/color mode (see note below)

0 = Monochrome mode

1 = Color mode

D4 = Mode type

0 = Text mode

1 = Graphics mode

D5-D15 = Reserved