Introduction to VESA programming

Purpose

To standardize a common software interface to Super VGA video adapters in order to provide a simplified software application access to advanced VGA products.

Summary

The standard provides a set of functions which an application program can use to:
  1. Obtain information about the capabilities and characteristics of a specific Super VGA implementation
  2. To control the operation of such hardware in terms of video mode, initialization, and video memory access.
The functions are provided as an extension to the VGA BIOS video services, accessed through interrupt 10h.

Contents

1.              Introduction
2.              Goals and Objectives
                2.1     Video environment information
                2.2     Programming support
                2.3     Compatibility
                2.4     Scope of standard
3.              Standard VGA BIOS
4.              Super VGA Mode Numbers
5.              CPU Video Memory Control
                5.1     Hardware design consideration
                        5.1.1   Limited to 64k/128k of CPU address space
                        5.1.2   Crossing CPU video memory window boundaries
                        5.1.3   Operating on data from different areas
                        5.1.4   Combining data from two different windows
                5.2     Different types of hardware windows
                        5.2.1   Single window systems
                        5.2.2   Dual window systems
6.              Extended VGA BIOS
                6.1     Status Information
                6.2     00h - Return super VGA information
                6.3     01h - Return super VGA mode information
                6.4     02h - Set super VGA mode
                6.5     03h - Return super VGA mode
                6.6     04h - Save/restore super VGA video state
                6.7     05h - Super VGA video memory window control
                6.8     06h - Set/get logical scan line length
                6.9     07h - Set/get display start
                6.10    08h - Set/get DAC palette control
7.              Application Example

1. Introduction

This document contains a specification for a standardized interface to extended VGA video modes and functions. The specification consists of mechanisms for supporting standard extended video modes and functions that have been approved by the main VESA committee and non-standard video modes that an individual VGA supplier may choose to add, in a uniform manner that application software can utilize without having to understand the intricate details of the particular VGA hardware.

The primary topics of this specification are definitions of extended VGA video modes and the functions necessary for application software to understand the characteristics of the video mode and manipulate the extended memory associated with the video mode.

Readers of this document should already be familiar with programming VGAs at the hardware level and Intel iAPX real mode assembly language. Readers who are unfamiliar with programming the VGA should first read one of the many VGA programming tutorials before attempting to understand these extensions to the standard VGA.

2. Goals and Objectives

The IBM VGA has become a defacto standard in the PC graphics world. A multitude of different VGA offerings exist in the marketplace, each one providing BIOS or register compatibility with the IBM VGA. More and more of these VGA compatible products implements various supersets of the VGA standard. These extensions range from higher resolutions and more colors to improved performance and even some graphics processing capabilities. Intense competition has dramatically improved the price/performance ratio, to the benefit of the end user.

However, several serious problems face a software developer who intends to take advantage of these "Super VGA" environments. Because there is no standard hardware implementation, the developer is faced with widely disparate Super VGA hardware architectures. Lacking a common software interface, designing applications for these environments is costly and technically difficult. Except for applications supported by OEM-specific display drivers, very few software packages can take advantage of the power and capabilities of Super VGA products.

The purpose of the VESA VGA BIOS Extension is to remedy this situation. Being a common software interface to Super VGA graphics products, the primary objective is to enable application and system software to adapt to and exploit the wide range of features available in these VGA extensions.

Specifically, the VESA BIOS Extension attempts to address the following issues:

  1. Return information about the video environment to the application
  2. Assist the application in initializing and programming the hardware

2.1 Video Environment Information

Today, an application has no standard mechanism to determine what Super VGA hardware it is running on. Only by knowing OEM-specific features can an application determine the presence of a particular video board. This often involves reading and testing registers located at I/O addresses unique to each OEM. By not knowing what hardware an application is running on, few, if any, of the extended features of the underlying hardware can be used.

The VESA BIOS Extension provides several functions to return information about the video environment. These functions return system level information as well as video mode specific details. Function 00h returns general system level information, including an OEM identification string. The function also returns a pointer to the supported video modes. Function 01h may be used by the application to obtain information about each supported video mode. Function 03h returns the current video mode.

2.2 Programming Support

Due to the fact that different Super VGA products have different hardware implementations, application software has great difficulty in adapting to each environment. However, since each is based on the VGA hardware architecture, differences are most common in video mode initialization and memory mapping. The rest of the architecture is usually kept intact, including I/O mapped registers, video buffer location in the CPU address space, DAC location and function, etc.

The VESA BIOS Extension provides several functions to interface to the different Super VGA hardware implementations. The most important of these is Function 02h, Set Super VGA video mode. This function isolates the application from the tedious and complicated task of setting up a video mode. Function 05h provides an interface to the underlying memory mapping hardware. Function 04h enables an application to save and restore a Super VGA state without knowing anything of the specific implementation.

2.3 Compatibility

A primary design objective of the VESA BIOS Extension is to preserve maximum compatibility to the standard VGA environment. In no way should the BIOS extensions compromise compatibility or performance. Another but related concern is to minimize the changes necessary to an existing VGA BIOS. Ram, as well as ROM-based implementations of the BIOS extension should be possible.

2.4 Scope of Standard

The purpose of the VESA BIOS Extension is to provide support for extended VGA environments. Thus, the underlying hardware architecture is assumed to be a VGA. Graphics software that drives a Super VGA board will perform its graphics output in generally the same way it drives a standard VGA, i.e. writing directly to a VGA style frame buffer, manipulating graphics controller registers, directly programming the palette, etc. No significant graphics processing will be done in hardware. For this reason, the VESA BIOS Extension does not provide any graphics output functions, such as BitBlt, line or circle drawing, etc.

An important constraint of the functionalities that can be placed into the VESA BIOS Extension is that ROM space is severely limited in certain existing BIOS implementations.

Outside the scope of this VESA BIOS Extension is the handling of different monitors and monitor timings. Such items are dealt with in other VESA fora. The purpose of the VESA BIOS Extension is to provide a standardized software interface to Super VGA graphics modes, independent of monitor and monitor timing issues.

3. Standard VGA BIOS

A primary design goal with the VESA BIOS Extension is to minimize the effects on the standard VGA BIOS. Standard VGA BIOS functions should need to be modified as little as possible. This is important since ROM, as well as RAM based versions of the extensions, may be implemented.

However, two standard VGA BIOS functions are affected by the VESA extension. These are Function 00h (Set video mode) and Function 0Fh (Read current video state). VESA-aware applications will not set the video mode using VGA BIOS function 00h. Nor will such applications use VGA BIOS function 0Fh. VESA BIOS functions 02h (Set Super VGA mode) and 03h (Get Super VGA mode) will be used instead.

To make such applications work, VESA recommends that whatever value returned by VGA BIOS function 0Fh (it is up to the OEM to define this number) should be used to reinitialize the video mode through VGA BIOS function 00h. Thus, the BIOS should keep track of the last Super VGA mode in effect.

It is recommended, but not mandatory, to support output functions (such as TTY-output, scroll, set pixel, etc.) in Super VGA modes. If the BIOS extension doesn't support such output functions, bit D2 (Output functions supported) of the ModeAttributes field (returned by VESA BIOS function 01h) should be clear.

4. Super VGA Mode Numbers

Standard VGA mode numbers are 7 bits wide and presently range from 00h to 13h. OEMs have defined extended video modes in the range 14h to 7Fh. Values in the range 80h to FFh cannot be used, since VGA BIOS function 00h (Set video mode) interprets bit 7 as a flag to clear/not clear video memory.

Due to the limitations of 7 bit mode numbers, VESA video mode numbers are 15 bits wide. To initialize a Super VGA mode, its number is passed in the BX register to VESA BIOS function 02h (Set Super VGA mode).

The format of VESA mode numbers is as follows:

D0-D8  = Mode number
              If D8 == 0, this is not a VESA defined mode
              If D8 == 1, this is a VESA defined mode
D9-D14 = Reserved by VESA for future expansion (= 0)
D15    = Reserved (= 0)
Thus, VESA mode numbers begin at 100h. This mode numbering scheme implements standard VGA mode numbers as well as OEM-defined mode numbers as subsets of the VESA mode number. That means that regular VGA modes may be initialized through VESA BIOS function 02h (Set Super VGA mode), simply by placing the mode number in BL and clearing the upper byte (BH). OEM-defined modes may be initialized in the same way.

To date, VESA has defined a 7-bit video mode number, 6Ah, for the 800x600, 16-color, 4-plane graphics mode. The corresponding 15-bit mode number for this mode is 102h.

The following VESA mode numbers have been defined:

                GRAPHICS                                TEXT

15-bit   7-bit    Resolution   Colors   15-bit   7-bit    Columns   Rows
mode     mode                           mode     mode
number   number                         number   number
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
100h     -        640x400      256      108h     -        80        60
101h     -        640x480      256
                                        109h     -        132       25
102h     6Ah      800x600      16       10Ah     -        132       43
103h     -        800x600      256      10Bh     -        132       50
                                        10Ch     -        132       60
104h     -        1024x768     16
105h     -        1024x768     256

106h     -        1280x1024    16
107h     -        1280x1024    256

10Dh     -        320x200      32K   (1:5:5:5)
10Eh     -        320x200      64K   (5:6:5)
10Fh     -        320x200      16.8M (8:8:8)
110h     -        640x480      32K   (1:5:5:5)
111h     -        640x480      64K   (5:6:5)
112h     -        640x480      16.8M (8:8:8)
113h     -        800x600      32K   (1:5:5:5)
114h     -        800x600      64K   (5:6:5)
115h     -        800x600      16.8M (8:8:8)
116h     -        1024x768     32K   (1:5:5:5)
117h     -        1024x768     64K   (5:6:5)
118h     -        1024x768     16.8M (8:8:8)
119h     -        1280x1024    32K   (1:5:5:5)
11Ah     -        1280x1024    64K   (5:6:5)
11Bh     -        1280x1024    16.8M (8:8:8)
11Ch     -        1600x1200    256
11Dh     -        1600x1200    32K   (1:5:5:5) (Unverified)  
11Eh     -        1600x1200    64K   (5:6:5)   (Unverified) 

5. CPU Video Memory Windows

A standard VGA sub-system provides 256k bytes of memory and a corresponding mechanism to address this memory. Super VGAs and their modes require more than the standard 256k bytes of memory but also require that the address space for this memory be restricted to the standard address space for compatibility reasons. CPU video memory windows provide a means of accessing this extended VGA memory within the standard CPU address space.

This chapter describes how several hardware implementations of CPU video memory windows operate, their impact on application software design, and relates them to the software model presented by the VESA VGA BIOS extensions.

The VESA CPU video memory windows functions have been designed to put the performance insensitive, non-standard hardware functions into the BIOS while putting the performance sensitive, standard hardware functions into the application. This provides portability among VGA systems together with the performance that comes from accessing the hardware directly. In particular, the VESA BIOS is responsible for mapping video memory into the CPU address space while the application is responsible for performing the actual memory read and write operations.

This combination software and hardware interface is accomplished by informing the application of the parameters that control the hardware mechanism of mapping the video memory into the CPU address space and then letting the application control the mapping within those parameters.

5.1 Hardware

5.1.1 Limited to 64k/128k of CPU address space

The first consideration in implementing extended video memory is to give access to the memory to application software.

The standard VGA CPU address space for 16 color graphics modes is typically at segment A000h for 64k. This gives access to the 256k bytes of a standard VGA, i.e. 64k per plane. Access to the extended video memory is accomplished by mapping portions of the video memory into the standard VGA CPU address space.

Every Super VGA hardware implementation provides a mechanism for software to specify the offset from the start of video memory which is to be mapped to the start of the CPU address space. Providing both read and write access to the mapped memory provides a necessary level of hardware support for an application to manipulate the extended video memory.

5.1.2 Crossing CPU video memory boundaries

The organization of most software algorithms which perform video operations consists of a pair of nested loops: and outer loop over rows or scan lines and an inner loop across the row or scan line. The latter is the proverbial inner loop, which is the bottle neck to high performance software.

If a target rectangle is large enough, or poorly located, part of the required memory may be with within the video memory mapped into the CPU address space and part of it may not be addressable by the CPU without changing the mapping. It is desirable that the test for remapping the video memory is located outside of the inner loop.

This is typically accomplished by selecting the mapping offset of the start of video memory to the start of the CPU address space so that at least one entire row or scan line can be processed without changing the video memory mapping. There are currently no Super VGAs that allow this offset to be specified on a byte boundary and there is a wide range among Super VGAs in the ability to position a desired video memory location at the start of the CPU address space.

The number of bytes between the closest two bytes in video memory that can be placed on any single CPU address is defined as the granularity of the window function. Some Super VGA systems allow any 4k video memory boundary to be mapped to the start of the CPU address space, while other Super VGA systems allow any 64k video memory boundary to be mapped to the start of the CPU address space. These two example systems would have granularities of 4k and 64k, respectively. This concept is very similar to the bytes that can be accessed with a 16 bit pointer in an Intel CPU before a segment register must be changed (the granularity of the segment register or mapping here is 16 bytes).

Notes

If the granularity is equal to the length of the CPU address space, i.e. the least significant address bit of the hardware mapping function is more significant than the most significant bit of the CPU address, then the inner loop will have to contain the test for crossing the end or beginning of the CPU address space. This is because if the length of the CPU address space (which is the granularity in this case) is not evenly divisible by the length of a scan line, then the scan line at the end of the CPU address will be in two different video memory which cannot be mapped into the CPU address space simultaneously.

5.1.3 Operating on data from different areas

It is sometimes required or convenient to move or combine data from two different areas of video memory. One example of this is storing menus in the video memory beyond the displayed memory because there is hardware support in all VGAs for transferring 32 bits of video data with an 8 bit CPU read and write. Two separately mappable CPU video memory windows must be used if the distance between the source and destination is larger than the size of the CPU video memory window.

5.1.4 Combining data from two different windows

The above example of moving data from one CPU video memory window to another CPU video memory only required read access to one window and only required write access to the other window. Sometimes it is convenient to have read access to both windows and write access to one window. An example of this would be a raster operation where the resulting destination is the source data logically combined with the original destination data.

5.2 Different types of hardware windows

Different hardware implementations of CPU video memory windows can be supported by the VESA BIOS extension. The information necessary for an application to understand the type of hardware implementation is provided by the BIOS to the application. There are three basic types of hardware windowing implementations and they are described below.

The types of windowing schemes described below do not include differences in granularity.

Also note that is possible for a VGA to use a CPU address space of 128k starting at segment A000h.

5.2.1 Single window systems

Some hardware implementations only provide a single window. This single window will be readable as well as writeable. However, this causes a significant performance degradation when moving data in video memory a distance that is larger than the CPU address space.

5.2.2 Dual window systems

Many Super VGAs provide two windows to facilitate moving data within video memory. There are two separate methods of providing two windows.

5.2.2.1 Overlapping windows

Some hardware implementations distinguish window A and window B by determining if the CPU is attempting to do a memory read or a memory write operation. When the two windows are distinguished by whether the CPU is trying to read or write they can, and usually do, share the same CPU address space. However, one window will be read only and the other will be write only.

5.2.2.2 Non-overlapping windows

Another mechanism used by two window systems to distinguish window A and window B is by looking at the CPU address within the total VGA CPU address space. When the two windows are distinguished by the CPU address within the VGA CPU address space the windows cannot share the same address space, but they can each be both read and written.

6. 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.

6.1 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.

6.2 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:

VgaInfoBlock    STRUC
      VESASignature   db   'VESA'      ; 4 signature bytes
      VESAVersion     dw   ?           ; VESA version number
      OEMStringPtr    dd   ?           ; Pointer to OEM string
      Capabilities    db   4 dup(?)    ; capabilities of the video environment
      VideoModePtr    dd   ?           ; pointer to supported Super VGA modes
      TotalMemory     dw   ?           ; Number of 64kb memory blocks on board
      Reserved        db   236 dup(?)  ; Remainder of VgaInfoBlock
VgaInfoBlock    ENDS

The VESASignature field contains the characters 'VESA' if this is a valid block.

The VESAVersion is a binary field which specifies what level of the VESA standard the Super VGA BIOS conforms to. The higher byte specifies the major version number. The lower byte specifies the minor version number. The current VESA version number is 1.2. Applications written to use the features of a specific version of the VESA BIOS Extension, are guaranteed to work in later versions. The VESA BIOS Extension will be fully upwards compatible.

The OEMStringPtr is a far pointer to a null terminated OEM-defined string. The string may used to identify the video chip, video board, memory configuration, etc. to hardware specific display drivers. There are no restrictions on the format of the string.

The Capabilities field describes what general features are supported in the video environment. The bits are defined as follows:

        D0      = DAC is switchable
                  0 = DAC is fixed width, with 6-bits per primary color
                  1 = DAC width is switchable
        D1-31   = Reserved

The VideoModePtr points to a list of supported Super VGA (VESA-defined as well as OEM-specific) mode numbers. Each mode number occupies one word (16 bits). The list of mode numbers is terminated by a -1 (0FFFFh). Please refer to chapter 2 for a description of VESA mode numbers. The pointer could point into either ROM or RAM, depending on the specific implementation. Either the list would be a static string stored in ROM, or the list would be generated at run-time in the information block (see above) in RAM. It is the application's responsibility to verify the current availability of any mode returned by this Function through the Return Super VGA mode information (Function 1) call. Some of the returned modes may not be available due to the video board's current memory and monitor configuration.

The TotalMemory field indicates the amount of memory installed on the VGA board. Its value represents the number of 64kb blocks of memory currently installed.

6.3 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 mode information block has the following structure:

ModeInfoBlock   STRUC

; mandatory information

        ModeAttributes      dw  ?  ; mode attributes
        WinAAttributes      db  ?  ; window A attributes
        WinBAttributes      db  ?  ; window B attributes
        WinGranularity      dw  ?  ; window granularity
        WinSize             dw  ?  ; window size
        WinASegment         dw  ?  ; window A start segment
        WinBSegment         dw  ?  ; window B start segment
        WinFuncPtr          dd  ?  ; pointer to windor function
        BytesPerScanLine    dw  ?  ; bytes per scan line

; formerly optional information (now mandatory)

        XResolution         dw  ?  ; horizontal resolution
        YResolution         dw  ?  ; vertical resolution
        XCharSize           db  ?  ; character cell width
        YCharSize           db  ?  ; character cell height
        NumberOfPlanes      db  ?  ; number of memory planes
        BitsPerPixel        db  ?  ; bits per pixel
        NumberOfBanks       db  ?  ; number of banks
        MemoryModel         db  ?  ; memory model type
        BankSize            db  ?  ; bank size in kb
        NumberOfImagePages  db  ?  ; number of images
        Reserved            db  1  ; reserved for page function

; new Direct Color fields

        RedMaskSize         db  ?  ; size of direct color red mask in bits
        RedFieldPosition    db  ?  ; bit position of LSB of red mask
        GreenMaskSize       db  ?  ; size of direct color green mask in bits
        GreenFieldPosition  db  ?  ; bit position of LSB of green mask
        BlueMaskSize        db  ?  ; size of direct color blue mask in bits
        BlueFieldPosition   db  ?  ; bit position of LSB of blue mask
        RsvdMaskSize        db  ?  ; size of direct color reserved mask in bits
        DirectColorModeInfo db  ?  ; Direct Color mode attributes
        Reserved            db  216 dup(?)      ; remainder of ModeInfoBlock
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

Note: Monochrome modes have their CRTC address at 3B4h. Color modes have their CRTC address at 3D4h. Monochrome modes have attributes in which only bit 3 (video) and bit 4 (intensity) of the attribute controller output are significant. Therefore, monochrome text modes have attributes of off, video, high intensity, blink, etc. Monochrome graphics modes are two plane graphics modes and have attributes of off, video, high intensity, and blink. Extended two color modes that have their CRTC address at 3D4h are color modes with one bit per pixel and one plane. The standard VGA modes 06h and 11h would be classified as color modes, while the standard VGA modes 07h and 0Fh would be classified as monochrome modes.

The BytesPerScanline field specifies how many bytes each logical scanline consists of. The logical scanline could be equal to or larger then the displayed scanline.

The WinAAttributes and WinBAttributes describe the characteristics of the CPU windowing scheme such as whether the windows exist and are read/writeable, as follows:

        D0 = 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

If windowing is not supported (bit D0 = 0 for both Window A and Window B), then an application can assume that the display memory buffer resides at the standard CPU address appropriate for the MemoryModel of the mode.

WinGranularity specifies the smallest boundary, in KB, on which the window can be placed in the video memory. The value of this field is undefined if Bit D0 of the appropriate WinAttributes field is not set.

WinSize specifies the size of the window in KB.

WinASegment and WinBSegment address specify the segment addresses where the windows are located in CPU address space.

WinFuncAddr specifies the address of the CPU video memory windowing function. The windowing function can be invoked either through VESA BIOS function 05h, or by calling the function directly. A direct call will provide faster access to the hardware paging registers than using Int 10h, and is intended to be used by high performance applications. If this field is Null, then Function 05h must be used to set the memory window, if paging is supported.

The XResolution and YResolution specify the width and height of the video mode. In graphics modes, this resolution is in units of pixels. In text modes, this resolution is in units of characters. Note that text mode resolutions, in units of pixels, can be obtained by multiplying XResolution and YResolution by the cell width and height, if the extended information is present.

The XCharCellSize and YCharSellSize specify the size of the character cell in pixels.

The NumberOfPlanes field specifies the number of memory planes available to software in that mode. For standard 16-color VGA graphics, this would be set to 4. For standard packed pixel modes, the field would be set to 1. The BitsPerPixel field specifies the total number of bits that define the color of one pixel. For example, a standard VGA 4 Plane 16-color graphics mode would have a 4 in this field and a packed pixel 256-color graphics mode would specify 8 in this field. The number of bits per pixel per plane can normally be derived by dividing the BitsPerPixel field by the NumberOfPlanes field.

The MemoryModel field specifies the general type of memory organization used in this mode. The following models have been defined:

        00h =           Text mode
        01h =           CGA graphics
        02h =           Hercules graphics
        03h =           4-plane planar
        04h =           Packed pixel
        05h =           Non-chain 4, 256 color
        06h =           Direct Color
        07h =           YUV
        08h-0Fh =       Reserved, to be defined by VESA
        10h-FFh =       To be defined by OEM

In Version 1.1 and earlier of the VESA Super VGA BIOS Extension, OEM defined Direct Color video modes with pixel formats 1:5:5:5, 8:8:8, and 8:8:8:8 were described as a Packed Pixel model with 16, 24, and 32 bits per pixel, respectively. In Version 1.2 and later of the VESA Super VGA BIOS Extension, it is recommended that Direct Color modes use the Direct Color MemoryModel and use the MaskSize and FieldPosition fields of the ModeInfoBlock to describe the pixel format. BitsPerPixel is always defined to be the total memory size of the pixel, in bits.

The NumberOfBanks field specifies the number of banks in which the scan lines are grouped. The remainder from dividing the scan line number by the number of banks is the bank that contains the scan line and the quotient is the scan line number within the bank. For example, CGA graphics modes have two banks and Hercules graphics mode has four banks. For modes that don't have scanline banks (such as VGA modes 0Dh-13h), this field should be set to 1.

The BankSize field specifies the size of a bank (group of scan lines) in units of 1KB. For CGA and Hercules graphics modes this is 8, as each bank is 8192 bytes in length. For modes that don't have scanline banks (such as VGA modes 0Dh-13h), this field should be set to 0.

The NumberOfImagePages field specifies the number of additional complete display images that will fit into the VGA's memory, at one time, in this mode. The application may load more than one image into the VGA's memory if this field is non-zero, and flip the display between them.

The Reserved field has been defined to support a future VESA BIOS extension feature and will always be set to one in this version.

The RedMaskSize, GreenMaskSize, BlueMaskSize, and RsvdMaskSize fields define the size, in bits, of the red, green, and blue components of a direct color pixel. A bit mask can be constructed from the MaskSize fields using simple shift arithmetic. For example, the MaskSize values for a Direct Color 5:6:5 mode would be 5, 6, 5, and 0, for the red, green, blue, and reserved fields, respectively. Note that in the YUV MemoryModel, the red field is used for V, the green field is used for Y, and the blue field is used for U. The MaskSize fields should be set to 0 in modes using a MemoryModel that does not have pixels with component fields.

The RedFieldPosition, GreenFieldPosition, BlueFieldPosition, and RsvdFieldPosition fields define the bit position within the direct color pixel or YUV pixel of the least significant bit of the respective color component. A color value can be aligned with its pixel field by shifting the value left by the FieldPosition. For example, the FieldPosition values for a Direct Color 5:6:5 mode would be 11, 5, 0, and 0, for the red, green, blue, and reserved fields, respectively. Note that in the YUV MemoryModel, the red field is used for V, the green field is used for Y, and the blue field is used for U. The FieldPosition fields should be set to 0 in modes using a MemoryModel that does not have pixels with component fields.

The DirectColorModeInfo field describes important characteristics of direct color modes. Bit D0 specifies whether the color ramp of the DAC is fixed or programmable. If the color ramp is fixed, then it can not be changed. If the color ramp is programmable, it is assumed that the red, green, and blue lookup tables can be loaded using a standard VGA DAC color registers BIOS call (AX=1012h). Bit D1 specifies whether the bits in the Rsvd field of the direct color pixel can be used by the application or are reserved, and thus unusable.

        D0 = Color ramp is fixed/programmable
                0 = Color ramp is fixed
                1 = Color ramp is programmable
        D1 = Bits in Rsvd field are usable/reserved
                0 = Bits in Rsvd field are reserved
                1 = Bits in Rsvd field are usable by the application

Notes

Version 1.1 and later VESA BIOS extensions will zero out all unused fields in the Mode Information Block, always returning exactly 256 bytes. This facilitates upward compatibility with future versions of the standard, as any newly added fields will be designed such that values of zero will indicate nominal defaults or non-implementation of optional features (for example, a field containing a bit-mask of extended capabilities would reflect the absence of all such capabilities). Applications that wish to be backwards compatible to Version 1.0 VESA BIOS extensions should pre-initialize the 256 byte buffer before calling Return Super VGA mode information.

6.4 Function 02h - Set Super VGA video mode

This function initializes a video mode. The BX register contains the mode to set. The format of VESA mode numbers is described in chapter 2. If the mode cannot be set, the BIOS should leave the video environment unchanged and return a failure error code.

        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)

6.5 Function 03h - Return current video mode

This function returns the current video mode in BX. The format of VESA video mode numbers is described in chapter 2 of this document.

        Input:  AH = 4Fh        Super VGA support
                AL = 03h        Return current video mode

        Output: AX = Status
                BX = Current video mode
                (All other registers are preserved)

Notes

In a standard VGA BIOS, function 0Fh (Read current video state) returns the current video mode in the AL register. In D7 of AL, it also returns the status of the memory clear bit (D7 of 40:87). This bit is set if the mode was set without clearing memory. In this Super VGA function, the memory clear bit will not be returned in BX since the purpose of the function is to return the video mode only. If an application wants to obtain the memory clear bit, it should call VGA BIOS function 0Fh.

6.6 Function 04h - Save/Restore Super VGA video state

These functions provide a mechanism to save and restore the Super VGA video state. The functions are a superset of the three subfunctions under standard VGA BIOS function 1Ch (Save/restore video state). The complete Super VGA video state (except video memory) should be saveable/restoreable by setting the requested states mask (in the CX register) to 000Fh.
        Input:  AH = 4Fh        Super VGA support
                AL = 04h        Save/restore Super VGA video state
                DL = 00h        Return save/restore state buffer size
                CX = Requested states
                        D0 = Save/restore video hardware state
                        D1 = Save/restore video BIOS data state
                        D2 = Save/restore video DAC state
                        D3 = Save/restore Super VGA state

        Output: AX = Status
                BX = Number of 64-byte blocks to hold the state buffer
                (All other registers are preserved)


        Input:  AH = 4Fh        Super VGA support
                AL = 04h        Save/restore Super VGA video state
                DL = 01h        Save Super VGA video state
                CX = Requested states (see above)
                ES:BX = Pointer to buffer

        Output: AX = Status
                (All other registers are preserved)


        Input:  AH = 4Fh        Super VGA support
                AL = 04h        Save/restore Super VGA video state
                DL = 02h        Restore Super VGA video state
                CX = Requested states (see above)
                ES:BX = Pointer to buffer

        Output: AX = Status
                (All other registers are preserved)

Notes

Due to the goal of complete compatibility with the VGA environment, the standard VGA BIOS function 1Ch (Save/restore VGA state) has not been extended to save the Super VGA video state. VGA BIOS compatibility requires that function 1Ch returns a specific buffer size with specific contents, in which there is no room for the Super VGA state.

6.7 Function 05h - CPU Video Memory Window Control

This function sets or gets the position of the specified window in the video memory. The function allows direct access to the hardware paging registers. To use this function properly, the software should use VESA BIOS Function 01h (Return Super VGA mode information) to determine the size, location, and granularity of the windows.
        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
                (See notes below)


        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)
                (See notes below)

Notes

This function is also directly accessible through a far call from the application. The address of the BIOS function may be obtained by using VESA BIOS Function 01h, Return Super VGA mode information. A field in the ModeInfoBlock contains the address of this function. Note that this function may be different among video modes in a particular BIOS implementation, so the function pointer should be obtained after each set mode.

In the far call version, no status information is returned to the application. Also, the AX and DX registers will be destroyed. Therefore, if AX and/or DX must be preserved, the application must do so priot to making the far call.

The application must load the input arguments in BH, BL, and DX (for set window) but does not need to load either AH or AL in order to use the far call version of this function.

6.8 Function 06h - Set/Get Logical Scan Line Length

This function sets or gets the length of a logical scan line. This function allows an application to set up a logical video memory buffer that is wider than the displayed area. Function 07h then allows the application to set the starting position that is to be displayed.
        
        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

Notes

The desired width in pixels may not be achieveable because of VGA hardware considerations. The next larger value will be selected that will accommodate the desired number of pixels, and the actual number of pixels will be returned in CX. BX returns a value that, when added to a pointer into video memory, will point to the next scan line. For example, in a mode 13h this would be 320, but in mode 12h this would be 80. DX returns the number of logical scan lines based upon the new scan line length and the total memory installed and useable in this display mode. This function is also valid in text modes. In text modes, the application should find out the current character cell width through normal BIOS functions, multiply that times the desired number of characters per line, and pass the value in the CX register.

6.9 Function 07h - Set/Get Display Start

This function selects the pixel to be displayed in the upper left corner of the display from the logical page. This function can be used to pan and scroll around logical screens that are larger than the displayed screen. This function can also be used to rapidly switch between two different displayed screens for double buffered animation effects.
        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

Notes

This function is also valid in text modes. In text modes, the application should find out the current character cell width through normal BIOS functions, multiply that times the desired starting character column, and pass that value in the CX register. It should also multiply the current character cell height times the desired starting character row, and pass that value in the DX register.

6.10 Function 08h - Set/Get DAC Palette Control

This function queries and selects the operating mode of the DAC palette. Some DACs are configurable to provide 6-bits, 8-bits, or more of color definition per red, green, and blue primary color. The DAC palette width is assumed to be reset to standard VGA 6-bits per primary during a standard or VESA Set Super VGA Mode (AX = 4F02h) call.
        Input:  AH = 4Fh        Super VGA support
                AL = 08h        Set/Get DAC Palette Control
                BL = 00h        Set DAC palette width
                BH = Desired number of bits of color per primary
                     (Standard VGA = 6)

        Output: AX = Status
                BH = Current number of bits of color per primary
                (Standard VGA = 6)


        Input:  AH = 4Fh        Super VGA support
                AL = 08h        Set/Get DAC Palette Control
                BL = 01h        Get DAC palette width

        Output: AX = Status
                BH = Current number of bits of color per primary
                (Standard VGA = 6)

Notes

An application can find out if DAC switching is available by querying Bit D0 of the Capabilities field of the VgaInfoBlock structure returned by VESA Return Super VGA Information (AX = 4F00h). The application can then attempt to set the DAC palette width to the desired value. If the Super VGA is not capable of selecting the requested palette width, then the next lower value that the Super VGA is capable of will be selected. The resulting palette width is returned.

7. Application Example

The following sequence illustrates how an application interface to the VESA BIOS Extension. The hypothetical application is VESA-aware and calls the VESA BIOS functions. However, the application is not limited to supporting just VESA-defined video modes. This it will inquire what video modes are available before setting up the video mode.
  1. The application would first allocate a 256 byte buffer. This buffer will be used by the VESA BIOS to return information about the video environment. Some applications will statically allocate this buffer, others will use system calls to temporarily obtain buffer space.
  2. The application would then call VESA BIOS function 00h (Return Super VGA information). If the AX register does not contain 004Fh on return from the function call, the application can determine that the VESA BIOS Extension is not present and handle such situation.
    If no error code is passed in AX, the function call was successful. The buffer has been filled by the VESA BIOS Extension with various information. The application can verify that indeed this is a valid VESA block by identifying the characters 'VESA' in the beginning of the block. The application can inspect the VESAVersion field to determine whether the VESA BIOS Extension ha sufficient functionality. The application may use the OEMStringPtr to locate OEM-specific information.
    Finally, the application can obtain a list of the supported Super VGA modes by using the VideoModePtr. This field points to a list of the video modes supported by the video environment.
  3. The application would then create a new buffer and call the VESA BIOS function 01h (Return Super VGA mode information) to obtain information about the supported video modes. Using the VideoModePtr obtained in step 2) above, the application would call this function with a new mode number until a suitable video mode is found. If no appropriate video mode is found, it is up to the application to handle this situation.
    The Return Super VGA mode information function fills a buffer specified by the application with information describing the features of the video mode. The data block contains all the information an application needs to take advantage of the video mode.
    The application would examine the ModeAttributes field. To verify that the mode indeed is supported, the application would inspect bit D0. If D0 is clear, then the mode is not supported by the hardware. This might happen is a specific mode requires a certain type of monitor but that monitor is not present.
  4. After the application has selected a video mode, the next step is to initialize the mode. However, the application might first want to save the present video mode. When the application exits, this mode would be restored. To obtain the present video mode, the VESA BIOS function 03h (Get Super VGA mode) would be used. If a non-VESA (standard VGA or OEM-specific) mode is in effect, only the lower byte in the mode number is filled. The upper byte is cleared.
  5. To initialize the video mode, the application would use VESA BIOS function 02h (Set Super VGA mode). The application has from this point on full access to the VGA hardware and video memory.
  6. When the application is about to terminate, it would restore the prior video mode. The prior video mode, obtained in step 4) above could be either a standard VGA mode, OEM-specific mode, or VESA-supported mode. It would reinitialize the video mode by calling VESA BIOS function 02h (Set Super VGA mode). The application would then exit.


Transcribed and converted to HTML by Mark Iuzzolino using VI (the editor).
„Muži nedovedou ocenit vlastní ženy. To přenechávají druhým.“ Oscar Wilde