Getting VESA information from Protected Mode or Real Mode

VESA Function Calls and Structures

There are two important VESA structures that are required for programmers to be able to write hardware independent applications. The first is a record that contains information about the available modes of the machine, the version, manufacturer, etc.
To get this information you must call Interrupt 10h function 4f00h.

Input: AH = 4fh Super VGA Support AL = 00h Return Super VGA information ES:DI = Real Mode pointer to buffer Output: AX = Status (All other registers are preserved)

The information is returned in this record format:

type TWordArray=array[byte] of word; TVESARec = record VBESignature : array [0..3] of Char; {'VESA' or 'VBE2'} minVersion : Byte; majVersion : Byte; OEMStringPtr : Pchar; Capabilities : LongInt; VideoModePtr : ^TWordArray; TotalMemory : word; {VESA 2.0} OemSoftwareRev : word; OemVendorNamePtr : Pchar; OemProductNamePtr : Pchar; OemProductRevPtr : Pchar; Paddington : array [35..512] of Byte; end;

For a detailed description of the above record download the VESA 1.2 or VESA 2.0 specification.

The second structure contains information about a specific mode. The function call is Interrupt 10h, function 4f01h.

Input: AH = 4fh Super VGA Support AL = 01h Return Super VGA Mode information CX = Mode Number ES:DI = Real Mode pointer to buffer Output: AX = Status (All other registers are preserved)

The interrupt call returns info in this format:

type TModeRec = record ModeAttributes : Word; WindowAFlags : Byte; WindowBFlags : Byte; Granularity : Word; WindowSize : Word; WindowASeg : Word; WindowBSeg : Word; BankSwitch : Pointer; BytesPerLine : Word; XRes,YRes : Word; CharWidth : Byte; CharHeight : Byte; NumBitplanes : Byte; BitsPerPixel : Byte; NumberOfBanks : Byte; MemoryModel : Byte; BankSize : Byte; NumOfImagePages : byte; Reserved : byte; {Direct Colour fields (required for Direct/6 and YUV/7 memory models} RedMaskSize : byte; RedFieldPosition : Byte; GreenMaskSize : Byte; GreenFieldPosition : Byte; BlueMaskSize : Byte; BlueFieldPosition : Byte; RsvdMaskSize : Byte; RsvdFieldPosition : Byte; DirectColourMode : Byte; {VESA 2.0 stuff} PhysBasePtr : longint; OffScreenMemOffset : pointer; OffScreenMemSize : word; paddington : array [49..512] of Byte; end;

Protected Mode Considerations

The VBE Interrupt calls are meant for Real-Mode applications. All the pointers that are returned via function calls 4F00h and 4F01h are RM pointers, including the direct bank switch call. There is a DPMI function call that will convert a RM address to a Protected Mode address:

{Input: A Real-Mode pointer; Output: The equivilant Protected-Mode pointer Usage: PM_Pointer:=ConvertPtr(RM_Pointer); Alternate: MyPointer:=ConvertPtr(MyPointer); Restrictions: Must be in Protected Mode; Must have a DPMI host. } function ConvertPtr(RMPointer:Pointer):pointer; assembler; asm mov ax,0002h mov bx,word ptr RMPointer+2 {Convert the RM segment to PM selector} int 31h {Call DPMI} mov dx,ax {Return pointer in DX:AX} mov ax,word ptr RMPointer {Offset is the same} end;

Incompatibilities

When attempting to code an application that should be universal, there will always be some configuration that won't work. Here are some potential problems with any of the VESA code you download from us:

No VBE installed. If there isn't a VESA driver, obviously our code won't work. It should, however, return an error code indicating that the call to see if VESA was installed failed.
Older SciTech drivers. It seems that older implementations of SciTech's UniVBE TSR wrote the current bank to the code segment. In Real Mode this isn't a problem. However, in Protected Mode this causes a GPF. In this case you must use the original Int 10h call. A work-around does exist to this problem, but we haven't implemented it yet.

Click on the parchment below to download a sample program that displays VESA information in either RM or PM.