Video Control

With the advent of more sophisticated video control (with 43 and 50 line support) and with yet more sophistication to come (with the rumoured 132 column support) it is instructive to examine the system routines and control records provided for the manipulation of the video screen. One thing that it is timeous to point out is that whilst some of these routines (such as VCARD) provide information that is accessible elsewhere, the use of calls to standardised routines (such as GET_VID_INFO) is encouraged by RevTI as it permits the construction of software "black boxes" which can be radically changed on the inside without changing the result returned to the calling program.

There is a record in the SYSTEM file which contains the setup information required when configuring the video mode. It has five multi-valued fields as follows

Called subroutine taking (and returning) one parameter, the kind of video card installed. Calling syntax is X = "" ; CALL VCARD(X). Types identified in Technical Bulletin 74 are, 0 - MDA, 1 - CGA, 2 - EGA, 3 - PGA, 4 - VGA, 5 - MCGA, 6 - Unknown.

This is a function taking one parameter - the branch, returning a result of the information being asked for. The branches and returned results are as follows

This assembler subroutine controls the video setting of the screen and can be used to "flip" modes in real time. It takes 5 parameters, of which only the first two and the last one seem to be used. As it is an assembler routine it is difficult to be able to state with any confidence exactly what all calls do, however there seem to be three major kinds of call, with each kind taking a specific action modifier. The calling sequence is therefore

     VidCtrl( Action, Mode, UK1, UK2, Flag)

where

Using this routine permits the programmer to reset the screen into an alternative video mode. Note that when this is done there are several caveats to be aware of :-

• Changing the screen size necessitates the changing of the status line location. This is done by resetting @CRTHIGH and @CRTMAXHIGH to the appropriate size and the calling the system routine INIT.STATUS.
• When video modes change, the screen is left blank. A background must therefore be filled in using Video.RW. In the code that follows the default background of \B01B\ is used. If you do not use the default and you wish to construct a generic routine, you will have to extract the correct settings from the environment.
• As mentioned in the discussion of VIDEOCONFIG, certain video modes do not work with certain video cards. The program following makes no allowance for this, but for genericism it could be coded to check the current video card against the requested mode by examining the VIDEOCONFIG record.
• As we wish to return to the normal status line when leaving the temporarily chosen mode, it is necessary to capture the screen image before the new mode is invoked, and restore this image upon return.

The code following represents a generic routine for calling a window from a menu and having that window appear in a non-default video mode. It may be called directly as a subroutine, or via catalyst using * to separate the arguments. The arguments are the window name and the mode to use. The mode can be the number or the text equivalent.

  Subroutine Call_Window(Window, Mode)
    Declare Subroutine VidCtrl, Catalyst, Video.Rw
    Declare Function Guar_Assign, Get_Vid_Info
    Mode = Guar_Assign(Mode)
    If Index(Window, "*", 1) Then
     Mode = Window[-1, "B*"]
     Window = Window[1,"*"]
    End
    VidCard = "" ; Call VCard(VidCard)
    ModeNums  = "0,1,2,3,4,5,6"
    Names = "TEXT,EGA25,EGA43,VGA25,VGA45,VGA50,VGA16"
    ModeHigh  = "25,25,43,25,45,50,16"
    GoSub Save
    Locate Mode In ModeNums Using "," Setting Pos
  Then
     DoIt = 1
    End Else
     Locate Mode In Names Using "," Setting Pos Then
  DoIt = 1
     End
    End
    If DoIt Then
     GoSub SetUp
     Catalyst("W", Window)
     GoSub Restore
    End
  Return

  SetUp:
    Flag = "" ; VidCtrl(VidCard, 0, "", "", Flag)
    Flag = ""
    VidCtrl(Field(ModeNums,",",Pos), 1, "", "", Flag)
    @CrtMaxHigh = Field(ModeHigh, ",", Pos)
    @CrtHigh    = @CrtMaxHigh
    Video.RW(0,0,(@CrtWide-1),(@CrtHigh-1),"C",\B01B\)
    Call Init.Status
  Return

  Save:
    SaveMode    = Get_Vid_Info(1)
    SaveHigh    = @CrtHigh
    SaveMaxHigh = @CrtMaxHigh
    Video.RW(0,0,@CrtWide-1,@CrtMaxHigh-1,"R",Image)
  Return

  Restore:
    Flag = "" ; VidCtrl(VidCard, 0, "", "", Flag)
    Flag = "" ; VidCtrl(SaveMode, 1, "", "", Flag)
    @CrtMaxHigh = SaveMaxHigh
    @CrtHigh = SaveHigh
    Call Init.Status
    Video.RW(0,0,@CrtWide-1,@CrtMaxHigh-1,"W", Image)
  Return

(Volume 4, Issue 3, Pages 8-10)