By: Matt Pritchard
Re: Vga mode x
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Hello Everyone,

    Lately I've seen a lot of interest in Mode X programming here from 
people like Matt Heck,  Sean Palmer,  Shawn Mills,  Jerry Coffin, Matt 
Rains, Mike Koss, etc.

Well,  I've decided to pull  my head out of the sand,  and donate some 
code to the echo.  All of these are Optimized  code,  fully supporting 
the virtual screen sizes and multiple pages.                                    
                                                                    
It was designed for MASM 5.10A, but TASM will probably work as well.

Take a look at what I have to offer:

* Complete Code: Full source with *LOTS* of Comments 
* COMPLETE MODE SET CODE - All *8* variations of MODE X are included 
* MULTIPLE PAGES/PAGE FLIPPING - All the code's included 
* VIRTUAL SCREENS/ SMOOTH SCROLLING - All of it's here 
* DAC READ/WRITE CODE - all the code needed to play with the DACs 
* HIGH SPEED GRAPHICS PRIMITIVES: 
  =   CLEAR PAGE 
  =   SET PIXEL 
  =   READ PIXEL 
  =   FILL BLOCK (4 times faster than mode 13h) 
  =   DRAW LINE (Fast! + Optimized Horizontal & vertical)

Despite the comments included,  I'm sure people will  have  questions. 
Please send the  to me  over the echo  (I Don't have net mail *sigh*), 
as I really want feedback on this.

The following is a FAQ (Frequently Asked Question) summary of informa-
tion and assembly routines for Mode "X" Graphics.

An overview of Mode "X" for the VGA Adaptor:

1) Mode "X" is a 256 color graphics mode that is  available  on  *ANY* 
VGA card with the  minimum  of  256K  video  RAM.  It  is  capable  of 
providing higher resoultions  than the only "Official" 256 color mode, 
mode 13h. (In quickbasic that is mode 13)

2) Mode  "X"  comes in 8 different flavors: 320 or 360 pixels horizon-
tally, and 200, 240, 400, and 480 pixels vertically.

3) Since mode X is not supported by the VGA BIOS, there is no built in 
support for it.  A program  must provide  its own  routines  for *ALL* 
operations in Mode "X", including setting up the video mode.

4) Unlike Mode 13h, which has one display page, Mode "X" allows from 1 
to 4 video pages, depending upon the  resoultion selected.  The reason 
that Mode 13h has but one  page is that  it  activates a VGA  hardware 
feature known as CHAIN4, which prevents access to all but 64K of VGA's 
video RAM.  CHAIN4 is what provides mode 13h's linear addres space.

5) Unlike Mode 13h,  where each 256 color (1-byte)  pixel has a unique 
address in the E000: segement,  in Mode X there are Four (4) Pixels at 
each address in E000: segment.  The VGA's control registers allow  you 
to control which of the 4 pixels is currently available at an address.

6) It is possible to use the VGA's control registers to  operate  on 2 
or more of the Pixels at the same address at  the  same time.  The CPU 
can write one color value,  and set up to 4 pixels with that value  at 
the same time.

7) Video RAM that is not being used for the current screen display can 
be used to store images and patterns.  These  images and  patterns can 
be copied to other parts of the Video RAM 4 bytes (32 bits) at a time, 
which is much faster than the 8 bits (1 byte) at time that is possible 
over the ISA BUS.(16 Bit BUS operations can produce erroneous results)

If anything is unclear,  inadequate,  or you just plain  want to  know 
more or have other specific questions, please send me a message.

-Matt Pritchard