DOS EGA Graphics Programming in Mode 0Dh

Question

I'm doing a bit of retro programming for fun. What I want to create is a DOS game using EGA graphics but I am having a bit of trouble finding a good reference on the web. Everybody who talks about doing DOS programming assumes that the programmer will use mode 13h, and although some pages mention the other graphics modes, I haven't found one yet that discusses their proper use.

Here's what I'm trying to get working right now:

//------------------------------------------------------------------------------
//  DOS graphics test
//
//  Thanks to the following links:
//    http://gamebub.com/cpp_graphics.php
//
//  Written for Digital Mars C compiler to be compiled as a DOS 16 bit binary.
//------------------------------------------------------------------------------

#include <dos.h>
#include <stdio.h>

#define SCREEN_WIDTH  320;
#define SCREEN_HEIGHT 200;

unsigned char far *vram = (unsigned char far *)0xA0000000L;

//------------------------------------------------------------------------------
void set_video_mode(unsigned char mode)
{
    union REGS in, out;
    in.h.ah = 0;
    in.h.al = mode;
    int86(0x10, &in, &out);
}

//------------------------------------------------------------------------------
void plot_pixel(unsigned int x, unsigned int y, unsigned char color)
{
    // this is wrong because it's only 4 bpp not 8
    vram[y * 320 + x] = color;
    //vram[((y<<8)+(y<<6))+x] = color;
}

//------------------------------------------------------------------------------
int main(int argc, char* argv[])
{
    // EGA 320 x 200 x 16
    set_video_mode(0x0d);

    for (unsigned char i = 0; i < 255; i++)
    {
        vram[i] = i;
    }

    //plot_pixel(10, 10, 1);

    getc(stdin);
    return 0;
}

This sample code works great if you change set_video_mode() to take 0x13 instead of 0x0d, but like I said I'm trying to get EGA graphic here, not VGA. :) I realize that in order to do four bits-per-pixel I'm going to either need to assume that plot_pixel writes two pixels at the same time or do a some bit twiddling to make sure I only write the four bits that I actually want.

My problem is that I'm not seeing what I expect as output--in particular, no colors! Everything seems to be mono colored, which is not what I want at all. Is there some kind of different procedure for employing a color palette in this graphics mode than in 13h? Or have I somehow invoked a completely different graphics mode from the one I intended? Guidance would be very much appreciated.

I don't think my compiler args would be relevant, but just in case:

..\dm\bin\dmc test.c -o test -mm

Answer 1

Mode 0x0d is planar based while 0x13 is not (without special configuration as Mode X). You should check more document to see how to work in planar mode.

Answer 2

Hmmm..dunno if this is a worthy answer, if I recall, you need to use inport,inportb,outport,outportb to the ports to control the EGA registers, this was taken from the excellent FAQSYSW (a vast collection of materials ranging from graphics demo, maths, fractals to compressions), the original archive was found here and here.:

3C0h:  Attribute Controller: Address register
bit 0-4  Address of data register to write to port 3C0h.
      5  If set screen output is enabled and the palette can not be modified,
         if clear screen output is disabled and the palette can be modified.


Port 3C0h is special in that it is both address and data-write register.
An internal flip-flop remembers whether it is currently acting as address or
data register. Accesses to the attribute controller must be separated by at
least 250ns. Reading port 3dAh will reset the flip-flop to address mode.


3C0h index 0-Fh  (W):  Attribute: Palette
bit   0  Primary Blue
      1  Primary Green
      2  Primary Red
      3  Secondary Blue
      4  Secondary Green
      5  Secondary Red

3C0h index 10h (W):  Attribute: Mode Control Register
bit   0  Graphics mode if set, Alphanumeric mode else.
      1  Monochrome mode if set, color mode else.
      2  9-bit wide characters if set.
         The 9th bit of characters C0h-DFh will be the same as
         the 8th bit. Otherwise it will be the background color.
      3  If set Attribute bit 7 is blinking, else high intensity.

3C0h index 11h (W):  Attribute: Overscan Color Register.
bit 0-5  Color of screen border. Color is defined as in the palette registers.
Note: The EGA requires the Overscan color to be 0 in high resolution modes.

3C0h index 12h (W):  Attribute: Color Plane Enable Register
bit   0  Bit plane 0 is enabled if set.
      1  Bit plane 1 is enabled if set.
      2  Bit plane 2 is enabled if set.
      3  Bit plane 3 is enabled if set.
    4-5  Video Status MUX. Diagnostics use only.
         Two attribute bits appear on bits 4 and 5 of the Input Status
         Register 1 (3dAh).  0: Red/Blue, 1: Blue(I)/Green, 2: Red(I)/Green(I)

3C0h index 13h (W):  Attribute: Horizontal PEL Panning Register
bit 0-3  Indicates number of pixels to shift the display left
         Value  9bit textmode   256color mode   Other modes
           0          1               0              0
           1          2              n/a             1
           2          3               1              2
           3          4              n/a             3
           4          5               2              4
           5          6              n/a             5
           6          7               3              6
           7          8              n/a             7
           8          0              n/a            n/a

3C2h (R):  Input Status #0 Register
bit   4  Status of the switch selected by the Miscellaneous Output
         Register 3C2h bit 2-3. Switch high if set.
      5  Pin 19 of the Feature Connector (FEAT0) is high if set
      6  Pin 17 of the Feature Connector (FEAT1) is high if set
      7  If set IRQ 2 has happened due to Vertical Retrace. Should be cleared
         by IRQ 2 interrupt routine by clearing port 3d4h index 11h bit 4.

3C2h (W):  Miscellaneous Output Register
bit   0  If set Color Emulation. Base Address=3Dxh else Mono Emulation. Base
         Address=3Bxh.
      1  Enable CPU Access to video memory if set
    2-3  Clock Select. 0: 14MHz, 1: 16MHz, 2: External
      4  Disable internal video drivers if set
      5  When in Odd/Even modes Select High 64k bank if set
      6  Horizontal Sync Polarity. Negative if set
      7  Vertical Sync Polarity. Negative if set
         Bit 6-7 indicates the number of lines on the display:
           0: 200 lines, 2: 350 lines
Note: Set to all zero on a hardware reset.

3C4h index  0  (W):  Sequencer: Reset
bit   0  Asynchronous Reset if clear
      1  Synchronous Reset if clear

3C4h index  1  (W):  Sequencer: Clocking Mode
bit   0  If set character clocks are 8 dots wide, else 9.
      1  If set the CRTC uses 2/5 of the clock cycles, else 4/5.
      2  If set loads video serializers every other character
         clock cycle, else every one.
      3  If set the Dot Clock is Master Clock/2, else same as Master Clock
          (See 3C2h bit 2-3). (Doubles pixels).

3C4h index  2  (W):  Sequencer: Map Mask Register
bit   0  Enable writes to plane 0 if set
      1  Enable writes to plane 1 if set
      2  Enable writes to plane 2 if set
      3  Enable writes to plane 3 if set

3C4h index  3  (W):  Sequencer: Character Map Select Register
bit 0-1  Selects EGA Character Map (0..3) if bit 3 of the character
         attribute is clear.
    2-3  Selects EGA Character Map (0..3) if bit 3 of the character
         attribute is set.
Note: Character Maps are placed as follows:
      Map 0 at 0k, map 1 at 16k, map 2 at 32k and map 3 at 48k

3C4h index  4  (W):  Sequencer: Memory Mode Register
bit   0  Set if in an alphanumeric mode, clear in graphics modes.
      1  Set if more than 64kbytes on the adapter.
      2  Enables Odd/Even addressing mode if set. Odd/Even mode places all odd
         bytes in plane 1&3, and all even bytes in plane 0&2.

3CAh (W):  Graphics 2 Position
bit 0-1  Select which bit planes should be controlled by Graphics Controller
         #2. Always set to 1.

3CCh (W):  Graphics 1 Position
bit 0-1  Select which bit planes should be controlled by Graphics Controller
         #1. Always set to 0.

3CEh index  0  (W):  Graphics: Set/Reset Register
bit   0  If in Write Mode 0 and bit 0 of 3CEh index 1 is set a write to
         display memory will set all the bits in plane 0 of the byte to this
         bit, if the corresponding bit is set in the Map Mask Register (3CEh
         index 8).
      1  Same for plane 1 and bit 1 of 3CEh index 1.
      2  Same for plane 2 and bit 2 of 3CEh index 1.
      3  Same for plane 3 and bit 3 of 3CEh index 1.

3CEh index  1  (W):  Graphics: Enable Set/Reset Register
bit   0  If set enables Set/reset of plane 0 in Write Mode 0.
      1  Same for plane 1.
      2  Same for plane 2.
      3  Same for plane 3.

3CEh index  2  (W):  Graphics: Color Compare Register
bit 0-3  In Read Mode 1 each pixel at the address of the byte read is compared
         to this color and the corresponding bit in the output set to 1 if
         they match, 0 if not. The Color Don't Care Register (3CEh index 7)
         can exclude bitplanes from the comparison.

3CEh index  3  (W):  Graphics: Data Rotate
bit 0-2  Number of positions to rotate data right before it is written to
         display memory. Only active in Write Mode 0.
    3-4  In Write Mode 2 this field controls the relation between the data
         written from the CPU, the data latched from the previous read and the
         data written to display memory:
           0: CPU Data is written unmodified
           1: CPU data is ANDed with the latched data
           2: CPU data is ORed  with the latch data.
           3: CPU data is XORed with the latched data.

3CEh index  4  (W):  Graphics: Read Map Select Register
bit 0-1  Number of the plane Read Mode 0 will read from.

3CEh index  5  (W):  Graphics: Mode Register
bit 0-1  Write Mode: Controls how data from the CPU is transformed before
         being written to display memory:
           0: Mode 0 works as a Read-Modify-Write operation.
              First a read access loads the data latches of the EGA with the
              value in video memory at the addressed location. Then a write
              access will provide the destination address and the CPU data
              byte. The data written is modified by the function code in the
              Data Rotate register (3CEh index 3) as a function of the CPU
              data and the latches, then data is rotated as specified by the
              same register.
           1: Mode 1 is used for video to video transfers.
              A read access will load the data latches with the contents of
              the addressed byte of video memory. A write access will write
              the contents of the latches to the addressed byte. Thus a single
              MOVSB instruction can copy all pixels in the source address byte
              to the destination address.
           2: Mode 2 writes a color to all pixels in the addressed byte of
              video memory. Bit 0 of the CPU data is written to plane 0 et
              cetera. Individual bits can be enabled or disabled through the
              Bit Mask register (3CEh index 8).
      2  Forces all outputs to a high impedance state if set.
      3  Read Mode
           0: Data is read from one of 4 bit planes depending on the Read Map
              Select Register (3CEh index 4).
           1: Data returned is a comparison between the 8 pixels occupying the
              read byte and the color in the Color Compare Register (3CEh
              index 2). A bit is set if the color of the corresponding pixel
              matches the register.
      4  Enables Odd/Even mode if set (See 3C4h index 4 bit 2).
      5  Enables CGA style 4 color pixels using even/odd bit pairs if set.

3CEh index  6  (W):  Graphics: Miscellaneous Register
bit   0  Indicates Graphics Mode if set, Alphanumeric mode else.
      1  Enables Odd/Even mode if set.
    2-3  Memory Mapping:
           0: use A000h-BFFFh
           1: use A000h-AFFFh   EGA Graphics modes
           2: use B000h-B7FFh   Monochrome modes
           3: use B800h-BFFFh   CGA modes

3CEh index  7  (W):  Graphics: Color Don't Care Register
bit   0  Ignore bit plane 0 in Read mode 1 if clear.
      1  Ignore bit plane 1 in Read mode 1 if clear.
      2  Ignore bit plane 2 in Read mode 1 if clear.
      3  Ignore bit plane 3 in Read mode 1 if clear.

3CEh index  8  (W):  Graphics: Bit Mask Register
bit 0-7  Each bit if set enables writing to the corresponding bit of a byte in
         display memory.

3d4h index  0  (W):  CRTC: Horizontal Total Register
bit 0-7  Horizontal Total Character Clocks-2

3d4h index  1  (W):  CRTC: Horizontal Display End Register
bit 0-7  Number of Character Clocks Displayed -1

3d4h index  2  (W):  CRTC: Start Horizontal Blanking Register
bit 0-7  The count at which Horizontal Blanking starts

3d4h index  3  (W):  CRTC: End Horizontal Blanking Register
bit 0-4  Horizontal Blanking ends when the last 5 bits of the character
         counter equals this field.
    5-6  Number of character clocks to delay start of display after Horizontal
         Total has been reached.

3d4h index  4  (W):  CRTC: Start Horizontal Retrace Register
bit 0-7  Horizontal Retrace starts when the Character Counter reaches this
         value.

3d4h index  5  (W):  CRTC: End Horizontal Retrace Register
bit 0-4  Horizontal Retrace ends when the last 5 bits of the character counter
         equals this value.
    5-6  Number of character clocks to delay start of display after Horizontal
         Retrace.
      7  Provides Smooth Scrolling in Odd/Even mode. When set display starts
         from an odd byte.

3d4h index  6  (W):  CRTC: Vertical Total Register
bit 0-7  Lower 8 bits of the Vertical Total. Bit 8 is found in 3d4h index 7
         bit 0.

3d4h index  7  (W):  CRTC: Overflow Register
bit   0  Bit 8 of Vertical Total (3d4h index 6)
      1  Bit 8 of Vertical Display End (3d4h index 12h)
      2  Bit 8 of Vertical Retrace Start (3d4h index 10h)
      3  Bit 8 of Start Vertical Blanking (3d4h index 15h)
      4  Bit 8 of Line Compare Register (3d4h index 18h)

3d4h index  8  (W):  CRTC: Preset Row Scan Register
bit 0-4  Number of lines we have scrolled down in the first character row.
         Provides Smooth Vertical Scrolling.

3d4h index  9  (W):  CRTC: Maximum Scan Line Register
bit 0-4  Number of scan lines in a character row -1. In graphics modes this is
         the number of times (-1) the line is displayed before passing on to
         the next line (0: normal, 1: double, 2: triple...).
         This is independent of bit 7, except in CGA modes which seems to
         require this field to be 1 and bit 7 to be set to work.

3d4h index 0Ah (W):  CRTC: Cursor Start Register
bit 0-4  First scanline of cursor within character.

3d4h index 0Bh (W):  CRTC: Cursor End Register
bit 0-4  Last scanline of cursor within character
    5-6  Delay of cursor data in character clocks.

3d4h index 0Ch (W):  CRTC: Start Address High Register
bit 0-7  Upper 8 bits of the start address of the display buffer

3d4h index 0Dh (W):  CRTC: Start Address Low Register
bit 0-7  Lower 8 bits of the start address of the display buffer

3d4h index 0Eh (W):  CRTC: Cursor Location High Register
bit 0-7  Upper 8 bits of the address of the cursor

3d4h index 0Fh (W):  CRTC: Cursor Location Low Register
bit 0-7  Lower 8 bits of the address of the cursor

3d4h index 10h (R):  CRTC: Light Pen High Register
bit 0-7  Upper 8 bits of the address of the lightpen position.

3d4h index 10h (W):  CRTC: Vertical Retrace Start Register
bit 0-7  Lower 8 bits of Vertical Retrace Start. Vertical Retrace starts when
         the line counter reaches this value. Bit 8 is found in 3d4h index 7
         bit 2.

3d4h index 11h (R):  CRTC: Light Pen Low Register
bit 0-7  Lower 8 bits of the address of the lightpen position.

3d4h index 11h (W):  CRTC: Vertical Retrace End Register
bit 0-3  Vertical Retrace ends when the last 4 bits of the line counter equals
         this value.
      4  if clear Clears pending Vertical Interrupts.
      5  Vertical Interrupts (IRQ 2) disabled if set. Can usually be left
         disabled, but some systems (including PS/2) require it to be enabled.

3d4h index 12h (W):  CRTC: Vertical Display End Register
bit 0-7  Lower 8 bits of Vertical Display End. The display ends when the line
         counter reaches this value. Bit 8 is found in 3d4h index 7 bit 1.

3d4h index 13h (W):  CRTC: Offset register
bit 0-7  Number of bytes in a scanline / K. Where K is 2 for byte mode, 4 for
         word mode and 8 for Double Word mode.

3d4h index 14h (W):  CRTC: Underline Location Register
bit 0-4  Position of underline within Character cell.

3d4h index 15h (W):  CRTC: Start Vertical Blank Register
bit 0-7  Lower 8 bits of Vertical Blank Start. Vertical blanking starts when
         the line counter reaches this value. Bit 8 is found in 3d4h index 7
         bit 3.

3d4h index 16h (W):  CRTC: End Vertical Blank Register
bit 0-4  Vertical blanking stops when the lower 5 bits of the line counter
         equals this field.

3d4h index 17h (W):  CRTC: Mode Control Register
bit   0  If clear use CGA compatible memory addressing system
         by substituting character row scan counter bit 0 for address bit 13,
         thus creating 2 banks for even and odd scan lines.
      1  If clear use Hercules compatible memory addressing system by
         substituting character row scan counter bit 1 for address bit 14,
         thus creating 4 banks.
      2  If set increase scan line counter only every second line.
      3  If set increase memory address counter only every other character
         clock.
      4  If set disable the EGA output drivers.
      5  When in Word Mode bit 15 is rotated to bit 0 if this bit is set else
         bit 13 is rotated into bit 0.
      6  If clear system is in word mode. Addresses are rotated 1 position up
         bringing either bit 13 or 15 into bit 0.
      7  Clearing this bit will reset the display system until the bit is set
         again.

3d4h index 18h (W):  CRTC: Line Compare Register
bit 0-7  Lower 8 bits of the Line Compare. When the Line counter reaches this
         value, the display address wraps to 0. Provides Split Screen
         facilities. Bit 8 is found in 3d4h index 7 bit 4.

3dAh (R):  Input Status #1 Register
bit   0  Either Vertical or Horizontal Retrace active if set
      1  Light Pen has triggered if set
      2  Light Pen switch is open if set
      3  Vertical Retrace in progress if set
    4-5  Shows two of the 6 color outputs, depending on 3C0h index 12h.
          Attr: Bit 4-5:   Out bit 4  Out bit 5
                   0          Blue       Red
                   1        I Blue       Green
                   2        I Red      I Green

3dAh (W):  Feature Control Register
bit   0  Output to pin 21 of the Feature Connector.
      1  Output to pin 20 of the Feature Connector.