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| | * [http://www.ctyme.com/intr/rb-2558.htm INT 21h AH=6] | | * [http://www.ctyme.com/intr/rb-2558.htm INT 21h AH=6] |
| | * [http://www.ctyme.com/intr/rb-2562.htm INT 21h AH=9] | | * [http://www.ctyme.com/intr/rb-2562.htm INT 21h AH=9] |
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| − | === Outputting in mode 13h (320x200) ===
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| − | ==== Basic pixel output ====
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| − | The videomemory for mode 13h is located at segment 0xA000, so you need to assign this value to a segment register. Also, after the start of your program you are normally still in textmode, so you need to switch to the videomode. The following snippet does both:
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| − | <syntaxhighlight lang="nasm">mov al,0x13
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| − | int 0x10 ; AH = 0 means : set video mode to AL = 0x13 (320 x 200 pixels in 256 colors)
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| − | push 0xA000 ; put value on the stack
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| − | pop es ; pop the top stack value into segment register ES</syntaxhighlight>
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| − | You're free to use any of the segment register / opcode combinations to write to the screen
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| − | * <code>ES</code> (<code>stosb</code>)
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| − | * <code>DS</code> (<code>mov</code>)
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| − | * <code>SS</code> (<code>push</code>)
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| − | Let's add some code that actually draws something on the screen, the following program occupies 23 bytes and draws a fullscreen XOR texture
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| − | [[File:Mode13h-example-xor.png|left|bottom|thumb|mode13h-example-xor]]
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| − | <syntaxhighlight lang="nasm">mov al,0x13
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| − | int 0x10
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| − | push 0xa000
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| − | pop es
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| − | X: cwd ; "clear" DX (if AH < 0x7F)
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| − | mov ax,di ; get screen position into AX
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| − | mov bx,320 ; get screen width into BX
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| − | div bx ; divide, to get row and column
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| − | xor ax,dx ; the famous XOR pattern
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| − | and al,32+8 ; a more interesting variation of it
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| − | stosb ; finally, draw to the screen
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| − | jmp short X ; rinse and repeat</syntaxhighlight>
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| − | Note that there is a different way of preparing the segment register, instead of :
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| − | <syntaxhighlight lang="nasm">push 0xa000
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| − | pop es</syntaxhighlight>
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| − | you can also do :
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| − | <syntaxhighlight lang="nasm">mov ah,0xA0
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| − | mov es,ax</syntaxhighlight>
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| − | both variations occupy 4 bytes, but the latter is executable on processor architectures where ''push <word>'' is not available.
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| − | ==== Alternative way of pixel plotting and optimization ====
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| − | Now let's optimize on the snippet. First, we can adapt the "LES" trick from the textmode section. We just exchange
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| − | <syntaxhighlight lang="nasm">push 0xa000
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| − | pop es</syntaxhighlight>
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| − | with:
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| − | <syntaxhighlight lang="nasm">les bx,[bx]</syntaxhighlight>
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| − | to save two bytes. This works because BX is 0x0000 at start and thus, accesses the region ''before'' our code, which is called [https://en.wikipedia.org/wiki/Program_Segment_Prefix Program Segment Prefix]. The two bytes that are put into the segment register ES are bytes 2 and 3 = ''"Segment of the first byte beyond the memory allocated to the program"'' which is usually 0x9FFF. That is just off by one to our desired 0xA000. Unfortunately that means a 16 pixel offset, so if screen alignment means something to you, you can't use this optimization. Also, said two bytes are not always 0x9FFF; for example, if resident programs are above the ''"memory allocated to the program"'' (FreeDos), their content is overwritten if we take their base as our video memory base.
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| − | Second, we can use an alternative way of putting pixels to the screen, subfunction AH = 0x0C of int 0x10. Also, instead of constructing row and column from the screen pointer, we can use some interesting properties of the screenwidth regarding logical operations. This results in the following 16 byte program:
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| − | <syntaxhighlight lang="nasm">cwd ; "clear" DX for perfect alignment
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| − | mov al,0x13
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| − | X: int 0x10 ; set video mode AND draw pixel
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| − | inc cx ; increment column
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| − | mov ax,cx ; get column in AH
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| − | xor al,ah ; the famous XOR pattern
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| − | mov ah,0x0C ; set subfunction "set pixel" for int 0x10
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| − | and al,32+8 ; a more interesting variation of it
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| − | jmp short X ; rinse and repeat</syntaxhighlight>
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| − | The first optimization is the double usage of the same "int 0x10" as setting the videomode and drawing the pixel. The subfunction AH = 0x0C expects row and column in DX and CX. Since the screenwidth is 320, which is 5 * 64, we can ignore the row and just works with the column, if we use logical operations and just use bit 0-6 of the result. The subfunction AH = 0x0C allows for unbounded column values in CX (up to 65535) and correctly "wraps" it internally without an error.
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| − | The major drawback of the "subfunction AH = 0x0C" approach is performance loss. While DosBox and many emulators perform just fine, real hardware will draw much much slower based on the Video BIOS.
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| − |
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| − | ==== Basic animation and user interaction ====
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| − | Now let's add the convenient check for the ESC key and also add a simple animation. The <code>DI</code> register is used as frame counter and incremented after the pixel counter <code>CX</code> ran through all 65536 values via <code>LOOP</code>. This frame counter is then added to the column. The resulting program is now 25 bytes in size :
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| − | [[File:Xor anim example.gif|thumb]]
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| − |
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| − | <syntaxhighlight lang="nasm">cwd ; "clear" DX for perfect alignment
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| − | mov al,0x13
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| − | X: int 0x10 ; set video mode AND draw pixel
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| − | mov ax,cx ; get column in AH
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| − | add ax,di ; offset by framecounter
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| − | xor al,ah ; the famous XOR pattern
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| − | and al,32+8 ; a more interesting variation of it
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| − | mov ah,0x0C ; set subfunction "set pixel" for int 0x10
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| − | loop X ; loop 65536 times
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| − | inc di ; increment framecounter
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| − | in al,0x60 ; check keyboard ...
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| − | dec al ; ... for ESC
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| − | jnz X ; rinse and repeat
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| − | ret ; quit program</syntaxhighlight>
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| − |
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| − | ( ↑ This example is the blueprint in the [[Floating-point_Opcodes#FPU_Basics| FPU Basics Section]].)
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| − | === Using Custom Colors ===
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| − | ==== Shades of Hue within the Default VGA palette ====
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| − | You might have noticed there is a bit of structure to [https://i.stack.imgur.com/OSZ6D.png the default VGA Palette], which you can exploit for some interesting results. Looking at the pallete there is a rainbow of different hue values that start at index 32 that are repeated in a slightly different luma seperated by 72 indices. If you are okay with limiting the amount of shades you need, you can get a small colorramp for all kinds of hue values by simply calculating your color-index like this:
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| − | <code>color=((shade%3)*72)+32+huevalue</code>
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| − | For an example of how this looks for all kinds of hue values, see [https://www.pouet.net/prod.php?which=63520 Popcast] by Hellmood/Desire.
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| − | ==== Setting a Custom Palette ====
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| − | Sometimes, when the [https://i.stack.imgur.com/OSZ6D.png Default VGA Palette] doesn't quite match the look you are looking for, it can be useful to set your own palette using the VGA registers, the basic setup loop looks like this:
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| − | <syntaxhighlight lang="nasm">
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| − | palloop:
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| − | mov ax,cx
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| − | mov dx,0x3c8
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| − | out dx,al ; select palette color
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| − | inc dx
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| − | out dx,al ; write red value (0..63)
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| − | out dx,al ; write green value (0..63)
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| − | out dx,al ; write blue value (0..63)
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| − | loop palloop
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| − | </syntaxhighlight>
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| − | The above code sets a simple grayscale palette, assumes CX Register to be at 0) and is compatible with all DOS platforms.
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| − | In some cases you can ommit the <code>mov dx,0x3c8, out dx,al, inc dx</code> and directly access the data register by just using <code>mov dx,0x3c9</code> instead.
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| − |
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| − | To get more interesting colors than just grayscale, you can alter the value of the AL register in between setting the red, green and blue values. For example by shifting, adding, substracting or performing logical operations. Just get creative and check if the result is sufficient for your usecase.
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| − | TomCat will show the most common color palettes grouped by functionality. Check his article: [https://abaddon.hu/256b/colors.html Colors (in tiny intros)]
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org 100h ; we start at CS:100h
xchg bp,ax ; already a trick, puts 09h into AH
mov dx,text ; DX expects the adress of a $ terminated string
int 21h ; call the DOS function (AH = 09h)
ret ; quit
text:
db 'Hello World!$'
Of course, this gets shorter with each byte you remove from the text itself. Now let's look into arbitrary screen access. Right after the start of your program you are in mode 3, that is 80x25 in 16 colors. See the
So, to show something on the screen, you would need to set a segment register to 0xB800, then write values into this segment.
The following three snippets showcase how to draw a red smiley in three different ways. All example snippets are meant to be standalone programs, starting with the first instruction and nothing before it. The target coordinate (40,12) is about the middle of the screen. We need a multiplier 2 since one char needs two bytes in memory (char and color is a byte each). The high byte 0x04 means red (4) on black (0) while the 0x01 is the first ASCII char - a smiley.
push 0xb800
pop ds
mov bx,(80*12+40)*2
mov ax, 0x0401
mov [bx],ax
ret
push 0xb800
pop es
mov di,(80*12+40)*2
mov ax, 0x0401
stosw
ret
push ss
push 0xb800
pop ss
mov sp,(80*12+40)*2
mov ax, 0x0401
push ax
pop ss
int 0x20
Besides the direct way of accessing memory there are also other ways of bringing char to the screen (f.e)
