Indian Spirit - Revision history

HellMood: Added explaination

2019-04-26T08:49:29Z

Added explaination

Trixter at 17:35, 8 April 2019

2019-04-08T17:35:22Z

Trixter: Created page with "{{Tinyprog|title=Indian Spirit|author=Hellmood|bsize=32|link=http://www.pouet.net/prod.php?which=80341}} Category:Case Study Indian Spirit implements a 640x400 American In..."

2019-04-08T17:33:40Z

Created page with "{{Tinyprog|title=Indian Spirit|author=Hellmood|bsize=32|link=http://www.pouet.net/prod.php?which=80341}} Category:Case Study Indian Spirit implements a 640x400 American In..."

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{{Tinyprog|title=Indian Spirit|author=Hellmood|bsize=32|link=http://www.pouet.net/prod.php?which=80341}}
[[Category:Case Study]]
Indian Spirit implements a 640x400 American Indian cloth tapestry while playing music inspired by that culture. Original Readme:

<nowiki>"Indian Spirit" - HellMood / DSR
released just for fun 02/2019

DOSBOX - 32 bytes - PC Speaker

Somewhat inspired by Adoks "Indian" Series
i coded for one hour to see what i can come up
with, but in 32 bytes, and with sound. At the
same time i exploited the "secret mode" 0x69
which switches to 640x400 in 256 colors, but
without using the size-unfriendly VESA functions.
For that reason, this demo only works in DOSBOX
or on a computer which as the EXACT graphic card
dosbox emulates.

Youtube Capture :
https://www.youtube.com/watch?v=TeIkW31Blf0</nowiki>

<syntaxhighlight lang=nasm>
pop ds
cwd
Y:
xor al,0x69
int 0x10
mov dl,cl
mov al,[0x46C]
neg al
xor dl,al
and al,32+16+4+2
or al,0x41
out 0x42,al
out 0x61,al
mov ah,12
loop Y
inc bl
jmp short Y
</syntaxhighlight>

← Older revision		Revision as of 08:49, 26 April 2019
Line 40:		Line 40:
	jmp short Y		jmp short Y
	</syntaxhighlight>		</syntaxhighlight>
		+
		+	<code>pop ds</code> puts 0 into DS. The top stack value at the start of a DOS program is known to be zero. <code>cwd</code> is used to clear DX, it is needed because later, pixel setting via BIOS is used (int10h, AH=12), where DX is the row number. Since DX is the same as DS at start, we need to clear it, otherwise there is an offset for the whole graphic effect. <code>xor al,0x69</code> is a part of the graphic effect as well as assigning a graphic mode number to AL. This is a non standard short cut to the usual way of setting a high resolution graphic mode (VESA), which needs 4 bytes more. The mode numbers differ depending on the graphics cards used, here it works for the graphics card DOSBOX is emulating, so it should work on real hardware. A proof of concept version for a GT105M from NVIDIA [https://www.youtube.com/watch?v=uTCnMpNHBIw can be found here]. The idea of the graphic effect is to <code>loop</code> over 65536 values of <code>CX</code> which denotes the column at the same time, while changing the row in <code>DL</code> to create an interesting pattern. The row has to be kept in reasonable range, otherwise uncaught illegal memory accesses can occur. Here, the row number itself cannot exceed 255, since only DL is modified in an otherwise empty DX. Due to the linear memory modell of graphics memory, a column of 65535 would translate to about 102 rows, so we reach up to 357 rows, which fills about 75% of the screen. Syncing to the timer in [0:0x46C] and generating PC speaker output both are explained [http://www.sizecoding.org/wiki/Output#The_drum_channel here] and [http://www.sizecoding.org/wiki/Output#PC_Speaker here]. The melody itself is playing with some binary logic on the raw timing value, and heavily intertwined with the actual pixel color, as well as switching the PC speaker on and off.

@@ Line 22: / Line 22: @@
 <syntaxhighlight lang=nasm>
-pop ds
+        pop     ds
-cwd
+        cwd
 Y:
-xor al,0x69
+        xor     al,0x69
-int 0x10
+        int     0x10
-mov dl,cl
+        mov     dl,cl
-mov al,[0x46C]
+        mov     al,[0x46C]
-neg al
+        neg     al
-xor dl,al
+        xor     dl,al
-and al,32+16+4+2
+        and     al,32+16+4+2
-or al,0x41
+        or      al,0x41
-out 0x42,al
+        out     0x42,al
-out 0x61,al
+        out     0x61,al
-mov ah,12
+        mov     ah,12
-loop Y
+        loop    Y
-inc bl
+        inc     bl
-jmp short Y
+        jmp short Y
 </syntaxhighlight>