Difference between revisions of "Indian Spirit"

Latest revision as of 02:49, 26 April 2019

Indian Spirit was created by Hellmood and is 32 bytes in size. Indian Spirit implements a 640x400 American Indian cloth tapestry while playing music inspired by that culture. Original Readme:

"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

        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

pop ds puts 0 into DS. The top stack value at the start of a DOS program is known to be zero. cwd 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. xor al,0x69 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 can be found here. The idea of the graphic effect is to loop over 65536 values of CX which denotes the column at the same time, while changing the row in DL 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 here and 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.