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Atari demoscene BBS
| Re: Democyclopedie |
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Posted by: Alien / ST-CNX
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Apr,01.2015-18:39
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H comme Hardscroll: Un effet specifique a l'Atari ST
The atari ST could not change the address of the screen in increments of less than 256 bytes. Because a line took 160 bytes this meant that one couldn't scroll the screen left or right. Similarly one could only scroll the screen up or down by 8 lines at a time.
To scroll a screen smoothly, you have to update it every 50th of a second (50Hz). There are 160256 cycles available every 50th of a second. Since one could not simply change the screen's address on the Atari ST as one could on the Amiga, one had to use the CPU (the 68000) to copy the entire screen in memory. This took most of the available CPU cycles in normal mode (~80% if I recall correctly), and more than was available in Overscan. Therefore demos on the Atari ST which scrolled the screen in Overscan were impossible, until the invention of Hardscroll. This was also why many early games either had a reduced playfield (Goldrunner), or ran every 2 frames (at 25Hz).
Hardscroll used the wider lines provided by various overscan modes to produce an effect identical to allowing the address of the screen to be set in increments of 2 bytes. This allowed for much faster scrolling of the screen, and for the screen to be scrolled in Overscan.
Notable overscan demos:
* TCB in Cuddly and Sync's demo were the first overscan demos to do this.
* ULM's parallax distorter
* Let's do the twist in the Punish Your Machine demo added a 4 pixel horizontal scroll which made the demo possible to run on an Atari ST with only 512Mb of ram, while running 16Khz soundtracker replay.
Notable uses in non-overscan demos:
* Cuddly main menu
* Enchanted Land by Thalion (features sprites displayed without double buffering -- means you have to know what part of the screen is being displayed, and update it before it is displayed).
A competition ensued to find the hardscroll technique that used the fewest lines (and thus the least CPU time) and still resulted in a stable display.
Atari recognized the issue and added hardware support for 1-pixel horizontal scrolling, and single line vertical scrolling in their STe series.
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Topic
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Posted by
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Date
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Democyclopedie
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s_t_s
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Mar,14.2015-19:47
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Re: Democyclopedie
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gwEm/PHF
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Mar,16.2015-11:36
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Re: Democyclopedie
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huggyone76
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Mar,16.2015-20:06
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Re: Democyclopedie
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s_t_s
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Mar,17.2015-10:20
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Re: Democyclopedie
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ggn
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Mar,17.2015-11:39
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Re: Democyclopedie
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s_t_s
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Mar,18.2015-10:22
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Re: Democyclopedie
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wietze
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Mar,26.2015-15:07
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Re: Democyclopedie
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defjam
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Mar,18.2015-23:44
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Re: Democyclopedie
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Splash/S1
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Mar,19.2015-00:56
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Re: Democyclopedie
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s_t_s
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Mar,19.2015-18:09
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Re: Democyclopedie
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wietze
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Mar,26.2015-16:09
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Re: Democyclopedie
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Splash/S1
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Mar,26.2015-21:40
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Re: Democyclopedie
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s_t_s
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Mar,27.2015-10:49
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Re: Democyclopedie
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s_t_s
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Mar,27.2015-10:41
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Re: Democyclopedie
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gwEm/PHF
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Mar,30.2015-10:43
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Re: Democyclopedie
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Bod/Effect
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Mar,30.2015-11:47
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Re: Democyclopedie
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s_t_s
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Mar,30.2015-21:31
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Re: Democyclopedie
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Alien / ST-CNX
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Apr,01.2015-18:39
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Re: Democyclopedie
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s_t_s
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Jul,23.2015-13:32
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Re: Democyclopedie
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gwEm/PHF
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Jul,24.2015-10:29
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Re: Democyclopedie
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gwEm/PHF
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Jul,24.2015-10:34
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Re: Democyclopedie
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havoc
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Jul,24.2015-12:52
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Re: Democyclopedie
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s_t_s
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Jul,24.2015-13:36
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Re: Democyclopedie
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gwEm/PHF
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Jul,24.2015-15:12
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Re: Democyclopedie
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s_t_s
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Jul,24.2015-14:12
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What's the anti-troll code?
That's your personal code to be able to add comments and messages on the dhs.nu site.
Don't have a code or forgot it? Fix it here.
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