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65 lines
2.7 KiB
Markdown
65 lines
2.7 KiB
Markdown
scroll text
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===========
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This example explores using the blitter to scroll text using it's shift function. Once again, this is probably not the best way to implement this outcome.
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Basically, we blit a whole screen row (font height) in place using a left shift to scroll. Characters are rendered to the right of the visible region such that text smoothly scrolls onto the screen.
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The first thing to note about shifting with the blitter is that if we want to left shift we need to use [descending mode](http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node0120.html).
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Descending mode is pretty easy to set up:
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```
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Descending mode is turned on by setting bit one of BLTCON1 (defined as
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BLITREVERSE). If you use descending mode the address pointers will be
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decremented by two (bytes) instead of incremented by two for each word
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fetched. In addition, the modulo values will be subtracted rather than
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added. Shifts are then towards the left, rather than the right, the
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first word mask masks the last word in a row (which is still the first
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word fetched), and the last word mask masks the first word in a row.
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Thus, for a standard memory copy, the only difference in blitter setup
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(assuming no shifting or masking ) is to initialize the
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address pointer registers to point to the last word in a block, rather
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than the first word. The modulo values , blit size , and all other
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parameters should be set the same.
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```
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So we move all of the bitplane pointers to the last word:
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```
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;; a0 - dest bitplane pointer
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;; a1 - source bitplane pointer
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;; d1 - height in pixels
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;; d2 - y destination in pixels
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add.l d1,d2 ; point to end of data for descending mode
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mulu.w #BITPLANE_WIDTH_BYTES*SCREEN_BIT_DEPTH,d2
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add.l d2,a0 ; end of dest bitplane
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add.l d2,a1 ; end of source bitplane
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```
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then enable descending mode:
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```
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swap d0 ; lsl.l #ASHIFTSHIFT,d0
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lsr.l #4,d0 ; d0 has ASHIFTSHIFT bits set
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ori.w #BC1F_DESC,d0 ; BLTCON1 value. shift and descending mode
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move.w d0,BLTCON1(a6) ;
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```
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Now we can do a blit with left shift.
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We use a simplified version of the text renderer used in the previous example to render a single character off screen, we then wait until that character has been fully shifted on screen and render the next one. The logic can be seen in [scroll_text.s](scroll_text.s). Finally we throw in a copper list to make it look a little less boring.
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screenshot
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----------
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try it
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------
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* [Download disk image](bin/scroll_text.adf?raw=true)
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* <a href="http://alpine9000.github.io/ScriptedAmigaEmulator/#amiga_examples/scroll_text.adf" target="_blank">Run in SAE</a>
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