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284 lines
11 KiB
Plaintext
284 lines
11 KiB
Plaintext
In my last writeup I described a demo for the Apple II, but said
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I'd leave all the fancy cycle-counting and modeswitching to the
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French Touch group.
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But then I realized that no, I was jealous of the Atari and other hackers,
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and I wanted to race the beam too.
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On an Apple II, however, the challenge isn't racing the beam, it's
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*finding* the beam.
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Graphics mode on the Apple II
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Lemonade stand, hires?, text?
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The original apple II has three video modes.
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Text, which is 40x24. The font is in ROM and cannot be changed.
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There are two pages, one at $400 and one at $800.
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Lores graphics, which is 40x48 blocks in 15 NTSC artifact colors
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(there are two identical greys). This re-used the same
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RAM as text mode, but interprets each byte as upper and lower
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4-bit colored blocks.
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Hires graphics, which is 280*192 (sort of) 6-color graphics with all
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sorts of restrictions. Two pages, one at $2000 and one at $4000.
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The graphics modes can optionally be set to display the bottom 4 lines
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of text mode text.
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Later Apple II models, starting with the IIe, can do some more advanced
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things but we are targeting the older models here.
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Some more fun with the graphics displays.
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They are not linear framebuffers. To save a few gates, and to get DRAM
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refresh for free, Woz scattered the addresses about so the video refresh
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circuitry (which runs in the half of the 6502 clock phase where the CPU
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is not accessing memory) touches each DRAM page.
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This means a table lookup (or costly math) any time you want to move
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to a new Y position.
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Also the LORES PAGE0 has ``holes'' in the address space that may be
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used by peripherals (original Apple II only 4k of RAM so had to have
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scratch space low) so you have to be careful not to over-write these by mistake.
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Finally in hires mode the pixel patterns are complex. Two 00 next to
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each other always is black.
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Two 11 is always white. 01 starting in an even pixel is one color, 10 is
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another. Which color (orange/blue or purple/green) depends on the high
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bit of the byte. There are 7 bits left in the byte, so some colors
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end up split between two bytes.
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The black and white colors happen any time you get consecutive 0s or 1s
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so ther tend to be white or black edge artifacts whenever you have colors
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touching.
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Anyway, enough about the challenges of drawing these modes.
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Can we make this work, and switch modes mid-screen to create graphics
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combinations mostly undreamed of?
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Yes! ANd it turns out this is a really old technique, Bob Bishop
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introduced it in 1982(?) in an article in Softtalk, and Don Lancaster
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expanded on it at length a few years later.
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So why weren't these effects exploited back in the day?
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Mostly because it's a pain to program. Also because Apple would never
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guarantee the way of finding things would always work.
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Fitting the music.
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I waited a bit late to find some music, but managed to find someone
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at the last minute.
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Dascon was kind enough to put together a 3-channel Amiga MOD file which
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I poorly converted by hand to a Vortex Tracker PT3 file, the kind played
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on ZX spectrums.
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It is possible to write very small trackers to play this format, but alas
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none seem to be available for the 6502, and if they were, it's unlikely
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they'd be cycle-invariant.
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So in the end I have to give up the dream of fitting in 48k, and
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managed to get the audio plus AY-3-8910/Mockingboard sound player
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to fit in the 4k of space I had leftover for music, plus 16k of the
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Language Card.
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The Language Card was Apple's bank switching hack of an expansion card
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that allowed swapping out the ROM for RAM in a 12k chunk at \$D000 and an
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alternate 4k chunk at \$D000
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(it's not possible to swap out the address space at \$C000 as that's where
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the expansion card ROM and softswitches live).
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The code runs fine even if you don't have one, the code will try to play
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your ROM as music
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to much less satisfying results.
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THe music is compressed to only have 8 of the AY-3-8910s registers
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needing updated (no envelope effects).
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The tracker pattern buffer is used to play the 31 patterns, each of which
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fit in four 256 byte chunks.
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Deduplication was used to make this fit in the roughly 17k we had available.
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Much of that was done by hand due to lack of time to automate it.
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Notes on the MEGADEMO
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Finding HBLANK/VBLANK on the Apple II:
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To do fancy mode switching, we need to switch graphic modes
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(by writing to a soft-switch address) at the exact time we want
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to switch modes.
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Finding this is difficult on the original Apple II.
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Unlike other machines,
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there is no register or interrupt that tells where the scan
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currently is. (Later IIe, IIc and IIgs models do add such a
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register, but each is incompatible with the others.
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Also, even those only gave you roughly +/- 7 cycles of
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VBLANK starting, not exact notification)
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The way this is found is to use a weird quirk of the Apple II:
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the "floating bus". If you read from a softswitch that doesn't
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drive the bus, you get back the last value written due to the
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residual capacitance on the bus lines. The Apple II writes the
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display each half cycle, so the values on this bus are the last
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written video display value.
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By drawing a pattern on the screen you can repeatedly read
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this floating bus and figure out where on the screen you are,
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and then calculate from there.
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This is a well known feature of the bus (it was described
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in the early 80s by at various times Bishop, Sather and Lancaster)
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but was not widely used, partly because it was a pain to do,
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but also because Apple made no guarantees this accidental
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behavrior would continue to be available.
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Cycle counting on the 6502:
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Cycle counting on an old 8-bit chip is a lot easier than on
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modern hardware, as the cycle counts are mostly deterministic
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(unlike a modern system that has out-of-order, caches, etc).
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There are some issues that can get you:
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* Conditional branches, taken take 3 cycles, not-taken 2 cycles.
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* Some load/store instructions can take an extra cycle if the
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indexing crosses a 256-byte boundary. This means your code
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might suddenly take longer if it ends up misaligned.
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* Branches also take longer if they cross a 256-byte page boundary.
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* The 65C02 chip found in newer Apple IIs have some timing
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differences from the NMOS 6502. One that is easy to get
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caught on is using JMP indirect (used for jump tables).
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There is a workaround for jump tables by pushing the value
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to be jumped onto the stack and then doing a "rts" return
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instruction to jump to it.
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Notes on each screen:
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+ Opening C64 screen
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HGR/Text split. The curtain opening effect isn't as great as
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it could be. The fastest you can switch modes on the II is
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4 cycles, and in 4 cycles 28 pixels are output in hiresmode
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(4 text chars wide).
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+ Falling Apple II
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This is done in the 40x96 forced lores mode, where it switches
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between the two lo-res pages halfway through to double the
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vertical resolution.
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This was supposed to scroll into place but that got sidetracked,
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especially as it's not possible to copy a full 1k lores screen
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in 60Hz.
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+ Incoming Message
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This flips between page1/page2 again, but in text mode which allows
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faking some lowercase looking characters on the original Apple II
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which has no lowercase support. Things like lowercase O can be
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made with the bottom half of an 8. Unfortunately the split to
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get "o" makes it not possible to get things like umlauts using
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".
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One tricky thing here is between the Apple II and the IIe
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they changed how font generation worked and all the characters
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were shifted up one vertical line. To fix that the demo
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detects the newer hardware and self-modifies the code to work
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on both.
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This screen also does a Text/lores split, and the lores is
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in the 40x96 mode.
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+ Starring
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The first part is flipping between Lores Page1/Page2 and Hires Page1
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to create a cheap animated effect.
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In theory the hires colors are a subset of lores so you can make
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exact matches, but in practice the generation in those modes is
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off a bit so the text shifts a bit.
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In theory you could add in Hires Page2 as well to get 4 frames,
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but that would take another 8k of memory which we can't afford.
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The timing code for this is the only place where I actually do
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the trick of jumping into the middle of a BIT instruction
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which gets interpreted as a harmless nop for timing reasons.
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+ Cast of characters
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This is a Lores/Hires split, with some fancy copying from offscreen
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memory to do the name flip.
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+ Leaving
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This has a split of text on top, lores on bottom.
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These animated scenes are actually harder than some of the others.
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Any time you have if/then/else type setups, you have to make
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sure both branches (then vs else) take the *exact* same number
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of cycles, and that's difficult to do.
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So little scenes with a lot of movement in complex directions
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quickly become a pain.
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+ Bird in front of Mountain
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This is a text/hires/lores three-way split with some animation
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of sprites going on.
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The sprites take different amounts of time to draw depending
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on the pattern so the code has to account for this.
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The text scrolling is actually fairly easy to do, no real
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tricks with that.
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+ Waterfall
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This effect does some tricky lores 1/2 shifting ot where
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the split happens to create an automated water effect, and
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there are gaps which give a fake transparency effect when
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the sprite walks behind the waterfall.
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This code wasn't too awful to write, but making it small
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using self-modifying code.
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+ Rocket Takeoff
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HGR/GR split, though it's subtle, but notice how the top
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of the rocket's tail has a smooth diagonal.
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Again scripting these behavios cycle exact is a pain with
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state-machines, jump-tables, and self modifying code at times.
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+ Mode7 flying
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This is a SNES-style mode7 pseudo-3d effect. If you're
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interested in how this works see my PoC||GTFO 0x18 article.
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+ Saturn flyby
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This is a TEXT/GR/HGR, but the GR/HGR part is mid-screen to
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give sort of a tiered look.
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It is tricky to do this on the fly.
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The original goal was to have the rasterbars coming in
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be 40x96 mode giving a much more impressive look, but it turns
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out switching HIRES to LORES and also PAGE0 to PAGE1 at the
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same time takes too many cycles.
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It might still be possible to do this effect if the HGR picture
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was mirrored in both PAGE1 and PAGE2, but we are using PAGE2 for
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code and don't have the room to do this.
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+ Arrival
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Very similar to the leaving.
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There are more effects I'd like to do but ran out of time.
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+ Fireworks
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This is a HIRES/LORES split, but with the bottom of the screen
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doing interlaced every-other line LORES to give the gradient
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effect.
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The code is based on a BASIC program by Fozztexx which was modified
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to have custom random routine, and also a deterministic HPLOT
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(hi-res plot) routine. The original code used HPLOT TO (to draw
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lines in some cases) but making that deterministic was too much
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of a hassle and was left off.
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The interleaved text scrolling effect looks nice and came more
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or less for free (well, for twice the string data).
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Other notes:
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Hires apple graphics conversion uses the BMP2DHR util by
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Bill Buckels which has knowledge of the weird Apple II hires
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modes so does a better job than a regular paint program would.
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