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25 lines
1.1 KiB
Markdown
25 lines
1.1 KiB
Markdown
# Video RAM (VRAM)
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* **Address Span:** `0x600_0000` to `0x601_7FFF` (96k)
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We've used this before! VRAM has a 16-bit bus and no wait. However, the same as
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with PALRAM, the "you might have to wait if the display controller is looking at
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it" rule applies here.
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Unfortunately there's not much more exact detail that can be given about VRAM.
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The use of the memory depends on the video mode that you're using.
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One general detail of note is that you can't write individual bytes to any part
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of VRAM. Depending on mode and location, you'll either get your bytes doubled
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into both the upper and lower parts of the 16-bit location targeted, or you
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won't even affect the memory. This usually isn't a big deal, except in two
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situations:
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* In Mode 4, if you want to change just 1 pixel, you'll have to be very careful
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to read the old `u16`, overwrite just the byte you wanted to change, and then
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write that back.
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* In any display mode, avoid using `memcopy` to place things into VRAM.
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It's written to be byte oriented, and only does 32-bit transfers under select
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conditions. The rest of the time it'll copy one byte at a time and you'll get
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either garbage or nothing at all.
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