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< a class = "header" href = "#video-memory-intro" id = "video-memory-intro" > < h1 > Video Memory Intro< / h1 > < / a >
< p > The GBA's Video RAM is 96k stretching from < code > 0x0600_0000< / code > to < code > 0x0601_7FFF< / code > .< / p >
< p > The Video RAM can only be accessed totally freely during a Vertical Blank
(aka " vblank" ). At other times, if the CPU tries to touch the same part of video
memory as the display controller is accessing then the CPU gets bumped by a
cycle to avoid a clash.< / p >
< p > Annoyingly, VRAM can only be properly written to in 16 and 32 bit segments (same
with PALRAM and OAM). If you try to write just an 8 bit segment, then both parts
of the 16 bit segment get the same value written to them. In other words, if you
write the byte < code > 5< / code > to < code > 0x0600_0000< / code > , then both < code > 0x0600_0000< / code > and ALSO
< code > 0x0600_0001< / code > will have the byte < code > 5< / code > in them. We have to be extra careful when
trying to set an individual byte, and we also have to be careful if we use
< code > memcopy< / code > or < code > memset< / code > as well, because they're byte oriented by default and
don't know to follow the special rules.< / p >
< a class = "header" href = "#rgb15" id = "rgb15" > < h2 > RGB15< / h2 > < / a >
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< p > As I said before, RGB15 stores a color within a < code > u16< / code > value using 5 bits for
each color channel.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
pub const RED: u16 = 0b0_00000_00000_11111;
pub const GREEN: u16 = 0b0_00000_11111_00000;
pub const BLUE: u16 = 0b0_11111_00000_00000;
#}< / code > < / pre > < / pre >
< p > In Mode 3 and Mode 5 we write direct color values into VRAM, and in Mode 4 we
write palette index values, and then the color values go into the PALRAM.< / p >
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< a class = "header" href = "#mode-3" id = "mode-3" > < h2 > Mode 3< / h2 > < / a >
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< p > Mode 3 is pretty easy. We have a full resolution grid of rgb15 pixels. There's
160 rows of 240 pixels each, with the base address being the top left corner. A
particular pixel uses normal " 2d indexing" math:< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
let row_five_col_seven = 5 + (7 * SCREEN_WIDTH);
#}< / code > < / pre > < / pre >
< p > To draw a pixel, we just write a value at the address for the row and col that
we want to draw to.< / p >
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< a class = "header" href = "#mode-4" id = "mode-4" > < h2 > Mode 4< / h2 > < / a >
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< p > Mode 4 introduces page flipping. Instead of one giant page at < code > 0x0600_0000< / code > ,
there's Page 0 at < code > 0x0600_0000< / code > and then Page 1 at < code > 0x0600_A000< / code > . The resolution
for each page is the same as above, but instead of writing < code > u16< / code > values, the
memory is treated as < code > u8< / code > indexes into PALRAM. The PALRAM starts at
< code > 0x0500_0000< / code > , and there's enough space for 256 palette entries (each a < code > u16< / code > ).< / p >
< p > To set the color of a palette entry we just do a normal < code > u16< / code > write_volatile.< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
(0x0500_0000 as *mut u16).offset(target_index).write_volatile(new_color)
#}< / code > < / pre > < / pre >
< p > To draw a pixel we set the palette entry that we want the pixel to use. However,
we must remember the " minimum size" write limitation that applies to VRAM. So,
if we want to change just a single pixel at a time we must< / p >
< ol >
< li > Read the full < code > u16< / code > it's a part of.< / li >
< li > Clear the half of the < code > u16< / code > we're going to replace< / li >
< li > Write the half of the < code > u16< / code > we're going to replace with the new value< / li >
< li > Write that result back to the address.< / li >
< / ol >
< p > So, the math for finding a byte offset is the same as Mode 3 (since they're both
a 2d grid). If the byte offset is EVEN it'll be the high bits of the < code > u16< / code > at
half the byte offset rounded down. If the offset is ODD it'll be the low bits of
the < code > u16< / code > at half the byte.< / p >
< p > Does that make sense?< / p >
< ul >
< li > If we want to write pixel (0,0) the byte offset is 0, so we change the high
bits of < code > u16< / code > offset 0. Then we want to write to (1,0), so the byte offset is
1, so we change the low bits of < code > u16< / code > offset 0. The pixels are next to each
other, and the target bytes are next to each other, good so far.< / li >
< li > If we want to write to (5,6) that'd be byte < code > 5 + 6 * 240 = 1445< / code > , so we'd
target the low bits of < code > u16< / code > offset < code > floor(1445/2) = 722< / code > .< / li >
< / ul >
< p > As you can see, trying to write individual pixels in Mode 4 is mostly a bad
time. Fret not! We don't < em > have< / em > to write individual bytes. If our data is
arranged correctly ahead of time we can just write < code > u16< / code > or < code > u32< / code > values
directly. The video hardware doesn't care, it'll get along just fine.< / p >
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< a class = "header" href = "#mode-5" id = "mode-5" > < h2 > Mode 5< / h2 > < / a >
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< p > Mode 5 is also a two page mode, but instead of compressing the size of a pixel's
data to fit in two pages, we compress the resolution.< / p >
< p > Mode 5 is full < code > u16< / code > color, but only 160w x 128h per page.< / p >
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< a class = "header" href = "#in-conclusion" id = "in-conclusion" > < h2 > In Conclusion...< / h2 > < / a >
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< p > So what got written into VRAM in < code > hello1< / code > ?< / p >
< pre > < pre class = "playpen" > < code class = "language-rust" >
# #![allow(unused_variables)]
#fn main() {
(0x06000000 as *mut u16).offset(120 + 80 * 240).write_volatile(0x001F);
(0x06000000 as *mut u16).offset(136 + 80 * 240).write_volatile(0x03E0);
(0x06000000 as *mut u16).offset(120 + 96 * 240).write_volatile(0x7C00);
#}< / code > < / pre > < / pre >
< p > So at pixels < code > (120,80)< / code > , < code > (136,80)< / code > , and < code > (120,96)< / code > we write three values. Once
again we probably need to < a href = "https://www.wolframalpha.com/" > convert them< / a > into
binary to make sense of it.< / p >
< ul >
< li > 0x001F: 0b11111< / li >
< li > 0x03E0: 0b11111_00000< / li >
< li > 0x7C00: 0b11111_00000_00000< / li >
< / ul >
< p > Ah, of course, a red pixel, a green pixel, and a blue pixel.< / p >
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