0xrsydn/gameboy-rust
1
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

fix(video): ignore enable bits for backgrounds a mode lacks

Browse source 
DISPCNT enable bits for BG3 in Mode 1, BG0/BG1 in Mode 2, and every background
but BG2 in Modes 3-5 failed the whole frame with UnsupportedLayers, and the
window turned that into a stopped session. Hardware has no such backgrounds and
ignores the bits, so the renderer now masks them. The error variant is removed.

Prohibited OBJ mode 3 and shape 3 stay diagnostics: their hardware output is
not established here.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhfHfHEuzv2AUTd6erW1gU
This commit is contained in:
0xrsydn 2026-10-14 08:35:32 +07:00
commit 0ef2719c6b
5 changed files with 31 additions and 36 deletions
Download patch file Download diff file Expand all files Collapse all files

11
crates/gba-core/src/video.rs
View file

@ -29,7 +29,6 @@ pub fn rgb555_to_rgb888(color: u16) -> u32 {
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum VideoError {
UnsupportedMode(u8),
UnsupportedLayers(u16),
UnsupportedTileAddress(usize),
UnsupportedMapAddress(usize),
UnsupportedObject { index: usize, reason: &'static str },
@ -51,9 +50,6 @@ impl fmt::Display for VideoError {
"background map address {address:#x} exceeds the supported 128 KiB BG VRAM window"
),
Self::UnsupportedObject { index, reason } => write!(f, "OBJ{index}: {reason}"),
Self::UnsupportedLayers(control) => {
write!(f, "unsupported display layers in DISPCNT {control:#06x}")
}
}
}
}
@ -227,16 +223,15 @@ pub(crate) fn render_rows(
};
let mut output = vec![0; length];
let mode = control & 7;
let unsupported = match mode {
// Each mode has a fixed background set. Enable bits for absent backgrounds are ignored.
let absent = match mode {
0 => 0,
1 => 0x800, // BG0/1 text, BG2 affine.
2 => 0x300, // BG2/3 affine.
3..=5 => 0xb00, // BG2 bitmap.
_ => return Err(VideoError::UnsupportedMode(mode as u8)),
};
if control & unsupported != 0 {
return Err(VideoError::UnsupportedLayers(control));
}
let control = control & !absent;
let mut layers: Vec<usize> = (0..4).filter(|bg| control & (0x100 << bg) != 0).collect();
// Lower priority number wins. BG0 wins ties, then BG1, BG2, BG3.
layers.sort_by_key(|&bg| (backgrounds[bg].control & 3, bg));

17
crates/gba-core/tests/affine_backgrounds.rs
View file

@ -1,7 +1,7 @@
use gba_core::{
io::*,
memory::{Memory, MemoryError, OAM_START as OAM, PALETTE_START as PAL, VRAM_START as VRAM},
video::{rgb555_to_rgb888, Framebuffer, VideoError, WIDTH},
video::{rgb555_to_rgb888, Framebuffer, WIDTH},
};
fn setup(bg: u32, size: u16, wrap: bool) -> Memory {
@ -252,17 +252,16 @@ fn transformed_backgrounds_composite_with_regular_and_affine_sprites() {
}
#[test]
fn unsupported_layers_leave_output_unchanged_and_disabled_layers_skip_validation() {
fn absent_layer_bits_are_ignored_and_disabled_layers_skip_validation() {
let mut m = setup(2, 3, false);
let mut f = Framebuffer::default();
f.clear(0x001f);
for control in [0xc01, 0xd02, 0xe02] {
// Mode 1 has no BG3 and mode 2 has no BG0 or BG1; their enable bits change nothing.
for (control, present) in [(0xc01, 0x401), (0xd02, 0xc02), (0xe02, 0xc02)] {
m.write16(DISPCNT, present).unwrap();
let expected = frame(&m);
m.write16(DISPCNT, control).unwrap();
assert_eq!(
m.render_frame(&mut f),
Err(VideoError::UnsupportedLayers(control))
);
assert!(f.pixels().iter().all(|&p| p == 0xff0000));
m.render_frame(&mut f).unwrap();
assert_eq!(f.pixels(), expected.pixels());
}
m.write16(DISPCNT, 2).unwrap();
m.render_frame(&mut f).unwrap();

18
crates/gba-core/tests/bitmap_modes.rs
View file

@ -1,7 +1,7 @@
use gba_core::{
io::{BG2CNT, BG2PA, BG2X, BG2Y, DISPCNT, GREENSWAP},
memory::{Memory, OAM_START as OAM, PALETTE_START as PAL, VRAM_START as VRAM},
video::{rgb555_to_rgb888, Framebuffer, VideoError, WIDTH},
video::{rgb555_to_rgb888, Framebuffer, WIDTH},
};
fn setup(mode: u16) -> Memory {
@ -423,20 +423,18 @@ fn cpu_bitmap_controls_pan_rotate_zoom_force_page_and_reset() {
}
#[test]
fn unsupported_bitmap_layers_leave_frame_unchanged() {
fn absent_bitmap_layer_bits_are_ignored() {
for mode in [4, 5] {
let mut m = setup(mode);
m.write16(VRAM + 0xa000, 0x0201).unwrap(); // Visible on the selected second page.
m.write16(DISPCNT, 0x410 | mode).unwrap();
let expected = frame(&m);
let mut f = Framebuffer::default();
f.clear(0x001f);
for extra in [0x100, 0x200, 0x800] {
let control = 0x410 | mode | extra;
m.write16(DISPCNT, control).unwrap();
assert_eq!(
m.render_frame(&mut f),
Err(VideoError::UnsupportedLayers(control))
);
m.write16(DISPCNT, 0x410 | mode | extra).unwrap();
m.render_frame(&mut f).unwrap();
assert_eq!(f.pixels(), expected.pixels());
}
assert!(f.pixels().iter().all(|&p| p == 0xff0000));
assert_eq!(m.cycles(), 0);
}
}

19
crates/gba-core/tests/graphics.rs
View file

@ -149,7 +149,7 @@ fn forced_blank_is_white_and_disabled_bg2_uses_palette_backdrop() {
}
#[test]
fn unsupported_modes_and_layers_report_errors_without_changing_output() {
fn unsupported_modes_report_errors_and_absent_layer_bits_are_ignored() {
let mut memory = mode3();
let mut frame = Framebuffer::default();
frame.clear(0x001f);
@ -160,14 +160,17 @@ fn unsupported_modes_and_layers_report_errors_without_changing_output() {
Err(VideoError::UnsupportedMode(mode as u8))
);
}
for layer in [0x100, 0x200, 0x800] {
memory.write16(DISPCNT, 0x403 | layer).unwrap();
assert_eq!(
memory.render_frame(&mut frame),
Err(VideoError::UnsupportedLayers(0x403 | layer))
);
}
assert!(frame.pixels().iter().all(|&pixel| pixel == 0xff0000));
// Mode 3 has only BG2. Stale BG0, BG1, and BG3 enables do not change the image.
memory.write16(VRAM_START, 0x03e0).unwrap();
memory.write16(DISPCNT, 0x403).unwrap();
let mut expected = Framebuffer::default();
memory.render_frame(&mut expected).unwrap();
for layer in [0x100, 0x200, 0x800, 0xb00] {
memory.write16(DISPCNT, 0x403 | layer).unwrap();
memory.render_frame(&mut frame).unwrap();
assert_eq!(frame.pixels(), expected.pixels());
}
}
#[test]