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don't use BPB_TotSec16 to determine FAT type #547

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don't use BPB_TotSec16 to determine FAT type #547

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8fa77f3
don't use BPB_TotSec16 to determine FAT type
Freax13 Mar 5, 2026
a6ea49b
add test with large RAM disk
Freax13 Mar 5, 2026
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6 changes: 2 additions & 4 deletions bios/stage-2/src/fat.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -48,16 +48,14 @@ impl Bpb {
let root_cluster;
let fat_size_32;

if (total_sectors_16 == 0) && (total_sectors_32 != 0) {
if fat_size_16 == 0 {
// FAT32
fat_size_32 = u32::from_le_bytes(raw[36..40].try_into().unwrap());
root_cluster = u32::from_le_bytes(raw[44..48].try_into().unwrap());
} else if (total_sectors_16 != 0) && (total_sectors_32 == 0) {
} else {
// FAT12 or FAT16
fat_size_32 = 0;
root_cluster = 0;
} else {
panic!("ExactlyOneTotalSectorsFieldMustBeZero");
}

Self {
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14 changes: 14 additions & 0 deletions tests/ramdisk.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,8 @@
use std::path::Path;

use bootloader_test_runner::run_test_kernel_with_ramdisk;
use tempfile::NamedTempFile;

static RAMDISK_PATH: &str = "tests/ramdisk.txt";

#[test]
Expand All @@ -26,3 +28,15 @@ fn memory_map() {
Some(Path::new(RAMDISK_PATH)),
);
}

#[test]
fn large_ramdisk() {
// Create a large file to act as the RAM disk.
let ramdisk = NamedTempFile::new().unwrap();
ramdisk.as_file().set_len(1024 * 1024 * 16).unwrap();

run_test_kernel_with_ramdisk(
env!("CARGO_BIN_FILE_TEST_KERNEL_RAMDISK_basic_boot"),
Some(ramdisk.as_ref()),
);
}
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