diff --git a/src/sfsTest.c b/src/sfsTest.c
new file mode 100644
index 0000000..571bc16
--- /dev/null
+++ b/src/sfsTest.c
@@ -0,0 +1,156 @@
+#include "ktest.h"
+#include "sfs.h"
+#include "ata.h"
+#include "string.h"
+#include "heap.h"
+#include "console.h"
+#include "rtc.h"
+#include "fs.h"
+
+/*
+ * Every test starts with an explicit sfsFormat() so results do not depend on
+ * whatever build/disk.img happened to contain from a previous `make test`
+ * run (the disk image is not recreated between runs unless deleted). That
+ * keeps the gate deterministic: a stale leftover file on disk must never be
+ * the reason a check passes or fails.
+ */
+
+KTEST(sfsFormatAndMountAreReal)
+{
+    if (!KT_TRUE(t, ataPresent()))
+        return;
+    KT_TRUE(t, sfsFormat());
+    KT_TRUE(t, sfsMounted());
+    KT_GT(t, sfsCapacityBytes(), 1024 * 1024);
+
+    int32 root = sfsLookup("");
+    KT_EQ(t, root, sfsRootInode);
+    KT_EQ(t, sfsTypeOf(root), sfsTypeDir);
+    KT_EQ(t, sfsSizeOf(root), 0);
+}
+
+KTEST(sfsCreateFileRoundTrip)
+{
+    if (!KT_TRUE(t, sfsFormat()))
+        return;
+
+    int32 f = sfsCreateFile("hello.txt");
+    KT_TRUE(t, f > 0);
+    KT_EQ(t, sfsTypeOf(f), sfsTypeFile);
+    KT_EQ(t, sfsSizeOf(f), 0);
+
+    const char* msg = "hello, disk!";
+    uint64 len = strlen(msg);
+    KT_EQ(t, sfsWriteAt(f, 0, msg, len), (int64)len);
+    KT_EQ(t, sfsSizeOf(f), len);
+
+    char back[32];
+    memset(back, 0, sizeof(back));
+    KT_EQ(t, sfsReadAt(f, 0, back, len), (int64)len);
+    KT_EQ(t, memcmp(back, msg, len), 0);
+}
+
+KTEST(sfsOverwriteReflectsLatestData)
+{
+    if (!KT_TRUE(t, sfsFormat()))
+        return;
+
+    int32 f = sfsCreateFile("a.bin");
+    KT_TRUE(t, f > 0);
+    KT_EQ(t, sfsWriteAt(f, 0, "AAAA", 4), 4);
+    KT_EQ(t, sfsWriteAt(f, 0, "BB", 2), 2);
+
+    char back[4];
+    KT_EQ(t, sfsReadAt(f, 0, back, 4), 4);
+    KT_EQ(t, memcmp(back, "BBAA", 4), 0);
+    KT_EQ(t, sfsSizeOf(f), 4);
+}
+
+KTEST(sfsWritePastEndZeroFillsGap)
+{
+    if (!KT_TRUE(t, sfsFormat()))
+        return;
+
+    int32 f = sfsCreateFile("gap.bin");
+    KT_TRUE(t, f > 0);
+    KT_EQ(t, sfsWriteAt(f, 10, "X", 1), 1);
+    KT_EQ(t, sfsSizeOf(f), 11);
+
+    uint8 back[11];
+    KT_EQ(t, sfsReadAt(f, 0, back, 11), 11);
+    for (int i = 0; i < 10; i++)
+        KT_EQ(t, back[i], 0);
+    KT_EQ(t, back[10], 'X');
+}
+
+KTEST(sfsMultiBlockFileSpansDirectPointers)
+{
+    if (!KT_TRUE(t, sfsFormat()))
+        return;
+
+    int32 f = sfsCreateFile("multi.bin");
+    KT_TRUE(t, f > 0);
+
+    /* Big enough to span several blocks: kmalloc'd rather than kept on the
+       stack, matching the rbt/slab stress-test convention (this ktest
+       battery runs on the fixed 16 KiB boot stack, not a per-thread one). */
+    #define sfsTestSpan (3 * sfsBlockSize + 200)
+    uint8* pattern = (uint8*)kmalloc(sfsTestSpan);
+    uint8* back = (uint8*)kmalloc(sfsTestSpan);
+    KT_NOTNULL(t, pattern);
+    KT_NOTNULL(t, back);
+    if (!pattern || !back) {
+        kfree(pattern);
+        kfree(back);
+        return;
+    }
+    for (int i = 0; i < sfsTestSpan; i++)
+        pattern[i] = (uint8)(i * 31 + 7);
+
+    KT_EQ(t, sfsWriteAt(f, 0, pattern, sfsTestSpan), sfsTestSpan);
+    KT_EQ(t, sfsSizeOf(f), sfsTestSpan);
+
+    memset(back, 0, sfsTestSpan);
+    KT_EQ(t, sfsReadAt(f, 0, back, sfsTestSpan), sfsTestSpan);
+    KT_EQ(t, memcmp(back, pattern, sfsTestSpan), 0);
+
+    kfree(pattern);
+    kfree(back);
+    #undef sfsTestSpan
+}
+
+KTEST(sfsFileGrowsThroughIndirectBlock)
+{
+    if (!KT_TRUE(t, sfsFormat()))
+        return;
+
+    int32 f = sfsCreateFile("big.bin");
+    KT_TRUE(t, f > 0);
+
+    /* 10 direct blocks (5120 bytes) plus a chunk that only the single
+       indirect block can address: this must reach past sfsDirectBlocks.
+       kmalloc'd (three of these at once would be ~18 KiB, more than the
+       whole 16 KiB boot stack ktest runs on). */
+    #define sfsBigSpan (12 * sfsBlockSize + 33)
+    uint8* pattern = (uint8*)kmalloc(sfsBigSpan);
+    uint8* back = (uint8*)kmalloc(sfsBigSpan);
+    uint8* verify = (uint8*)kmalloc(sfsBigSpan);
+    KT_NOTNULL(t, pattern);
+    KT_NOTNULL(t, back);
+    KT_NOTNULL(t, verify);
+    if (!pattern || !back || !verify) {
+        kfree(pattern);
+        kfree(back);
+        kfree(verify);
+        return;
+    }
+    for (int i = 0; i < sfsBigSpan; i++)
+        pattern[i] = (uint8)(i ^ 0x5A);
+
+    KT_EQ(t, sfsWriteAt(f, 0, pattern, sfsBigSpan), sfsBigSpan);
+    KT_EQ(t, sfsSizeOf(f), sfsBigSpan);
+
+    memset(back, 0, sfsBigSpan);
+    KT_EQ(t, sfsReadAt(f, 0, back, sfsBigSpan), sfsBigSpan);
+    KT_EQ(t, memcmp(back, pattern, sfsBigSpan), 0);
+