diff --git a/src/coreTest.c b/src/coreTest.c
new file mode 100644
index 0000000..04d29c2
--- /dev/null
+++ b/src/coreTest.c
@@ -0,0 +1,157 @@
+#include "ktest.h"
+#include "string.h"
+#include "heap.h"
+#include "pmm.h"
+#include "vfs.h"
+#include "fs.h"
+#include "sfs.h"
+
+/*
+ * Regression coverage for the subsystems that already existed before the
+ * in-kernel test harness: the freestanding string routines, the free-list
+ * heap, the physical frame allocator and the VFS/tmpfs path.  These run
+ * against the live kernel, so a regression in any of them fails the boot gate.
+ */
+
+KTEST(coreStringMem)
+{
+    char buf[32];
+
+    memset(buf, 'A', sizeof(buf));
+    KT_EQ(t, buf[0], 'A');
+    KT_EQ(t, buf[31], 'A');
+
+    memset(buf, 0, sizeof(buf));
+    KT_EQ(t, buf[0], 0);
+
+    const char* src = "frostbyte";
+    memcpy(buf, src, 10);
+    KT_STREQ(t, buf, "frostbyte");
+    KT_EQ(t, strlen(buf), 9);
+
+    KT_EQ(t, memcmp("abc", "abc", 3), 0);
+    KT_TRUE(t, memcmp("abc", "abd", 3) < 0);
+    KT_TRUE(t, memcmp("abd", "abc", 3) > 0);
+
+    KT_EQ(t, strcmp("abc", "abc"), 0);
+    KT_TRUE(t, strcmp("abc", "abd") < 0);
+    KT_EQ(t, strncmp("abcdef", "abcxxx", 3), 0);
+    KT_TRUE(t, strncmp("abcdef", "abcxxx", 4) != 0);
+}
+
+KTEST(coreMemmoveOverlap)
+{
+    char buf[16];
+    memcpy(buf, "0123456789", 11);
+
+    /* forward overlap: dest > src */
+    memmove(buf + 2, buf, 8);
+    buf[10] = 0;
+    KT_STREQ(t, buf, "0101234567");
+
+    memcpy(buf, "0123456789", 11);
+    /* backward overlap: dest < src */
+    memmove(buf, buf + 2, 8);
+    buf[8] = 0;
+    KT_STREQ(t, buf, "23456789");
+}
+
+KTEST(coreHeapAllocFree)
+{
+    uint64 freeBefore = heapFreeBytes();
+
+    void* a = kmalloc(100);
+    void* b = kmalloc(200);
+    void* c = kmalloc(100);
+    KT_NOTNULL(t, a);
+    KT_NOTNULL(t, b);
+    KT_NOTNULL(t, c);
+    KT_NE(t, a, b);
+    KT_NE(t, b, c);
+
+    /* freeing b and re-allocating a same-or-smaller block reuses the hole */
+    kfree(b);
+    void* d = kmalloc(150);
+    KT_EQP(t, d, b);
+
+    kfree(a);
+    kfree(c);
+    kfree(d);
+
+    /* everything returned: free bytes back to (at least) where we started */
+    KT_GE(t, heapFreeBytes(), freeBefore);
+}
+
+KTEST(coreHeapRealloc)
+{
+    char* p = (char*)kmalloc(8);
+    KT_NOTNULL(t, p);
+    memcpy(p, "1234567", 8);
+
+    p = (char*)krealloc(p, 64);
+    KT_NOTNULL(t, p);
+    KT_STREQ(t, p, "1234567");   /* contents preserved across grow */
+
+    p = (char*)krealloc(p, 4);
+    KT_NOTNULL(t, p);
+    KT_EQ(t, memcmp(p, "1234", 4), 0);
+
+    kfree(p);
+}
+
+KTEST(corePmmAllocDistinct)
+{
+    uint64 freeBefore = pmmFreeFrames();
+
+    uint64 f1 = pmmAlloc();
+    uint64 f2 = pmmAlloc();
+    uint64 f3 = pmmAlloc();
+    KT_NE(t, f1, 0);
+    KT_NE(t, f2, 0);
+    KT_NE(t, f3, 0);
+    KT_NE(t, f1, f2);
+    KT_NE(t, f2, f3);
+    KT_NE(t, f1, f3);
+    KT_EQ(t, f1 & (frameSize - 1), 0);   /* page aligned */
+
+    KT_EQ(t, pmmFreeFrames(), freeBefore - 3);
+
+    pmmFree(f1);
+    pmmFree(f2);
+    pmmFree(f3);
+    KT_EQ(t, pmmFreeFrames(), freeBefore);
+}
+
+KTEST(coreVfsTmpfsRoundtrip)
+{
+    int fd = vfsOpen("/tmp/coretest.txt", oReadWrite | oCreate | oTrunc);
+    KT_GE(t, fd, 0);
+    if (fd < 0)
+        return;
+
+    const char* msg = "hello vfs";
+    KT_EQ(t, vfsWrite(fd, msg, 9), 9);
+
+    KT_EQ(t, vfsLseek(fd, 0, seekSet), 0);
+    char rd[16];
+    memset(rd, 0, sizeof(rd));
+    KT_EQ(t, vfsRead(fd, rd, 9), 9);
+    KT_STREQ(t, rd, "hello vfs");
+
+    KT_EQ(t, vfsLseek(fd, 0, seekEnd), 9);
+    vfsClose(fd);
+
+    FbStat st;
+    KT_EQ(t, vfsStat("/tmp/coretest.txt", &st), 0);
+    KT_EQ(t, st.size, 9);
+    KT_EQ(t, st.type, fsTypeFile);
+
+    KT_EQ(t, vfsUnlink("/tmp/coretest.txt"), 0);
+    KT_EQ(t, vfsStat("/tmp/coretest.txt", &st), -1);   /* gone */
+}
+
+KTEST(coreVfsMkdirGetdents)
+{
+    KT_EQ(t, vfsMkdir("/tmp/cd"), 0);
+    KT_EQ(t, vfsMkdir("/tmp/cd"), -1);   /* already exists */
+