1 files changed, 42 insertions, 0 deletions
diff --git a/arch/riscv/syscall/syscall_md.cc b/arch/riscv/syscall/syscall_md.cc
new file mode 100644
index 0000000..26320c8
--- /dev/null
+++ b/arch/riscv/syscall/syscall_md.cc
@@ -0,0 +1,42 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* Copyright (C) 2026 Richard Qin */
+
+/*
+ * Syscall MD Layer — RISC-V Register Extraction
+ *
+ * Extracts syscall number (a7) and arguments (a0-a5) from the TrapFrame,
+ * calls the MI dispatcher, and writes the return value back to a0.
+ *
+ * Reference: docs/specs/syscall.md §5
+ */
+
+#include <lume/syscall.h>
+#include <lume/syscall_nr.h>
+#include <arch/trapframe.h>
+#include <arch/cpu.h>
+
+void syscall_dispatch_md(arch::TrapFrame* tf)
+{
+    uint64 nr = tf->a[7];   /* RISC-V ABI: syscall number in a7 */
+
+    uint64 args[6] = {
+        tf->a[0], tf->a[1], tf->a[2],
+        tf->a[3], tf->a[4], tf->a[5],
+    };
+
+    /* Enable interrupts so timer and preemption work during syscalls */
+    arch::cpu::intr_on();
+
+    int64 ret = syscall_dispatch(nr, args);
+
+    arch::cpu::intr_off();
+
+    /* sys_rt_sigreturn restores the entire TrapFrame from the signal
+     * frame on the user stack.  Writing ret into a[0] would clobber
+     * the restored value, so we must skip the writeback. */
+    if (nr == static_cast<uint64>(SYS_rt_sigreturn)){
+        return;
+    }
+    tf->a[0] = static_cast<uint64>(ret);  /* Return value in a0 */
+}
+