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diff --git a/rcc-legacy/include/rcc/rv64.h b/rcc-legacy/include/rcc/rv64.h
new file mode 100644
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+++ b/rcc-legacy/include/rcc/rv64.h
@@ -0,0 +1,155 @@
+/* Source code for RCC-Legacy.
+   Copyright (C) 2026 Richard Qin
+
+This file is part of RCC.
+
+RCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+RCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with RCC; see the file LICENSE.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+/// @file rv64.h
+/// @brief RISC-V 64 (lp64d ABI) backend per-target header.
+
+#pragma once
+
+#include "rcc/all.h"
+
+#include <cstdint>
+
+namespace rcc {
+
+// Register Numbers
+
+enum Rv64Reg : int {
+    // GPR: caller-save temps
+    T0 = RXX + 1,
+    T1,
+    T2,
+    T3,
+    T4,
+    T5,
+
+    // GPR: arguments / return values (caller-save)
+    A0,
+    A1,
+    A2,
+    A3,
+    A4,
+    A5,
+    A6,
+    A7,
+
+    // GPR: callee-save (S0 = FP, see below; therefore jumps to S1)
+    S1,
+    S2,
+    S3,
+    S4,
+    S5,
+    S6,
+    S7,
+    S8,
+    S9,
+    S10,
+    S11,
+
+    // Globally live
+    FP,  // = S0 alias, frame pointer
+    SP,  // stack pointer
+    GP,  // global pointer
+    TP,  // thread pointer
+    RA,  // return address
+
+    // FPR: caller-save temps
+    FT0,
+    FT1,
+    FT2,
+    FT3,
+    FT4,
+    FT5,
+    FT6,
+    FT7,
+    FT8,
+    FT9,
+    FT10,
+
+    // FPR: arguments / return values (caller-save)
+    FA0,
+    FA1,
+    FA2,
+    FA3,
+    FA4,
+    FA5,
+    FA6,
+    FA7,
+
+    // FPR: callee-save
+    FS0,
+    FS1,
+    FS2,
+    FS3,
+    FS4,
+    FS5,
+    FS6,
+    FS7,
+    FS8,
+    FS9,
+    FS10,
+    FS11,
+
+    // Reserved (emit.cc internal scratch, does not participate in rega)
+    T6,
+    FT11,
+
+    // Derived counters (layout-dependent)
+    NFPR = FS11 - FT0 + 1,      // Total FPR count (including reserved)
+    NGPR = RA - T0 + 1,         // Total GPR count (including reserved)
+    NGPS = A7 - T0 + 1,         // T0..A7: scratch + arg segment
+    NFPS = FA7 - FT0 + 1,       // FT0..FA7: scratch + arg segment
+    NCLR = (S11 - S1 + 1)       // Total callee-save count (GP)
+           + (FS11 - FS0 + 1),  //              + (FP)
+};
+
+MAKESURE(rv64_reg_not_tmp, FT11 < static_cast<int>(Tmp0));
+
+// Per-op attribute table (defined in targ.cc)
+
+struct Rv64Op {
+    uint8_t imm;  // 1 = op accepts immediate operand
+};
+
+// Global tables provided by targ.cc
+
+extern int rv64_rsave[];  // caller-save list (for rega)
+extern int rv64_rclob[];  // call boundary clobber list
+extern Rv64Op rv64_op[];  // Indexed by O* op enum value
+
+// rv64 target descriptor. Assigned `T = T_rv64` early in main startup (before worker thread
+// creation), read-only afterwards.
+extern Target T_rv64;
+
+// Backend pass entries
+
+// abi.cc - RCall encode/decode (pure query, no allocations)
+bits rv64_retregs(Ref, int p[2]);
+bits rv64_argregs(Ref, int p[2]);
+
+// abi.cc - ABI lowering (internal newtmp/emit/idup/vgrow/alloc, requires Ctx*)
+void rv64_abi(Ctx*, Fn*);
+
+// isel.cc - Instruction selection (internal newtmp/emit, requires Ctx*)
+void rv64_isel(Ctx*, Fn*);
+
+// emit.cc - Assembly emission
+void rv64_emitfn(Fn*, FILE*);
+
+}  // namespace rcc