diff --git a/rialto/idmap.S b/rialto/idmap.S
new file mode 100644
index 0000000..7281d9b
--- /dev/null
+++ b/rialto/idmap.S
@@ -0,0 +1,66 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+//
+// Initial TTBR0 idmap activated before first memory write.
+// Remains active until a new page table is created by early Rust.
+//
+
+.set .SZ_1K, 1024
+.set .SZ_4K, 4 * .SZ_1K
+.set .SZ_1M, 1024 * .SZ_1K
+.set .SZ_2M, 2 * .SZ_1M
+.set .SZ_1G, 1024 * .SZ_1M
+
+.set .PAGE_SIZE, .SZ_4K
+
+.set .ORIGIN_ADDR, 2 * .SZ_1G
+.set .DTB_ADDR, .ORIGIN_ADDR + (0 * .SZ_2M)
+.set .TEXT_ADDR, .ORIGIN_ADDR + (1 * .SZ_2M)
+.set .DATA_ADDR, .ORIGIN_ADDR + (2 * .SZ_2M)
+
+.set .L_TT_TYPE_BLOCK, 0x1
+.set .L_TT_TYPE_PAGE,  0x3
+.set .L_TT_TYPE_TABLE, 0x3
+
+.set .L_TT_AF, 0x1 << 10 // Access flag
+.set .L_TT_NG, 0x1 << 11 // Not global
+.set .L_TT_RO, 0x2 << 6
+.set .L_TT_XN, 0x3 << 53
+
+.set .L_TT_MT_DEV, 0x0 << 2			// MAIR #0 (DEV_nGnRE)
+.set .L_TT_MT_MEM, (0x1 << 2) | (0x3 << 8)	// MAIR #1 (MEM_WBWA), inner shareable
+
+.set .L_BLOCK_RO,  .L_TT_TYPE_BLOCK | .L_TT_MT_MEM | .L_TT_AF | .L_TT_RO | .L_TT_XN
+.set .L_BLOCK_DEV, .L_TT_TYPE_BLOCK | .L_TT_MT_DEV | .L_TT_AF | .L_TT_XN
+.set .L_BLOCK_MEM, .L_TT_TYPE_BLOCK | .L_TT_MT_MEM | .L_TT_AF | .L_TT_XN | .L_TT_NG
+.set .L_BLOCK_MEM_XIP, .L_TT_TYPE_BLOCK | .L_TT_MT_MEM | .L_TT_AF | .L_TT_NG | .L_TT_RO
+
+.section ".rodata.idmap", "a", %progbits
+.global idmap
+.balign .PAGE_SIZE
+idmap:
+	/* level 1 */
+	.quad		.L_BLOCK_DEV | 0x0		// 1 GiB of device mappings
+	.quad		0x0				// 1 GiB unmapped
+	.quad		.L_TT_TYPE_TABLE + 0f		// up to 1 GiB of DRAM
+	.balign .PAGE_SIZE, 0				// unmapped
+
+	/* level 2 */
+0:	.quad		.L_BLOCK_RO  | .DTB_ADDR	// DT provided by VMM
+	.quad		.L_BLOCK_MEM_XIP | .TEXT_ADDR	// 2 MiB of DRAM containing image
+	.quad		.L_BLOCK_MEM | .DATA_ADDR	// 2 MiB of writable DRAM
+	.balign .PAGE_SIZE, 0				// unmapped