diff --git a/sim/Makefile b/sim/Makefile
index 709ed34..333b814 100644
--- a/sim/Makefile
+++ b/sim/Makefile
@@ -20,7 +20,7 @@ CC=gcc
 CFLAGS=-O2 -g -Wall
 CPPFLAGS=-I../include
 
-SIMOBJS=sim.o
+SIMOBJS=sim.o pdp10-virt-mem.o
 LIBOBJS=../lib/pdp10-elf36.o ../lib/pdp10-extint.o ../lib/pdp10-stdio.o
 
 sim:	$(SIMOBJS) $(LIBOBJS)
diff --git a/sim/pdp10-virt-mem.c b/sim/pdp10-virt-mem.c
new file mode 100644
index 0000000..09f5f26
--- /dev/null
+++ b/sim/pdp10-virt-mem.c
@@ -0,0 +1,76 @@
+/*
+ * pdp10-virt-mem.c -- virtual memory simulation for PDP10
+ * Copyright (C) 2018  Mikael Pettersson
+ *
+ * This file is part of pdp10-tools.
+ *
+ * pdp10-tools 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 of the License, or
+ * (at your option) any later version.
+ *
+ * pdp10-tools 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 pdp10-tools.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <string.h>
+#include "pdp10-virt-mem.h"
+
+int pdp10_vmem_init(struct pdp10_vmem *vmem)
+{
+    memset(vmem, 0, sizeof *vmem);
+    return 0;
+}
+
+static inline pdp10_uint36_t pdp10_mword_read(uint64_t *mword)
+{
+    return *mword;
+}
+
+static inline void pdp10_mword_write(uint64_t *mword, pdp10_uint36_t word)
+{
+    /* TODO: assert high bits are clear */
+    *mword = word;
+}
+
+static inline uint64_t *deref(struct pdp10_vmem *vmem, uint32_t address)
+{
+    struct pdp10_section *section;
+    struct pdp10_page *page;
+
+    section = vmem->section[address >> 18];
+    if (section) {
+	page = section->page[(address >> 9) & ((1 << 9) - 1)];
+	if (page)
+	    return &page->mword[address & ((1 << 9) - 1)];
+    }
+    return NULL;
+}
+
+uint64_t pdp10_vmem_read(struct pdp10_vmem *vmem, uint32_t address)
+{
+    uint64_t *mword;
+
+    mword = deref(vmem, address);
+    if (!mword)
+	return (uint64_t)-1;
+
+    return pdp10_mword_read(mword);
+}
+
+int pdp10_vmem_write(struct pdp10_vmem *vmem, uint32_t address, pdp10_uint36_t word)
+{
+    uint64_t *mword;
+
+    mword = deref(vmem, address);
+    if (!mword)
+	return -1;
+
+    pdp10_mword_write(mword, word);
+    return 0;
+}
diff --git a/sim/pdp10-virt-mem.h b/sim/pdp10-virt-mem.h
new file mode 100644
index 0000000..5cd998d
--- /dev/null
+++ b/sim/pdp10-virt-mem.h
@@ -0,0 +1,60 @@
+/*
+ * pdp10-virt-mem.h -- virtual memory simulation for PDP10
+ * Copyright (C) 2018  Mikael Pettersson
+ *
+ * This file is part of pdp10-tools.
+ *
+ * pdp10-tools 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 of the License, or
+ * (at your option) any later version.
+ *
+ * pdp10-tools 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 pdp10-tools.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#ifndef PDP10_VIRT_MEM_H
+#define PDP10_VIRT_MEM_H
+
+#include <stdint.h>
+#include "pdp10-stdint.h"
+
+/*
+ * A fully-extended PDP10 has 30-bit addresses, divided into
+ * - 12 bit section number (4096 sections)
+ * - 9 bit page number (512 pages per section)
+ * - 9 bit page offset (512 36-bit words per page)
+ *
+ * Each 36-bit word is currently stored in the low 36 bits of an uint64_t.
+ *
+ * (Other options that pack the 36-bit words more tightly have drawbacks
+ * such as more complex address arithmetic, requiring multiple loads or stores
+ * per access, or requiring writes to perform read-modify-write operations.)
+ */
+
+struct pdp10_page {
+    uint64_t mword[512];
+};
+
+struct pdp10_section {
+    struct pdp10_page *page[512];
+};
+
+struct pdp10_vmem {
+    struct pdp10_section *section[4096];
+};
+
+/* Returns non-zero on failure, zero on success. */
+int pdp10_vmem_init(struct pdp10_vmem *vmem);
+
+/* Returns (uint64_t)-1 on failure (page fault), a pdp10_uint36_t word on success. */
+uint64_t pdp10_vmem_read(struct pdp10_vmem *vmem, uint32_t address);
+
+/* Returns non-zero on failure, zero on success. */
+int pdp10_vmem_write(struct pdp10_vmem *vmem, uint32_t address, pdp10_uint36_t word);
+
+#endif /* PDP10_VIRT_MEM_H */