diff --git a/sim/pdp10-core.h b/sim/pdp10-core.h
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+/*
+ * pdp10-core.h -- core definitions for PDP10 simulation
+ * 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_CORE_H
+#define PDP10_CORE_H
+
+#include <assert.h>
+#include "pdp10-stdint.h"
+#include "pdp10-virt-mem.h"
+
+/*
+ * Words are 36 bits wide.  The bits are numbered 0-35, left to right (most
+ * significant to least significant).  The left (more significant) half word
+ * is bits 0-17, and the right (less significant) half word is bits 18-36.
+ * Numbers use twos-complement representation, with bit 0 as the sign.
+ *
+ * A double word is two adjacent words treated as a single 72-bit entity, where
+ * the word with the lower address is the more significant.
+ *
+ * Bits are numbered in decimal, most other numbers are in octal, in particular
+ * accumulator numbers, addresses, and instruction codes are in octal.
+ *
+ * A virtual address on a fully extended processor is 30 bits, composed of a 12
+ * bit section number (upper 12 bits) and an 18-bit section offset (lower 18
+ * bits).  Paging hardware divides each section into 512 pages of 512 words each.
+ * The address bits are numbered according to the right-justified position of an
+ * address in a word.  Thus bits 6-17 contain the section number, and bits 18-35
+ * the section offset.
+ *
+ * Single-section processors (PDP6, KA10, KI10, non-extended KL10, KS10) have no
+ * section number in their virtual addresses, while the extended KL10 (KL10B)
+ * only has 5 bit section numbers (bits 13-17).
+ *
+ * The program counter register, PC, contains a virtual address.  Incrementing PC
+ * increments its segment offset without propagating any carry into the section
+ * number.
+ *
+ * Instructions are words.  In the basic instruction format, bits 0-8 specify the
+ * operation, and bits 9-12 (A) address an accumulator.  The rest of the word
+ * specifies how to calculate the effective address (E).  Bit 13 (I) specifies the
+ * type of addressing (indirect or not), bits 14-17 (X) specify an index register,
+ * and bits 18-35 (Y) specify a memory location.  Some instructions use bits 9-12
+ * to address flags or as extension of the instruction code.
+ *
+ * If an instruction does not use some part of the instruction word, that part is
+ * reserved and MUST BE ZERO, unless otherwise specified.
+ */
+
+/*
+ * BITS36(X, LEFT, RIGHT)
+ *
+ * Extract bits LEFT through RIGHT (inclusive) from a 36-bit number X.
+ * Requires 0 <= LEFT <= RIGHT <= 35.
+ */
+static inline pdp10_uint36_t BITS36(pdp10_uint36_t X, unsigned int LEFT, unsigned int RIGHT)
+{
+    assert(0 <= LEFT && LEFT <= RIGHT && RIGHT <= 35);
+    return (X >> (35 - RIGHT)) & ((1UL << (1 + RIGHT - LEFT)) - 1);
+}
+
+static inline uint32_t BITS36LOW18(pdp10_uint36_t X)
+{
+    return BITS36(X, 18, 35);
+}
+
+/*
+ * BIT36(X, BIT)
+ *
+ * Extract bit BIT from a 36-bit number X.
+ * Requires 0 <= BIT <= 35.
+ */
+static inline unsigned int BIT36(pdp10_uint36_t X, unsigned int BIT)
+{
+    return BITS36(X, BIT, BIT);
+}
+
+typedef uint32_t pdp10_vaddr_t; /* 30 bits: section number, section offset */
+typedef uint32_t pdp10_pc_t; /* 31 bits: global flag, vaddr */
+
+struct pdp10_cpu {
+    pdp10_pc_t PC;
+    pdp10_uint36_t AC[017];
+    unsigned int flags; /* 13 bits */
+    struct pdp10_vmem vmem;
+};
+
+#endif /* PDP10_CORE_H */