diff --git a/expression.cpp b/expression.cpp
new file mode 100644
index 0000000..abb1c58
--- /dev/null
+++ b/expression.cpp
@@ -0,0 +1,169 @@
+#include "expression.h"
+#include <vector>
+#include "obj816.h"
+
+/* a ** b */
+uint32_t power(uint32_t a, uint32_t b) {
+
+	uint32_t rv = 1;
+	for( ; b; b >>= 1) {
+		if (b & 0x01) rv *= a;
+		a = a * a;
+	}
+	return rv; 
+}
+
+bool unary_op(unsigned op, std::vector<expr> &v) {
+	if (v.size() >=1 ) {
+		expr &a = v.back();
+		if (a.tag == OP_VAL) {
+			switch(a.tag) {
+			case OP_NOT: a.value = !a.value; break;
+			case OP_NEG: a.value = -a.value; break;
+			case OP_FLP: a.value = ~a.value; break;
+			}
+			return true;
+		}
+	}
+	v.emplace_back(op);
+	return false;
+}
+
+
+bool binary_op(unsigned op, std::vector<expr> &v) {
+	if (v.size() >= 2) {
+		expr &a = v[v.size() - 2];
+		expr &b = v[v.size() - 1];
+
+		if (a.tag == OP_VAL && b.tag == OP_VAL) {
+			uint32_t value = 0;
+			switch(op) {
+			case OP_EXP: value = power(a.value, b.value); break;
+			case OP_MUL: value = a.value * b.value; break;
+			case OP_DIV: value = a.value / b.value; break;
+			case OP_MOD: value = a.value % b.value; break;
+			case OP_SHR: value = a.value >> b.value; break;
+			case OP_SHL: value = a.value << b.value; break;
+			case OP_ADD: value = a.value + b.value; break;
+			case OP_SUB: value = a.value - b.value; break;
+			case OP_AND: value = a.value & b.value; break;
+			case OP_OR: value = a.value | b.value; break;
+			case OP_XOR: value = a.value ^ b.value; break;
+			case OP_EQ: value = a.value == b.value; break;
+			case OP_GT: value = (int32_t)a.value > (int32_t)b.value; break;
+			case OP_LT: value = (int32_t)a.value < (int32_t)b.value; break;
+			case OP_UGT: value = a.value > b.value; break;
+			case OP_ULT: value = a.value < b.value; break;
+			}
+			v.pop_back();
+			v.back().value = value;
+			return true;
+		}
+		if (a.tag == OP_VAL && b.tag == OP_LOC) {
+			switch(op) {
+				case OP_ADD: {
+					// constant + symbol
+					expr tmp = b;
+					tmp.value = a.value + b.value;
+					v.pop_back();
+					v.pop_back();
+					v.emplace_back(tmp);
+					return true; 
+				}
+				#if 0
+				case OP_SUB: {
+					// constant - symbol .. does this even make sense?
+					expr tmp = b;
+					tmp.value = a.value - b.value;
+					v.pop_back();
+					v.pop_back();
+					v.emplace_back(tmp);
+					return true; 
+				}
+				#endif
+			}
+		}
+		if (a.tag == OP_LOC && b.tag == OP_VAL) {
+			switch(op) {
+				case OP_ADD: a.value += b.value; return true;
+				case OP_SUB: a.value -= b.value; return true;
+			}
+		}
+
+		if (a.tag == OP_LOC && b.tag == OP_LOC && a.section == b.section) {
+			uint32_t value = 0;
+			bool rv = false;
+			switch (op) {
+				case OP_SUB:
+					// end - start
+					value = a.value - b.value;
+					rv = true; break;
+				case OP_EQ:
+					value = a.value == b.value;
+					rv = true; break;
+				case OP_GT:
+				case OP_UGT:
+					value = a.value > b.value;
+					rv = true; break;
+				case OP_LT:
+				case OP_ULT:
+					value = a.value < b.value;
+					rv = true; break;
+			}
+			if (rv) {
+				v.pop_back();
+				v.pop_back();
+				v.emplace_back(OP_VAL, value);
+				return true;
+			}
+		}
+
+
+	}
+	v.emplace_back(op);
+	return false;
+}
+
+
+bool simplify_expression(expression &e) {
+	std::vector<expr> tmp;
+	bool rv = false;
+	for (auto &t : e.stack) {
+		if (t.tag >= OP_BIN) {
+			rv = binary_op(t.tag, tmp) || rv;
+		} else if (t.tag >= OP_UNA) {
+			rv = unary_op(t.tag, tmp) || rv;
+		} else {
+			tmp.emplace_back(t);
+		}
+	}
+	if (rv) e.stack = std::move(tmp);
+	return rv;
+}
+
+/* 
+ * if force is true, treat OP_LOC records as OP_VAL (for omf)
+ */
+
+// should return std::optional<uint32_t>...
+
+uint32_t evaluate_expression(expression &e, bool force) {
+	std::vector<expr> tmp;
+	for (const auto &t : e.stack) {
+		if (t.tag == OP_LOC && force) {
+			tmp.emplace_back(OP_VAL, t.value);
+			continue;				
+		}
+		if (t.tag >= OP_BIN) {
+			binary_op(t.tag, tmp);
+		} else if (t.tag >= OP_UNA) {
+			unary_op(t.tag, tmp);
+		} else {
+			tmp.emplace_back(t);
+		}
+	}
+
+	if (tmp.size() == 1 && tmp.front().tag == OP_VAL) return tmp.front().value;
+	return 0;
+}
+
diff --git a/expression.h b/expression.h
new file mode 100644
index 0000000..db3c56d
--- /dev/null
+++ b/expression.h
@@ -0,0 +1,32 @@
+#ifndef __expression_h__
+#define __expression_h__
+
+#include <stdint.h>
+#include <vector>
+
+struct expr {
+	expr(int t = 0, uint32_t v = 0, uint32_t s = 0)
+		: tag(t), value(v), section(s)
+	{}
+	int tag = 0;
+	uint32_t value = 0;
+	unsigned section = 0;
+};
+
+
+struct expression {
+	unsigned section = 0;
+	uint32_t offset = 0;
+	uint8_t size = 0;
+	uint8_t relative = false;
+	bool undefined = false;
+
+	std::vector<expr> stack;
+
+};
+
+uint32_t evaluate_expression(expression &e, bool force = false);
+
+bool simplify_expression(expression &e);
+
+#endif