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diff --git a/libalee/state.hpp b/libalee/state.hpp
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+/**
+ * Alee Forth: A portable and concise Forth implementation in modern C++.
+ * Copyright (C) 2023  Clyne Sullivan <clyne@bitgloo.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program 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 Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef ALEEFORTH_STATE_HPP
+#define ALEEFORTH_STATE_HPP
+
+#include "dictionary.hpp"
+#include "types.hpp"
+
+#include <csetjmp>
+#include <cstddef>
+
+constexpr unsigned DataStackSize = 16;
+constexpr unsigned ReturnStackSize = 16;
+
+struct State
+{
+    enum class Error : int {
+        none = 0,
+        push,
+        pop,
+        pushr,
+        popr,
+        top,
+        pick,
+        exit
+    };
+
+    Addr ip = 0;
+    Dictionary& dict;
+    void (*input)(State&);
+
+    Cell dstack[DataStackSize] = {};
+    Cell rstack[ReturnStackSize] = {};
+    Cell *dsp = dstack - 1;
+    Cell *rsp = rstack - 1;
+
+    std::jmp_buf jmpbuf = {};
+
+    constexpr State(Dictionary& d, void (*i)(State&)):
+        dict(d), input(i) {}
+
+    bool compiling() const;
+    void compiling(bool);
+
+    Error execute(Addr);
+
+    std::size_t size() const noexcept;
+    std::size_t rsize() const noexcept;
+
+    inline void push(Cell value) {
+        if (dsp == dstack + DataStackSize - 1)
+            std::longjmp(jmpbuf, static_cast<int>(Error::push));
+        *++dsp = value;
+    }
+
+    inline Cell pop() {
+        if (dsp < dstack)
+            std::longjmp(jmpbuf, static_cast<int>(Error::pop));
+        return *dsp--;
+    }
+
+    inline Cell beyondip() {
+        ip += sizeof(Cell);
+        return dict.read(ip);
+    }
+
+    inline void pushr(Cell value) {
+        if (rsp == rstack + ReturnStackSize - 1)
+            std::longjmp(jmpbuf, static_cast<int>(Error::pushr));
+        *++rsp = value;
+    }
+
+    inline Cell popr() {
+        if (rsp < rstack)
+            std::longjmp(jmpbuf, static_cast<int>(Error::popr));
+        return *rsp--;
+    }
+
+    inline Cell& top() {
+        if (dsp < dstack)
+            std::longjmp(jmpbuf, static_cast<int>(Error::top));
+        return *dsp;
+    }
+
+    inline Cell& pick(std::size_t i) {
+        if (dsp - i < dstack)
+            std::longjmp(jmpbuf, static_cast<int>(Error::pick));
+        return *(dsp - i);
+    }
+};
+
+#endif // ALEEFORTH_STATE_HPP
+