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95 lines
2.7 KiB
C++
95 lines
2.7 KiB
C++
//
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// Carry.hpp
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// Clock Signal
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//
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// Created by Thomas Harte on 30/08/2023.
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// Copyright © 2023 Thomas Harte. All rights reserved.
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//
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#ifndef Carry_hpp
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#define Carry_hpp
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namespace Numeric {
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/// @returns @c true if from @c bit there was:
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/// • carry after calculating @c lhs + @c rhs if @c is_add is true; or
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/// • borrow after calculating @c lhs - @c rhs if @c is_add is false;
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/// producing @c result.
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template <bool is_add, int bit, typename IntT> bool carried_out(IntT lhs, IntT rhs, IntT result) {
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// Additive:
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//
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// 0 and 0 => didn't.
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// 0 and 1 or 1 and 0 => did if 0.
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// 1 and 1 => did.
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//
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// Subtractive:
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//
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// 1 and 0 => didn't
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// 1 and 1 or 0 and 0 => did if 1.
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// 0 and 1 => did.
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if constexpr (!is_add) {
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rhs = ~rhs;
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}
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const bool carry = IntT(1 << bit) & (lhs | rhs) & ((lhs & rhs) | ~result);
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if constexpr (!is_add) {
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return !carry;
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} else {
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return carry;
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}
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}
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/// @returns @c true if there was carry into @c bit when computing either:
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/// • @c lhs + @c rhs; or
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/// • @c lhs - @c rhs;
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/// producing @c result.
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template <int bit, typename IntT> bool carried_in(IntT lhs, IntT rhs, IntT result) {
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// 0 and 0 or 1 and 1 => did if 1.
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// 0 and 1 or 1 and 0 => did if 0.
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return IntT(1 << bit) & (lhs ^ rhs ^ result);
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}
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//
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// BEGIN TEMPORARY COPY AND PASTE SECTION.
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//
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// The following are largely excised from the M68k PerformImplementation.hpp; if there proves to be no
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// reason further to specialise them, there'll be a factoring out. In some cases I've tightened the documentation.
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//
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/// @returns An int of type @c IntT with only the most-significant bit set.
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template <typename IntT> constexpr IntT top_bit() {
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static_assert(!std::numeric_limits<IntT>::is_signed);
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constexpr IntT max = std::numeric_limits<IntT>::max();
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return max - (max >> 1);
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}
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/// @returns The number of bits in @c IntT.
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template <typename IntT> constexpr int bit_size() {
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return sizeof(IntT) * 8;
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}
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/// @returns An int with the top bit indicating whether overflow occurred during the calculation of
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/// • @c lhs + @c rhs (if @c is_add is true); or
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/// • @c lhs - @c rhs (if @c is_add is false)
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/// and the result was @c result. All other bits will be clear.
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template <bool is_add, typename IntT>
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IntT overflow(IntT lhs, IntT rhs, IntT result) {
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const IntT output_changed = result ^ lhs;
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const IntT input_differed = lhs ^ rhs;
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if constexpr (is_add) {
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return top_bit<IntT>() & output_changed & ~input_differed;
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} else {
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return top_bit<IntT>() & output_changed & input_differed;
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}
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}
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// NOTE TO FUTURE SELF: the original 68k `overflow` treats lhs and rhs the other way
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// around, affecting subtractive overflow. Be careful.
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//
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// END COPY AND PASTE SECTION.
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//
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}
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#endif /* Carry_hpp */
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