Created
January 20, 2018 08:10
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Microcode for Ben Eater's breadboard computer at https://www.youtube.com/playlist?list=PLowKtXNTBypGqImE405J2565dvjafglHU
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| #include <cstddef> | |
| #include <array> | |
| #include <stdexcept> | |
| //// This would be a header file if Compiler Explorer supported those | |
| //// You can skip to the comment "Instructions set definition here" | |
| // Use put in a namespace to avoid poluting global namespace (Otherwise there are issues with multiple HLTs) | |
| namespace ControlBitsNS { | |
| enum ControlBits : uint16_t { | |
| NONE = 0, | |
| J = 1 << 1, | |
| CO = 1 << 2, | |
| CE = 1 << 3, | |
| OI = 1 << 4, | |
| BI = 1 << 5, | |
| SU = 1 << 6, | |
| EO = 1 << 7, | |
| AO = 1 << 8, | |
| AI = 1 << 9, | |
| II = 1 << 10, | |
| IO = 1 << 11, | |
| RO = 1 << 12, | |
| RI = 1 << 13, | |
| MI = 1 << 14, | |
| HLT = 1 << 15 | |
| }; | |
| // Make the bitwise operators return the right type | |
| ControlBits constexpr operator|(ControlBits lhs, ControlBits rhs) { | |
| return ControlBits(uint16_t(lhs) | uint16_t(rhs)); | |
| } | |
| ControlBits constexpr operator&(ControlBits lhs, ControlBits rhs) { | |
| return ControlBits(uint16_t(lhs) & uint16_t(rhs)); | |
| } | |
| ControlBits constexpr operator~(ControlBits bits) { | |
| return ControlBits(~ uint16_t(bits)); | |
| } | |
| } | |
| using ControlBitsNS::ControlBits; | |
| /// Verify that control bits are sensible | |
| ControlBits constexpr verify(ControlBits uop) { | |
| ControlBits output_ops = ControlBits::CO | ControlBits::EO | ControlBits::AO | |
| | ControlBits::IO | ControlBits::RO; | |
| ControlBits input_ops = ControlBits::J | ControlBits::OI | ControlBits::BI | |
| | ControlBits::AI | ControlBits::II | ControlBits::RI | |
| | ControlBits::MI; | |
| if (__builtin_popcount(uint16_t(uop & output_ops)) > 1) { | |
| throw std::invalid_argument("Contention on the bus"); | |
| } | |
| if ((uop & output_ops) && ! (uop & input_ops)) { | |
| throw std::invalid_argument("Bus output not being read"); | |
| } | |
| if ((uop & input_ops) && ! (uop & output_ops)) { | |
| throw std::invalid_argument("Reading floating bus input"); | |
| } | |
| // Note: We do not check that HLT is the only control bit if it's set | |
| // You might want to halt in subtract mode or with soething on the bus. | |
| return uop; | |
| } | |
| struct Microcode { | |
| constexpr Microcode(ControlBits t2 = ControlBits::NONE, ControlBits t3 = ControlBits::NONE, | |
| ControlBits t4 = ControlBits::NONE, ControlBits t5 = ControlBits::NONE, | |
| ControlBits t6 = ControlBits::NONE, ControlBits t7 = ControlBits::NONE) | |
| : uops{{verify(t2), verify(t3), verify(t4), verify(t5), verify(t6), verify(t7)}} {} | |
| // Initialization microcode; same for every instruction | |
| std::array<ControlBits, 2> init {{verify(ControlBits::MI | ControlBits::CO), | |
| verify(ControlBits::II | ControlBits::RO | ControlBits::CE)}}; | |
| std::array<ControlBits, 6> uops; | |
| }; | |
| static_assert(sizeof(Microcode) == 16, "Microcode object size incorrect"); | |
| using InstructionSet = std::array<Microcode, 16>; |
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