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228 lines
7.3 KiB
C++
228 lines
7.3 KiB
C++
//
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// 68000.hpp
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// Clock Signal
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//
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// Created by Thomas Harte on 08/03/2019.
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// Copyright © 2019 Thomas Harte. All rights reserved.
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//
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#ifndef MC68000_h
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#define MC68000_h
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#include <cstdint>
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#include <iomanip>
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#include <iostream>
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#include <tuple>
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#include <vector>
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#include "../../ClockReceiver/ClockReceiver.hpp"
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#include "../RegisterSizes.hpp"
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namespace CPU {
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namespace MC68000 {
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/*!
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A microcycle is an atomic unit of 68000 bus activity — it is a single item large enough
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fully to specify a sequence of bus events that occur without any possible interruption.
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Concretely, a standard read cycle breaks down into at least two microcycles:
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1) a 5 half-cycle length microcycle in which the address strobe is signalled; and
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2) a 3 half-cycle length microcycle in which at least one of the data strobes is
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signalled, and the data bus is sampled.
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That is, assuming DTack were signalled when microcycle (1) ended. If not then additional
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wait state microcycles would fall between those two parts.
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The 68000 data sheet defines when the address becomes valid during microcycle (1), and
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when the address strobe is actually asserted. But those timings are fixed. So simply
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telling you that this was a microcycle during which the address trobe was signalled is
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sufficient fully to describe the bus activity.
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(Aside: see the 68000 template's definition for options re: implicit DTack; if your
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68000 owner can always predict exactly how long it will hold DTack following observation
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of an address-strobing microcycle, it can just supply those periods for accounting and
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avoid the runtime cost of actual DTack emulation. But such as the bus allows.)
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*/
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struct Microcycle {
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/*
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The operation code is composed of several parts; a compound low part
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that can be masked off with TypeMask identifies the type of the cycle;
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some of the other status lines are also present in the top parts of the int.
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*/
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static const int TypeMask = 3;
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/// A NewAddress cycle is one in which the address strobe is initially low but becomes high;
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/// this correlates to states 0 to 5 of a standard read/write cycle.
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static const int Idle = 0;
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/// A NewAddress cycle is one in which the address strobe is initially low but becomes high;
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/// this correlates to states 0 to 5 of a standard read/write cycle.
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static const int NewAddress = 1;
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/// A SameAddress cycle is one in which the address strobe is continuously asserted, but neither
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/// of the data strobes are.
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static const int SameAddress = 2;
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/// A Reset cycle is one in which the RESET output is asserted.
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static const int Reset = 3;
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/// Indicates that the address and both data select strobes are active.
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static const int SelectWord = 1 << 2;
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/// Indicates that the address strobe and exactly one of the data strobes are active; you can determine
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/// which by inspecting the low bit of the provided address. The RW line indicates a read.
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static const int SelectByte = 1 << 3;
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/// If set, indicates a read. Otherwise, a write.
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static const int Read = 1 << 4;
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/// Contains the value of line FC0.
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static const int IsData = 1 << 5;
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/// Contains the value of line FC1.
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static const int IsProgram = 1 << 6;
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int operation = 0;
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HalfCycles length = HalfCycles(4);
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/*!
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For expediency, this provides a full 32-bit byte-resolution address — e.g.
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if reading indirectly via an address register, this will indicate the full
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value of the address register.
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The receiver should ignore bits 0 and 24+.
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*/
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const uint32_t *address = nullptr;
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RegisterPair16 *value = nullptr;
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bool operator ==(const Microcycle &rhs) const {
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return
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std::make_tuple(value, address, length, operation, value) ==
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std::make_tuple(rhs.value, rhs.address, rhs.length, rhs.operation, rhs.value);
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}
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// Various inspectors.
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/*! @returns true if any data select line is active; @c false otherwise. */
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inline bool data_select_active() const {
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return bool(operation & (SelectWord | SelectByte));
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}
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/*!
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@returns 0 if this byte access wants the low part of a 16-bit word; 8 if it wants the high part.
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*/
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inline unsigned int byte_shift() const {
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return (((*address) & 1) << 3) ^ 8;
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}
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/*!
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Obtains the mask to apply to a word that will leave only the byte this microcycle is selecting.
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@returns 0x00ff if this byte access wants the low part of a 16-bit word; 0xff00 if it wants the high part.
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*/
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inline uint16_t byte_mask() const {
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return uint16_t(0xff00) >> (((*address) & 1) << 3);
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}
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/*!
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Obtains the mask to apply to a word that will leave only the byte this microcycle **isn't** selecting.
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i.e. this is the part of a word that should be untouched by this microcycle.
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@returns 0xff00 if this byte access wants the low part of a 16-bit word; 0x00ff if it wants the high part.
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*/
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inline uint16_t untouched_byte_mask() const {
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return uint16_t(uint16_t(0xff) << (((*address) & 1) << 3));
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}
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/*!
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Assuming this cycle is a byte write, mutates @c destination by writing the byte to the proper upper or
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lower part, retaining the other half.
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*/
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uint16_t write_byte(uint16_t destination) const {
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return uint16_t((destination & untouched_byte_mask()) | (value->halves.low << byte_shift()));
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}
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/*!
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@returns non-zero if this is a byte read and 68000 LDS is asserted.
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*/
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inline int lower_data_select() const {
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return (operation & SelectByte) & ((*address & 1) << 3);
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}
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/*!
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@returns non-zero if this is a byte read and 68000 UDS is asserted.
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*/
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inline int upper_data_select() const {
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return (operation & SelectByte) & ~((*address & 1) << 3);
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}
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/*!
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@returns the address being accessed at the precision a 68000 supplies it —
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only 24 address bit precision, with the low bit shifted out. So it's the
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68000 address at word precision: address 0 is the first word in the address
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space, address 1 is the second word (i.e. the third and fourth bytes) in
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the address space, etc.
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*/
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uint32_t word_address() const {
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return (address ? (*address) & 0x00fffffe : 0) >> 1;
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}
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};
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/*!
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This is the prototype for a 68000 bus handler; real bus handlers can descend from this
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in order to get default implementations of any changes that may occur in the expected interface.
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*/
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class BusHandler {
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public:
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/*!
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Provides the bus handler with a single Microcycle to 'perform'.
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FC0 and FC1 are provided inside the microcycle as the IsData and IsProgram
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flags; FC2 is provided here as is_supervisor — it'll be either 0 or 1.
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*/
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HalfCycles perform_bus_operation(const Microcycle &cycle, int is_supervisor) {
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return HalfCycles(0);
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}
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void flush() {}
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};
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#include "Implementation/68000Storage.hpp"
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class ProcessorBase: public ProcessorStorage {
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};
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struct ProcessorState {
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uint32_t data[8];
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uint32_t address[7];
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uint32_t user_stack_pointer, supervisor_stack_pointer;
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uint32_t program_counter;
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uint16_t status;
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// TODO: More state needed to indicate current instruction, the processor's
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// progress through it, and anything it has fetched so far.
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// uint16_t current_instruction;
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};
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template <class T, bool dtack_is_implicit> class Processor: public ProcessorBase {
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public:
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Processor(T &bus_handler) : ProcessorBase(), bus_handler_(bus_handler) {}
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void run_for(HalfCycles duration);
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using State = ProcessorState;
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State get_state();
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void set_state(const State &);
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private:
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T &bus_handler_;
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};
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#include "Implementation/68000Implementation.hpp"
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}
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}
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#endif /* MC68000_h */
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