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94 lines
2.9 KiB
C++
94 lines
2.9 KiB
C++
//
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// LanguageCardSwitches.hpp
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// Clock Signal
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//
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// Created by Thomas Harte on 22/10/2020.
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// Copyright © 2020 Thomas Harte. All rights reserved.
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//
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#ifndef LanguageCardSwitches_h
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#define LanguageCardSwitches_h
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namespace Apple {
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namespace II {
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/*!
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Models the language card soft switches, present on any Apple II with a language card and provided built-in from the IIe onwards.
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Relevant memory accesses should be fed to this class; it'll call:
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* machine.set_language_card_paging() if the proper mapped state changes.
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*/
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template <typename Machine> class LanguageCardSwitches {
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public:
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struct State {
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/// Indicates which 4kb chunk of RAM should be visible at $Dxxx if RAM is visible at all.
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bool bank1 = false;
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/// @c true indicates that RAM should be readable in the range $D000–$FFFF;
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/// @c indicates ROM should be readable.
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bool read = false;
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/// @c true indicates that ROM is selected for 'writing' in the range $D000–$FFFF (i.e. writes are a no-op);
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/// @c false indicates that RAM is selected for writing.
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bool write = false;
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bool operator != (const State &rhs) const {
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return
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bank1 != rhs.bank1 ||
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read != rhs.read ||
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write != rhs.write;
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}
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};
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LanguageCardSwitches(Machine &machine) : machine_(machine) {}
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/// Used by an owner to forward any access to $c08x.
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void access(uint16_t address, bool is_read) {
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const auto previous_state = state_;
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// Quotes below taken from Understanding the Apple II, p. 5-28 and 5-29.
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// "A3 controls the 4K bank selection"
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state_.bank1 = address & 8;
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// "Access to $C080, $C083, $C084, $0087, $C088, $C08B, $C08C, or $C08F sets the READ ENABLE flip-flop"
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// (other accesses reset it)
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state_.read = !(((address&2) >> 1) ^ (address&1));
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// "The WRITE ENABLE' flip-flop is reset by an odd read access to the $C08X range when the PRE-WRITE flip-flop is set."
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if(pre_write_ && is_read && (address&1)) state_.write = false;
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// "[The WRITE ENABLE' flip-flop] is set by an even access in the $C08X range."
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if(!(address&1)) state_.write = true;
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// ("Any other type of access causes the WRITE ENABLE' flip-flop to hold its current state.")
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// "The PRE-WRITE flip-flop is set by an odd read access in the $C08X range. It is reset by an even access or a write access."
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pre_write_ = is_read ? (address&1) : false;
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// Apply whatever the net effect of all that is to the memory map.
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if(previous_state != state_) {
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machine_.set_language_card_paging();
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}
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}
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/// Provides read-only access to the current language card switch state.
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const State &state() {
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return state_;
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}
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private:
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Machine &machine_;
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State state_;
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// This is an additional flip flop contained on the language card, but
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// it is one step removed from current banking state, so I've excluded it
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// from the State struct.
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bool pre_write_ = false;
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};
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}
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}
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#endif /* LanguageCard_h */
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