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75 lines
2.3 KiB
C++
75 lines
2.3 KiB
C++
//
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// LanguageCard.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 LanguageCard_h
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#define LanguageCard_h
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namespace Apple {
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namespace II {
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template <typename Machine> class LanguageCard {
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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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/// Contains the state of the internal pre-write flip flop; it does not directly affect the current memory map.
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bool pre_write = false;
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};
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LanguageCard(Machine &machine) : machine_(machine) {}
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/// Should be 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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// 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(state_.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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state_.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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machine_.set_language_card_paging();
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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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};
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}
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}
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#endif /* LanguageCard_h */
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