1
0
mirror of https://github.com/TomHarte/CLK.git synced 2024-11-04 15:05:36 +00:00
CLK/Machines/Apple/AppleII/LanguageCardSwitches.hpp
2020-12-10 22:11:53 -05:00

117 lines
3.6 KiB
C++
Raw Blame History

This file contains ambiguous Unicode characters

This file contains Unicode characters that might be confused with other characters. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.

//
// LanguageCardSwitches.hpp
// Clock Signal
//
// Created by Thomas Harte on 22/10/2020.
// Copyright © 2020 Thomas Harte. All rights reserved.
//
#ifndef LanguageCardSwitches_h
#define LanguageCardSwitches_h
namespace Apple {
namespace II {
/*!
Models the language card soft switches, present on any Apple II with a language card and provided built-in from the IIe onwards.
Relevant memory accesses should be fed to this class; it'll call:
* machine.set_language_card_paging() if the proper mapped state changes.
*/
template <typename Machine> class LanguageCardSwitches {
public:
struct State {
/// When RAM is visible in the range $D000$FFFF:
/// @c true indicates that bank 2 should be used between $D000 and $DFFF;
/// @c false indicates bank 1.
bool bank2 = true;
/// @c true indicates that RAM should be readable in the range $D000$FFFF;
/// @c false indicates ROM should be readable.
bool read = false;
/// @c true indicates that ROM is selected for 'writing' in the range $D000$FFFF (i.e. writes are a no-op);
/// @c false indicates that RAM is selected for writing.
bool write = false;
bool operator != (const State &rhs) const {
return
bank2 != rhs.bank2 ||
read != rhs.read ||
write != rhs.write;
}
};
LanguageCardSwitches(Machine &machine) : machine_(machine) {}
/// Used by an owner to forward any access to $c08x.
void access(uint16_t address, bool is_read) {
const auto previous_state = state_;
// Quotes below taken from Understanding the Apple II, p. 5-28 and 5-29.
// "A3 controls the 4K bank selection"; 0 = bank 2, 1 = bank 1.
state_.bank2 = !(address & 8);
// "Access to $C080, $C083, $C084, $0087, $C088, $C08B, $C08C, or $C08F sets the READ ENABLE flip-flop"
// (other accesses reset it)
state_.read = !(((address&2) >> 1) ^ (address&1));
// "The WRITE ENABLE' flip-flop is reset by an odd read access to the $C08X range when the PRE-WRITE flip-flop is set."
if(pre_write_ && is_read && (address&1)) state_.write = false;
// "[The WRITE ENABLE' flip-flop] is set by an even access in the $C08X range."
if(!(address&1)) state_.write = true;
// ("Any other type of access causes the WRITE ENABLE' flip-flop to hold its current state.")
// "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."
pre_write_ = is_read ? (address&1) : false;
// Apply whatever the net effect of all that is to the memory map.
if(previous_state != state_) {
machine_.set_language_card_paging();
}
}
/// Provides read-only access to the current language card switch state.
const State &state() const {
return state_;
}
/// Provides relevant parts of the IIgs interface.
void set_state(uint8_t value) {
const auto previous_state = state_;
// Bit 3: 1 => enable ROM, 0 => enable RAM.
state_.read = !(value & 0x08);
// Bit 2: 1 => select bank 2, 0 => select bank 1. [per errata to the Hardware Reference
// correcting the original, which lists them the other way around]
state_.bank2 = value & 0x04;
if(previous_state != state_) {
machine_.set_language_card_paging();
}
}
uint8_t get_state() const {
return
(state_.read ? 0x00 : 0x08) |
(state_.bank2 ? 0x04 : 0x00);
}
private:
Machine &machine_;
State state_;
// This is an additional flip flop contained on the language card, but
// it is one step removed from current banking state, so I've excluded it
// from the State struct.
bool pre_write_ = false;
};
}
}
#endif /* LanguageCard_h */