mirror of https://github.com/stella-emu/stella.git
220 lines
7.3 KiB
C++
220 lines
7.3 KiB
C++
//============================================================================
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//
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// SSSS tt lll lll
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// SS SS tt ll ll
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// SS tttttt eeee ll ll aaaa
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// SSSS tt ee ee ll ll aa
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// SS tt eeeeee ll ll aaaaa -- "An Atari 2600 VCS Emulator"
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// SS SS tt ee ll ll aa aa
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// SSSS ttt eeeee llll llll aaaaa
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//
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// Copyright (c) 1995-2018 by Bradford W. Mott, Stephen Anthony
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// and the Stella Team
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//
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// See the file "License.txt" for information on usage and redistribution of
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// this file, and for a DISCLAIMER OF ALL WARRANTIES.
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//============================================================================
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#ifndef CARTRIDGEFE_HXX
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#define CARTRIDGEFE_HXX
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class System;
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#include "bspf.hxx"
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#include "Cart.hxx"
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#ifdef DEBUGGER_SUPPORT
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#include "CartFEWidget.hxx"
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#endif
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/**
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Bankswitching method used by Activision's Robot Tank and Decathlon.
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This scheme was originally designed to have up to 8 4K banks, and is
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triggered by monitoring the address bus for address $01FE. All released
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carts had only two banks, and this implementation assumes that (ie, ROM
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is always 8K, and there are two 4K banks).
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The following is paraphrased from the original patent by David Crane,
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European Patent Application # 84300730.3, dated 06.02.84:
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---------------------------------------------------------------------------
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The twelve line address bus is connected to a plurality of 4K by eight bit
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memories.
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The eight line data bus is connected to each of the banks of memory, also.
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An address comparator is connected to the bus for detecting the presence of
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the 01FE address. Actually, the comparator will detect only the lowest 12
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bits of 1FE, because of the twelve bit limitation of the address bus. Upon
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detection of the 01FE address, a one cycle delay is activated which then
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actuates latch connected to the data bus. The three most significant bits
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on the data bus are latched and provide the address bits A13, A14, and A15
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which are then applied to a 3 to 8 de-multiplexer. The 3 bits A13-A15
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define a code for selecting one of the eight banks of memory which is used
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to enable one of the banks of memory by applying a control signal to the
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enable, EN, terminal thereof. Accordingly, memory bank selection is
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accomplished from address codes on the data bus following a particular
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program instruction, such as a jump to subroutine.
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---------------------------------------------------------------------------
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Note that in the general scheme, we use D7, D6 and D5 for the bank number
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(3 bits, so 8 possible banks). However, the scheme as used historically
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by Activision only uses two banks. Furthermore, the two banks it uses
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are actually indicated by binary 110 and 111, and translated as follows:
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binary 110 -> decimal 6 -> Upper 4K ROM (bank 1) @ $D000 - $DFFF
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binary 111 -> decimal 7 -> Lower 4K ROM (bank 0) @ $F000 - $FFFF
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Since the actual bank numbers (0 and 1) do not map directly to their
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respective bitstrings (7 and 6), we simply test for D5 being 0 or 1.
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This is the significance of the test '(value & 0x20) ? 0 : 1' in the code.
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NOTE: Consult the patent application for more specific information, in
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particular *why* the address $01FE will be placed on the address
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bus after both the JSR and RTS opcodes.
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@author Stephen Anthony; with ideas/research from Christian Speckner and
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alex_79 and TomSon (of AtariAge)
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*/
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class CartridgeFE : public Cartridge
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{
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friend class CartridgeFEWidget;
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public:
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/**
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Create a new cartridge using the specified image
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@param image Pointer to the ROM image
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@param size The size of the ROM image
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@param settings A reference to the various settings (read-only)
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*/
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CartridgeFE(const BytePtr& image, uInt32 size, const Settings& settings);
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virtual ~CartridgeFE() = default;
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public:
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/**
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Reset device to its power-on state
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*/
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void reset() override;
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/**
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Install cartridge in the specified system. Invoked by the system
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when the cartridge is attached to it.
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@param system The system the device should install itself in
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*/
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void install(System& system) override;
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/**
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Install pages for the specified bank in the system.
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@param bank The bank that should be installed in the system
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*/
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bool bank(uInt16 bank) override;
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/**
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Get the current bank.
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*/
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uInt16 getBank() const override;
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/**
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Query the number of banks supported by the cartridge.
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*/
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uInt16 bankCount() const override;
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/**
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Patch the cartridge ROM.
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@param address The ROM address to patch
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@param value The value to place into the address
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@return Success or failure of the patch operation
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*/
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bool patch(uInt16 address, uInt8 value) override;
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/**
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Access the internal ROM image for this cartridge.
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@param size Set to the size of the internal ROM image data
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@return A pointer to the internal ROM image data
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*/
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const uInt8* getImage(uInt32& size) const override;
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/**
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Save the current state of this cart to the given Serializer.
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@param out The Serializer object to use
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@return False on any errors, else true
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*/
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bool save(Serializer& out) const override;
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/**
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Load the current state of this cart from the given Serializer.
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@param in The Serializer object to use
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@return False on any errors, else true
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*/
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bool load(Serializer& in) override;
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/**
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Get a descriptor for the device name (used in error checking).
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@return The name of the object
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*/
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string name() const override { return "CartridgeFE"; }
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#ifdef DEBUGGER_SUPPORT
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/**
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Get debugger widget responsible for accessing the inner workings
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of the cart.
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*/
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CartDebugWidget* debugWidget(GuiObject* boss, const GUI::Font& lfont,
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const GUI::Font& nfont, int x, int y, int w, int h) override
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{
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return new CartridgeFEWidget(boss, lfont, nfont, x, y, w, h, *this);
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}
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#endif
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public:
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/**
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Get the byte at the specified address.
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@return The byte at the specified address
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*/
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uInt8 peek(uInt16 address) override;
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/**
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Change the byte at the specified address to the given value.
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@param address The address where the value should be stored
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@param value The value to be stored at the address
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@return True if the poke changed the device address space, else false
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*/
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bool poke(uInt16 address, uInt8 value) override;
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private:
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/**
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Perform bankswitch when necessary, by monitoring for $01FE
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on the address bus and getting the bank number from the data bus.
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*/
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void checkBankSwitch(uInt16 address, uInt8 value);
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private:
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// The 8K ROM image of the cartridge
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uInt8 myImage[8192];
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// Indicates the offset into the ROM image (aligns to current bank)
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uInt16 myBankOffset;
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// Whether previous address by peek/poke equals $01FE (hotspot)
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bool myLastAccessWasFE;
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private:
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// Following constructors and assignment operators not supported
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CartridgeFE() = delete;
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CartridgeFE(const CartridgeFE&) = delete;
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CartridgeFE(CartridgeFE&&) = delete;
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CartridgeFE& operator=(const CartridgeFE&) = delete;
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CartridgeFE& operator=(CartridgeFE&&) = delete;
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};
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#endif
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