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◆ main openmsx / src / memory / RomPlain.cc
5.2 KiB · C++ · last changed chore(): initial commit 1 month ago source blame history raw
e8555c89 Andy McCall 1 
#include "RomPlain.hh"
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#include "MSXException.hh"
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#include "XMLElement.hh"
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#include "serialize.hh"
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#include "one_of.hh"
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#include "unreachable.hh"
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#include "xrange.hh"
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#include <array>
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namespace openmsx {
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// return x inside [start, start+len)
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[[nodiscard]] static constexpr bool isInside(unsigned x, unsigned start, unsigned len)
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{
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	unsigned tmp = x - start;
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	return tmp < len;
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}
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[[nodiscard]] static std::string toString(unsigned start, unsigned len)
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{
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	return strCat("[0x", hex_string<4>(start), ", "
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	               "0x", hex_string<4>(start + len), ')');
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}
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RomPlain::RomPlain(const DeviceConfig& config, Rom&& rom_, RomType type)
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	: Rom8kBBlocks(config, std::move(rom_))
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{
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	unsigned windowBase =  0x0000;
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	unsigned windowSize = 0x10000;
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	if (const auto* mem = config.findChild("mem")) {
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		windowBase = mem->getAttributeValueAsInt("base", 0);
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		windowSize = mem->getAttributeValueAsInt("size", 0);
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	}
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	auto romSize_ = rom.size();
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	if ((romSize_ > 0x10000) || (romSize_ & 0x1FFF)) {
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		throw MSXException(rom.getName(),
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			": invalid rom size: must be smaller than or equal to 64kB "
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			"and must be a multiple of 8kB.");
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	}
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	auto romSize = narrow<unsigned>(romSize_);
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	using enum RomType;
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	const int start = [&] {
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		switch (type) {
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			case MIRRORED: return -1;
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			case NORMAL:   return -1;
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			case MIRRORED0000: return 0x0000;
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			case MIRRORED4000: return 0x4000;
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			case MIRRORED8000: return 0x8000;
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			case MIRROREDC000: return 0xC000;
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			case NORMAL0000: return 0x0000;
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			case NORMAL4000: return 0x4000;
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			case NORMAL8000: return 0x8000;
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			case NORMALC000: return 0xC000;
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			default: UNREACHABLE;
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		}
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	}();
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	const bool mirrored = type == one_of(MIRRORED,
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	                                     MIRRORED0000, MIRRORED4000,
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	                                     MIRRORED8000, MIRROREDC000);
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	unsigned romBase = (start == -1)
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	                 ? guessLocation(windowBase, windowSize)
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	                 : unsigned(start);
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	if ((start == -1) &&
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	    (!isInside(romBase,               windowBase, windowSize) ||
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	     !isInside(romBase + romSize - 1, windowBase, windowSize))) {
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		// ROM must fall inside the boundaries given by the <mem>
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		// tag (this code only looks at one <mem> tag), but only
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		// check when the start address was not explicitly specified
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		throw MSXException(rom.getName(),
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			": invalid rom position: interval ",
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			toString(romBase, romSize), " must fit in ",
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			toString(windowBase, windowSize), '.');
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	}
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	if (romBase & 0x1FFF) {
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		throw MSXException(rom.getName(),
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			": invalid rom position: must start at a 8kB boundary.");
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	}
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	unsigned firstPage = romBase / 0x2000;
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	unsigned numPages = romSize / 0x2000;
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	for (auto page : xrange(8)) {
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		unsigned romPage = page - firstPage;
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		if (romPage < numPages) {
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			setRom(page, romPage);
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		} else {
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			if (mirrored) {
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				setRom(page, romPage & (numPages - 1));
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			} else {
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				setUnmapped(page);
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			}
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		}
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	}
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	// RomPlain is currently implemented as a subclass of Rom8kBBlocks,
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	// because of that it inherits a 8kB alignment requirement on the
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	// base/size attributes in the <mem> tag in the hardware description.
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	// But for example for the 'Boosted_audio' extension this requirement
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	// is too strict. Except for the calls to setRom() and setUnmapped() in
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	// this constructor, RomPlain doesn't dynamically change the memory
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	// layout. So if we undo the cache-pre-filling stuff done by setRom()
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	// it is OK to relax the alignment requirements again.
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	//
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	// An alternative is to not inherit from Rom8kBBlocks but from MSXRom.
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	// That would simplify the alignment stuff, but OTOH then we have to
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	// reimplement the debuggable stuff. Also the format of savestates will
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	// change (so it would require extra backwards compatibility code).
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	// Therefor, at least for now, I've not chosen this alternative.
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	invalidateDeviceRCache();
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}
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void RomPlain::guessHelper(unsigned offset, std::span<int, 3> pages) const
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{
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	if ((rom[offset + 0] == 'A') && (rom[offset + 1] == 'B')) {
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		for (auto i : xrange(4)) {
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			if (auto addr = rom[offset + 2 + 2 * i + 0] +
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			                rom[offset + 2 + 2 * i + 1] * 256) {
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				unsigned page = (addr >> 14) - (offset >> 14);
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				if (page <= 2) {
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					pages[page]++;
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				}
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			}
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		}
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	}
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}
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unsigned RomPlain::guessLocation(unsigned windowBase, unsigned windowSize) const
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{
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	std::array<int, 3> pages = {0, 0, 0};
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	// count number of possible routine pointers
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	if (rom.size() >= 0x0010) {
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		guessHelper(0x0000, pages);
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	}
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	if (rom.size() >= 0x4010) {
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		guessHelper(0x4000, pages);
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	}
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	// start address must be inside memory window
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	if (!isInside(0x0000, windowBase, windowSize)) pages[0] = 0;
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	if (!isInside(0x4000, windowBase, windowSize)) pages[1] = 0;
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	if (!isInside(0x8000, windowBase, windowSize)) pages[2] = 0;
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	// we prefer 0x4000, then 0x000 and then 0x8000
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	if (pages[1] && (pages[1] >= pages[0]) && (pages[1] >= pages[2])) {
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		return 0x4000;
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	} else if (pages[0] && pages[0] >= pages[2]) {
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		return 0x0000;
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	} else if (pages[2]) {
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		return 0x8000;
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	}
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	// heuristics didn't work, return start of window
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	return windowBase;
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}
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unsigned RomPlain::getBaseSizeAlignment() const
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{
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	// Because this mapper has no switchable banks, we can relax the
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	// alignment requirements again. See also the comment at the end of the
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	// constructor.
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	return MSXRom::getBaseSizeAlignment();
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}
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REGISTER_MSXDEVICE(RomPlain, "RomPlain");
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} // namespace openmsx