RomPlain.cc

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#include "RomPlain.hh"
#include "XMLElement.hh"
#include "MSXException.hh"
#include "one_of.hh"
#include "serialize.hh"
#include "unreachable.hh"
#include "xrange.hh"
#include <array>
 
namespace openmsx {
 
// return x inside [start, start+len)
[[nodiscard]] static constexpr bool isInside(unsigned x, unsigned start, unsigned len)
{
        unsigned tmp = x - start;
        return tmp < len;
}
 
[[nodiscard]] static std::string toString(unsigned start, unsigned len)
{
        return strCat("[0x", hex_string<4>(start), ", "
                       "0x", hex_string<4>(start + len), ')');
}
 
RomPlain::RomPlain(const DeviceConfig& config, Rom&& rom_, RomType type)
        : Rom8kBBlocks(config, std::move(rom_))
{
        unsigned windowBase =  0x0000;
        unsigned windowSize = 0x10000;
        if (const auto* mem = config.findChild("mem")) {
                windowBase = mem->getAttributeValueAsInt("base", 0);
                windowSize = mem->getAttributeValueAsInt("size", 0);
        }
 
        auto romSize_ = rom.size();
        if ((romSize_ > 0x10000) || (romSize_ & 0x1FFF)) {
                throw MSXException(rom.getName(),
                        ": invalid rom size: must be smaller than or equal to 64kB "
                        "and must be a multiple of 8kB.");
        }
        auto romSize = narrow<unsigned>(romSize_);
 
        const int start = [&] {
                switch (type) {
                        case ROM_MIRRORED: return -1;
                        case ROM_NORMAL:   return -1;
                        case ROM_MIRRORED0000: return 0x0000;
                        case ROM_MIRRORED4000: return 0x4000;
                        case ROM_MIRRORED8000: return 0x8000;
                        case ROM_MIRROREDC000: return 0xC000;
                        case ROM_NORMAL0000: return 0x0000;
                        case ROM_NORMAL4000: return 0x4000;
                        case ROM_NORMAL8000: return 0x8000;
                        case ROM_NORMALC000: return 0xC000;
                        default: UNREACHABLE; return -1;
                }
        }();
        const bool mirrored = type == one_of(ROM_MIRRORED,
                                             ROM_MIRRORED0000, ROM_MIRRORED4000,
                                             ROM_MIRRORED8000, ROM_MIRROREDC000);
 
        unsigned romBase = (start == -1)
                         ? guessLocation(windowBase, windowSize)
                         : unsigned(start);
        if ((start == -1) &&
            (!isInside(romBase,               windowBase, windowSize) ||
             !isInside(romBase + romSize - 1, windowBase, windowSize))) {
                // ROM must fall inside the boundaries given by the <mem>
                // tag (this code only looks at one <mem> tag), but only
                // check when the start address was not explicitly specified
                throw MSXException(rom.getName(),
                        ": invalid rom position: interval ",
                        toString(romBase, romSize), " must fit in ",
                        toString(windowBase, windowSize), '.');
        }
        if ((romBase & 0x1FFF)) {
                throw MSXException(rom.getName(),
                        ": invalid rom position: must start at a 8kB boundary.");
        }
 
        unsigned firstPage = romBase / 0x2000;
        unsigned numPages = romSize / 0x2000;
        for (auto page : xrange(8)) {
                unsigned romPage = page - firstPage;
                if (romPage < numPages) {
                        setRom(page, romPage);
                } else {
                        if (mirrored) {
                                setRom(page, romPage & (numPages - 1));
                        } else {
                                setUnmapped(page);
                        }
                }
        }
        // RomPlain is currently implemented as a subclass of Rom8kBBlocks,
        // because of that it inherits a 8kB alignment requirement on the
        // base/size attributes in the <mem> tag in the hardware description.
        // But for example for the 'Boosted_audio' extension this requirement
        // is too strict. Except for the calls to setRom() and setUnmapped() in
        // this constructor, RomPlain doesn't dynamically change the memory
        // layout. So if we undo the cache-pre-filling stuff done by setRom()
        // it is OK to relax the alignment requirements again.
        //
        // An alternative is to not inherit from Rom8kBBlocks but from MSXRom.
        // That would simplify the alignment stuff, but OTOH then we have to
        // reimplement the debuggable stuff. Also the format of savestates will
        // change (so it would require extra backwards compatibility code).
        // Therefor, at least for now, I've not chosen this alternative.
        invalidateDeviceRCache();
}
 
void RomPlain::guessHelper(unsigned offset, std::span<int, 3> pages)
{
        if ((rom[offset++] == 'A') && (rom[offset++] =='B')) {
                for (auto i : xrange(4)) {
                        if (auto addr = rom[offset + 2 * i + 0] +
                                        rom[offset + 2 * i + 1] * 256) {
                                unsigned page = (addr >> 14) - (offset >> 14);
                                if (page <= 2) {
                                        pages[page]++;
                                }
                        }
                }
        }
}
 
unsigned RomPlain::guessLocation(unsigned windowBase, unsigned windowSize)
{
        std::array<int, 3> pages = {0, 0, 0};
 
        // count number of possible routine pointers
        if (rom.size() >= 0x0010) {
                guessHelper(0x0000, pages);
        }
        if (rom.size() >= 0x4010) {
                guessHelper(0x4000, pages);
        }
 
        // start address must be inside memory window
        if (!isInside(0x0000, windowBase, windowSize)) pages[0] = 0;
        if (!isInside(0x4000, windowBase, windowSize)) pages[1] = 0;
        if (!isInside(0x8000, windowBase, windowSize)) pages[2] = 0;
 
        // we prefer 0x4000, then 0x000 and then 0x8000
        if (pages[1] && (pages[1] >= pages[0]) && (pages[1] >= pages[2])) {
                return 0x4000;
        } else if (pages[0] && pages[0] >= pages[2]) {
                return 0x0000;
        } else if (pages[2]) {
                return 0x8000;
        }
 
        // heuristics didn't work, return start of window
        return windowBase;
}
 
unsigned RomPlain::getBaseSizeAlignment() const
{
        // Because this mapper has no switchable banks, we can relax the
        // alignment requirements again. See also the comment at the end of the
        // constructor.
        return MSXRom::getBaseSizeAlignment();
}
 
REGISTER_MSXDEVICE(RomPlain, "RomPlain");
 
} // namespace openmsx