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// license:BSD-3-Clause
// copyright-holders:Nigel Barnes
/***************************************************************************

    Gemini DataGem ROM Carrier

***************************************************************************/

#include "emu.h"
#include "datagem.h"


namespace {

class bbc_datagem_device : public device_t, public device_bbc_rom_interface
{
public:
	bbc_datagem_device(const machine_config &mconfig, const char *tag, device_t *owner, uint32_t clock)
		: device_t(mconfig, BBC_DATAGEM, tag, owner, clock)
		, device_bbc_rom_interface(mconfig, *this)
		, m_bank(0)
	{
	}

protected:
	// device_t overrides
	virtual void device_start() override ATTR_COLD;

	// device_bbc_rom_interface overrides
	virtual uint32_t get_rom_size() override { return 0x4000; }
	virtual uint8_t read(offs_t offset) override;
	virtual void decrypt_rom() override;

private:
	uint8_t m_bank;
};


//-------------------------------------------------
//  device_start - device-specific startup
//-------------------------------------------------

void bbc_datagem_device::device_start()
{
	save_item(NAME(m_bank));
}

//-------------------------------------------------
//  decrypt_rom
//-------------------------------------------------

void bbc_datagem_device::decrypt_rom()
{
	uint8_t *rom = get_rom_base();

	for (int i = 0x0000; i < 0x6000; i++)
	{
		// decrypt 16K and 8K ROM data lines
		switch (rom[i] & 0x07)
		{
		case 0x01: case 0x06: rom[i] = bitswap<8>(rom[i], 7, 6, 5, 4, 3, 0, 1, 2); break;
		case 0x02: case 0x05: rom[i] = bitswap<8>(rom[i], 7, 6, 5, 4, 3, 2, 0, 1); break;
		case 0x03: case 0x04: rom[i] = bitswap<8>(rom[i], 7, 6, 5, 4, 3, 1, 2, 0); break;
		}

		// decrypt additional 8K ROM data lines
		if (i & 0x4000)
		{
			rom[i] = bitswap<8>(rom[i], 3, 6, 5, 4, 7, 2, 1, 0);
		}
	}
}

//-------------------------------------------------
//  read
//-------------------------------------------------

uint8_t bbc_datagem_device::read(offs_t offset)
{
	if (!machine().side_effects_disabled())
	{
		// switching zones for DataGem
		switch (offset & 0x3fff)
		{
		case 0x1ffe: m_bank = 1; break;
		case 0x1fff: m_bank = 0; break;
		}
	}

	if (offset & 0x2000)
	{
		return get_rom_base()[(offset & 0x3fff) + (m_bank << 13)];
	}
	else
	{
		return get_rom_base()[offset & 0x3fff];
	}
}

} // anonymous namespace


DEFINE_DEVICE_TYPE_PRIVATE(BBC_DATAGEM, device_bbc_rom_interface, bbc_datagem_device, "bbc_datagem", "Gemini DataGem ROM Carrier")