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◆ main openmsx / src / video / DisplayMode.hh
5.9 KiB · C++ · last changed chore(): initial commit 1 month ago source blame history raw
e8555c89 Andy McCall 1 
#ifndef DISPLAYMODE_HH
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#define DISPLAYMODE_HH
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#include "one_of.hh"
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#include <cstdint>
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namespace openmsx {
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/** Represents a VDP display mode.
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  * A display mode determines how bytes in the VRAM are converted to pixel
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  * colors.
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  * A display mode consists of a base mode with YJK filters on top.
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  * Only the V9958 supports YJK filters.
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  */
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class DisplayMode
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{
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private:
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	constexpr explicit DisplayMode(uint8_t mode_) : mode(mode_) {}
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	/** Display mode flags: YAE YJK M5..M1.
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	  * The YAE flag indicates whether YAE is active, not just the value of
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	  * the corresponding mode bit; so if YJK is 0, YAE is 0 as well.
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	  */
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	uint8_t mode = 0;
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public:
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	static constexpr uint8_t GRAPHIC1   = 0x00; // Graphic 1
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	static constexpr uint8_t TEXT1      = 0x01; // Text 1
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	static constexpr uint8_t MULTICOLOR = 0x02; // Multicolor
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	static constexpr uint8_t GRAPHIC2   = 0x04; // Graphic 2
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	static constexpr uint8_t TEXT1Q     = 0x05; // !!
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	static constexpr uint8_t MULTIQ     = 0x06; // !!
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	static constexpr uint8_t GRAPHIC3   = 0x08; // Graphic 3
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	static constexpr uint8_t TEXT2      = 0x09; // Text 2
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	static constexpr uint8_t GRAPHIC4   = 0x0C; // Graphic 4
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	static constexpr uint8_t GRAPHIC5   = 0x10; // Graphic 5
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	static constexpr uint8_t GRAPHIC6   = 0x14; // Graphic 6
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	static constexpr uint8_t GRAPHIC7   = 0x1C; // Graphic 7
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	/** Bits of VDP register 0 that encode part of the display mode. */
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	static constexpr uint8_t REG0_MASK = 0x0E;
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	/** Bits of VDP register 1 that encode part of the display mode. */
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	static constexpr uint8_t REG1_MASK = 0x18;
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	/** Bits of VDP register 25 that encode part of the display mode. */
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	static constexpr uint8_t REG25_MASK = 0x18;
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	/** Encoding of YJK flag. */
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	static constexpr uint8_t YJK = 0x20;
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	/** Encoding of YAE flag. */
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	static constexpr uint8_t YAE = 0x40;
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	/** Create the initial display mode.
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	  */
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	constexpr DisplayMode() = default;
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	/** Create a specific display mode.
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	  * @param reg0 The contents of VDP register 0.
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	  * @param reg1 The contents of VDP register 1.
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	  * @param reg25 The contents of VDP register 25;
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	  *     on non-V9958 chips, pass 0.
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	  */
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	constexpr DisplayMode(uint8_t reg0, uint8_t reg1, uint8_t reg25) {
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		if ((reg25 & 0x08) == 0) reg25 = 0; // If YJK is off, ignore YAE.
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		mode = uint8_t(
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			((reg25 & 0x18) << 2) | // YAE YJK
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			((reg0  & 0x0E) << 1) | // M5..M3
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			((reg1  & 0x08) >> 2) | // M2
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			((reg1  & 0x10) >> 4)); // M1
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	}
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	[[nodiscard]] constexpr DisplayMode updateReg25(uint8_t reg25) const {
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		if ((reg25 & 0x08) == 0) reg25 = 0; // If YJK is off, ignore YAE.
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		return DisplayMode(uint8_t(getBase() | ((reg25 & 0x18) << 2)));
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	}
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	/** Bring the display mode to its initial state.
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	  */
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	constexpr void reset() {
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		*this = DisplayMode();
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	}
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	/** Equals operator.
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	  */
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	[[nodiscard]] constexpr bool operator==(const DisplayMode&) const = default;
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	/** Get the display mode as a byte: YAE YJK M5..M1 combined.
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	  * @return The byte representation of this display mode,
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	  *     in the range [0..0x7F].
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	  */
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	[[nodiscard]] constexpr uint8_t getByte() const {
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		return mode;
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	}
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	/** Used for de-serialization. */
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	constexpr void setByte(uint8_t mode_) {
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		mode = mode_;
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	}
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	/** Get the base display mode as an integer: M5..M1 combined.
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	  * If YJK is active, the base mode is the underlying display mode.
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	  * @return The integer representation of the base of this display mode,
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	  *     in the range [0..0x1F].
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	  */
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	[[nodiscard]] constexpr uint8_t getBase() const {
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		return mode & 0x1F;
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	}
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	/** Was this mode introduced by the V9938?
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	  * @return True iff the base of this mode only is available on V9938/58.
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	  */
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	[[nodiscard]] constexpr bool isV9938Mode() const {
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		return (mode & 0x18) != 0;
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	}
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	/** Is the current mode a text mode?
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	  * Text1 and Text2 are text modes.
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	  * @return True iff the current mode is a text mode.
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	  */
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	[[nodiscard]] constexpr bool isTextMode() const {
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		return getBase() == one_of(TEXT1, TEXT2, TEXT1Q);
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	}
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	/** Is the current mode a bitmap mode?
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	  * Graphic4 and higher are bitmap modes.
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	  * @return True iff the current mode is a bitmap mode.
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	  */
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	[[nodiscard]] constexpr bool isBitmapMode() const {
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		return getBase() >= 0x0C;
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	}
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	/** Is VRAM "planar" in the current display mode?
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	  * Graphic 6 and 7 spread their bytes over two VRAM ICs,
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	  * such that the even bytes go to the first half of the address
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	  * space and the odd bytes to the second half.
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	  * @return True iff the current display mode has planar VRAM.
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	  */
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	[[nodiscard]] constexpr bool isPlanar() const {
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		// TODO: Is the display mode check OK? Profile undefined modes.
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		return (mode & 0x14) == 0x14;
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	}
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	/** Are sprite pixels narrow?
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	  */
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	[[nodiscard]] constexpr bool isSpriteNarrow() const {
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		// TODO: Check what happens to sprites in Graphic5 + YJK/YAE.
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		return mode == GRAPHIC5;
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	}
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	/** Get the sprite mode of this display mode.
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	  * @return The current sprite mode:
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	  *     0 means no sprites,
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	  *     1 means sprite mode 1 (MSX1 display modes),
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	  *     2 means sprite mode 2 (MSX2 display modes).
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	  */
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	[[nodiscard]] constexpr int getSpriteMode(bool isMSX1) const {
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		switch (getBase()) {
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		case GRAPHIC1: case MULTICOLOR: case GRAPHIC2:
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			return 1;
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		case MULTIQ: // depends on VDP type
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			return isMSX1 ? 1 : 0;
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		case GRAPHIC3: case GRAPHIC4: case GRAPHIC5:
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		case GRAPHIC6: case GRAPHIC7:
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			return 2;
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		case TEXT1: case TEXT1Q: case TEXT2:
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		default: // and all other (bogus) modes
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			// Verified on real V9958: none of the bogus modes
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			// show sprites.
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			// TODO check on TMSxxxx
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			return 0;
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		}
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	}
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	/** Get number of pixels on a display line in this mode.
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	  * @return 512 for Text 2, Graphic 5 and 6, 256 for all other modes.
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	  * TODO: Would it make more sense to treat Text 2 as 480 pixels?
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	  */
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	[[nodiscard]] constexpr unsigned getLineWidth() const {
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		// Note: Testing "mode" instead of "base mode" ensures that YJK
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		//       modes are treated as 256 pixels wide.
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		return mode == one_of(TEXT2, GRAPHIC5, GRAPHIC6)
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			? 512
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			: 256;
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	}
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};
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} // namespace openmsx
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#endif