gzdoom/game-music-emu/gme/Sap_Cpu.h

// Atari 6502 CPU emulator

// Game_Music_Emu https://bitbucket.org/mpyne/game-music-emu/
#ifndef SAP_CPU_H
#define SAP_CPU_H

#include "blargg_common.h"

typedef blargg_long sap_time_t; // clock cycle count
typedef unsigned sap_addr_t; // 16-bit address
enum { future_sap_time = INT_MAX / 2 + 1 };

class Sap_Cpu {
public:
	// Clear all registers and keep pointer to 64K memory passed in
	void reset( void* mem_64k );
	
	// Run until specified time is reached. Returns true if suspicious/unsupported
	// instruction was encountered at any point during run.
	bool run( sap_time_t end_time );
	
	// Registers are not updated until run() returns (except I flag in status)
	struct registers_t {
		uint16_t pc;
		uint8_t a;
		uint8_t x;
		uint8_t y;
		uint8_t status;
		uint8_t sp;
	};
	registers_t r;
	
	enum { idle_addr = 0xFEFF };
	
	// Time of beginning of next instruction to be executed
	sap_time_t time() const             { return state->time + state->base; }
	void set_time( sap_time_t t )       { state->time = t - state->base; }
	void adjust_time( int delta )       { state->time += delta; }
	
	sap_time_t irq_time() const         { return irq_time_; }
	void set_irq_time( sap_time_t );
	
	sap_time_t end_time() const         { return end_time_; }
	void set_end_time( sap_time_t );
	
public:
	Sap_Cpu() { state = &state_; }
	enum { irq_inhibit = 0x04 };
private:
	struct state_t {
		sap_time_t base;
		sap_time_t time;
	};
	state_t* state; // points to state_ or a local copy within run()
	state_t state_;
	sap_time_t irq_time_;
	sap_time_t end_time_;
	uint8_t* mem;
	
	inline sap_time_t update_end_time( sap_time_t end, sap_time_t irq );
};

inline sap_time_t Sap_Cpu::update_end_time( sap_time_t t, sap_time_t irq )
{
	if ( irq < t && !(r.status & irq_inhibit) ) t = irq;
	sap_time_t delta = state->base - t;
	state->base = t;
	return delta;
}

inline void Sap_Cpu::set_irq_time( sap_time_t t )
{
	state->time += update_end_time( end_time_, (irq_time_ = t) );
}

inline void Sap_Cpu::set_end_time( sap_time_t t )
{
	state->time += update_end_time( (end_time_ = t), irq_time_ );
}

#endif