//	applesystem.h
//	Core elements of the 6502 CPU, and flag manipulation.

typedef unsigned char byte;
typedef unsigned short address;
byte acc, X, Y, P, S = 0x00;
address PC = 0x0000;
byte Memory[4096]; // TO DO. Add expansion capability to memory.

//	FLAGS
const byte flag_N = 0x80;	// Negative
const byte flag_V = 0x40;	// Overflow
const byte flag_B = 0x10;	// BRK command
const byte flag_D = 0x08;	// Decimal mode
const byte flag_I = 0x04;	// IRQ disable
const byte flag_Z = 0x02;	// Zero
const byte flag_C = 0x01;	// Carry

byte getFlag(byte flag) {
	return ((P & flag) == flag) ? 1 : 0;
}

void setFlag(byte flag, int x) { //OVERLOAD TO ACCEPT INT AS WELL
	if (x == 1){
		if ((P & flag) == 0x0)	P += flag;
	}else if (x == 0){
		if ((P & flag) == flag)	P -= flag;
	}
	else{
		fprintf(stderr, "setFlag() passed arg neither 0 or 1");
	}
}

void flagSet(byte flag){
	P |= flag;
}

void flagClear(byte flag){
	P &= ~flag;
}


//	BCD
// need to actually look into BCD on the 6502
byte toBCD(byte x){
	if (x < 100){
		byte a = ((x / 10) << 4);
		byte b = (x % 10);
		return (a + b);
	}
	else{
		fprintf(stderr, "Number greater than 99 passed to toBCD()");
	}
}

byte fromBCD(byte x){
	byte a = ((x >> 4) * 10);
	byte b = (x & 0xF);
	return (a + b);
}


//	Functions which perform reusable routines for finding if a specific flag should be set.

void setFlagN(byte x){
	if (x & flag_N == flag_N)
		setFlag(flag_N, 1);
	else
		setFlag(flag_N, 0);
}

//Perform prior to any changes
void setFlagV(byte x, byte y){
	if ((x & flag_N) == (y & flag_N)){
		if (((x + y) & (flag_N ^ 0xFF)) > 0x7F) setFlag(flag_V, 1);
		else setFlag(flag_V, 0);
	}else{
		if (((x - y) & (flag_N ^ 0xFF)) > 0x7F) setFlag(flag_V, 1);
		else setFlag(flag_V, 0);
	}
}

/*void setFlagB(){ //WORK ON
	setFlag(flag_B, 1);
}*/ //Dont really need since its dependent on the BRK insturction

/*void setFlagD(){
	setFlag(flag_D, 1);
}*/

/*void setFlagI(){ //WORK ON
	setFlag(flag_Z, 1);
}*/

void setFlagZ(int x){
	if (x == 0)
		setFlag(flag_Z, 1);
	else
		setFlag(flag_Z, 0);
}

//Only 6 instructions, 2 not including stack instructions, use the carry flag.
// Need to look further into implementation details for this.
/*void setFlagC(){
	setFlag(flag_Z, 1);
}*/