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// 6502.h
// Core elements of the 6502 CPU
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
#define flag_N 0x80 // Negative
#define flag_V 0x40 // Overflow
#define flag_B 0x10 // BRK command
#define flag_D 0x08 // Decimal mode
#define flag_I 0x04 // IRQ disable
#define flag_Z 0x02 // Zero
#define flag_C 0x01 // Carry
byte getFlag(byte flag) {
return ((P & flag) == flag) ? 1 : 0;
}
void setFlag(byte flag, int x) {
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);
}*/
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