From ee742e6020248f9695cc9ce5bbace5f42814383e Mon Sep 17 00:00:00 2001
From: alekseiplusplus <alekseijeaves@protonmail.com>
Date: Fri, 21 Apr 2023 03:44:47 +1000
Subject: have to call it a night.

---
 headers/applesystem.h | 113 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 113 insertions(+)
 create mode 100644 headers/applesystem.h

(limited to 'headers/applesystem.h')

diff --git a/headers/applesystem.h b/headers/applesystem.h
new file mode 100644
index 0000000..6b2f818
--- /dev/null
+++ b/headers/applesystem.h
@@ -0,0 +1,113 @@
+//	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);
+}*/
+
+
+
+
+
+
+
-- 
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