Initial cpu and instruction parsing

lib/cpu.c

@@ -1,10 +1,64 @@
 #include <cpu.h>
+#include <bus.h>
+
+cpu_context ctx = {0};
 
 void cpu_init() {
+    ctx.regs.pc = 0x100;
+}
 
+static void fetch_instruction() {
+    ctx.cur_opcode = bus_read(ctx.regs.pc++);
+    ctx.cur_inst = instruction_by_opcode(ctx.cur_opcode);
+    if(ctx.cur_inst == NULL){
+        printf("Unknown Instruction! %d\n", ctx.cur_opcode);
+        exit(-7);
+    }
+}
+
+static void fetch_data() {
+    ctx.mem_dest = 0;
+    ctx.dest_is_mem = false;
+
+    switch(ctx.cur_inst->mode) {
+        case AM_IMP: return;
+        case AM_R: 
+            ctx.fetched_data = cpu_read_reg(ctx.cur_inst->reg_1);
+            return;
+        case AM_R_D8:
+            ctx.fetched_data = bus_read(ctx.regs.pc);
+            emu_cycles(1);
+            ctx.regs.pc++;
+            return;
+        case AM_D16: {
+            u16 lo = bus_read(ctx.regs.pc);
+            emu_cycles(1);
+            u16 hi = bus_read(ctx.regs.pc+1);
+            emu_cycles(1);
+            ctx.fetched_data = lo | (hi << 8);
+            ctx.regs.pc += 2;
+            return;
+        }
+        default:
+            printf("Unknown Addressing Mode! %d\n", ctx.cur_inst->mode);
+            exit(-7);
+            return;
+    }
+}
+
+static void execute() {
+    printf("\tNot executing yet...\n");
 }
 
 bool cpu_step() {
-    printf("Cpu not yet implemented.\n");
-    return false;
+    
+    if(!ctx.halted) {
+        u16 pc = ctx.regs.pc;
+        fetch_instruction();
+        fetch_data();
+        printf("Executing Instruction: %02X     PC: %04X\n", ctx.cur_opcode, pc);
+        execute();
+    }
+
+    return true;
 }