1 files changed, 55 insertions, 15 deletions

diff --git a/cpu.c b/cpu.c
index 076e887..65e60d4 100644
--- a/cpu.c
+++ b/cpu.c
@@ -67,7 +67,7 @@ peek16(uint16_t addr)
 static void
 memwrite(uint16_t addr, uint8_t byte)
 {
-        memory[addr] = b;
+        memory[addr] = byte;
 }
 
 static void
@@ -125,6 +125,50 @@ opcode_arg(enum addressing_mode mode)
         return val;
 }
 
+static uint16_t
+opcode_mem(enum addressing_mode mode)
+{
+        uint8_t arg;
+        uint16_t val;
+
+        if (mode != AM_ABS && mode != AM_ABS_X && mode != AM_ABS_Y)
+                arg = peek(regs.pc++);
+        else
+                arg = peek16(regs.pc), regs.pc += 2;
+
+        switch (mode) {
+        case AM_ZP:
+                val = arg % 256;
+                break;
+        case AM_ZP_X:
+                val = (arg + regs.x) % 256;
+                break;
+        case AM_ZP_Y:
+                val = (arg + regs.y) % 256;
+                break;
+        case AM_ABS:
+                val = arg;
+                break;
+        case AM_ABS_X:
+                val = arg + regs.x;
+                break;
+        case AM_ABS_Y:
+                val = arg + regs.y;
+                break;
+        case AM_IND_X:
+                val = peek((arg + regs.x) % 256) + peek((arg + regs.x + 1) % 256) * 256;
+                break;
+        case AM_IND_Y:
+                val = peek(arg) + peek((arg + 1) % 256) * 256 + regs.y;
+                break;
+        default:
+                fprintf(stderr, "INVALID ADDRESSING MODE\n");
+                abort();
+        }
+
+        return val;
+}
+
 static void
 adc(uint8_t arg)
 {
@@ -313,9 +357,9 @@ cpy(uint8_t arg)
 }
 
 static void
-dec(uint8_t arg)
+dec(uint16_t mem)
 {
-/* TODO: complete this */
+        memwrite(mem, peek(mem) - 1);
 }
 
 static void
@@ -839,19 +883,19 @@ interpret(void)
                         cycles += 2;
                         break;
                 case 0x18:
-                        clc(opcode_arg(AM_ACC));
+                        clc();
                         cycles += 2;
                         break;
                 case 0xd8:
-                        cld(opcode_arg(AM_ACC));
+                        cld();
                         cycles += 2;
                         break;
                 case 0x58:
-                        cli(opcode_arg(AM_ACC));
+                        cli();
                         cycles += 2;
                         break;
                 case 0xb8:
-                        clv(opcode_arg(AM_ACC));
+                        clv();
                         cycles += 2;
                         break;
                 case 0xc9:
@@ -914,24 +958,20 @@ interpret(void)
                         cpy(opcode_arg(AM_ABS));
                         cycles += 4;
                         break;
-                case 0x3a:
-                        dec(opcode_arg(AM_ACC));
-                        cycles += 2;
-                        break;
                 case 0xc6:
-                        dec(opcode_arg(AM_ZP));
+                        dec(opcode_mem(AM_ZP));
                         cycles += 5;
                         break;
                 case 0xd6:
-                        dec(opcode_arg(AM_ZP_X));
+                        dec(opcode_mem(AM_ZP_X));
                         cycles += 6;
                         break;
                 case 0xce:
-                        dec(opcode_arg(AM_ABS));
+                        dec(opcode_mem(AM_ABS));
                         cycles += 6;
                         break;
                 case 0xde:
-                        dec(opcode_arg(AM_ABS_X));
+                        dec(opcode_mem(AM_ABS_X));
                         cycles += 7;
                         break;
                 case 0xca: