did i just fix math

itsvic-dev · May 3, 2024 · 3856f84 · 3856f84
1 parent af9d49f
commit 3856f84
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Showing 2 changed files with 12 additions and 9 deletions.
diff --git a/src/opcodes/arith.cpp b/src/opcodes/arith.cpp
@@ -6,16 +6,16 @@ OPCODE(adc) {
   if (FLAG_IS_SET(cpu->status, STATUS_DECIMAL)) {
     v6502Internal::logWarning("ADC: decimal mode is not supported yet");
   }
-  int16_t result = cpu->a + cpu->fetchByteWithMode(mode) +
-                   FLAG_IS_SET(cpu->status, STATUS_CARRY);
+  uint8_t m = cpu->fetchByteWithMode(mode);
+  int16_t result = cpu->a + m + FLAG_IS_SET(cpu->status, STATUS_CARRY);
 
   cpu->clearFlags(STATUS_CARRY | STATUS_OVERFLOW);
-  if ((result & 0x80) == 0x80 && (cpu->a & 0x80) != 0x80) {
-    cpu->setFlags(STATUS_OVERFLOW);
-  }
   if (result > 0xff) {
     cpu->setFlags(STATUS_CARRY);
   }
+  if ((m ^ result) & (cpu->a ^ result) & 0x80) {
+    cpu->setFlags(STATUS_OVERFLOW);
+  }
 
   cpu->a = result & 0xFF;
   cpu->evaluateFlags(cpu->a);
@@ -55,14 +55,14 @@ OPCODE(sbc) {
   if (FLAG_IS_SET(cpu->status, STATUS_DECIMAL)) {
     v6502Internal::logWarning("SBC: decimal mode is not supported yet");
   }
-  int16_t result = cpu->a - cpu->fetchByteWithMode(mode) -
-                   !FLAG_IS_SET(cpu->status, STATUS_CARRY);
+  uint8_t m = cpu->fetchByteWithMode(mode);
+  int16_t result = cpu->a - m - !FLAG_IS_SET(cpu->status, STATUS_CARRY);
 
   cpu->clearFlags(STATUS_CARRY | STATUS_OVERFLOW);
   if (result >= 0) {
     cpu->setFlags(STATUS_CARRY);
   }
-  if ((result & 0x80) != 0x80 && (cpu->a & 0x80) == 0x80) {
+  if ((~m ^ result) & (cpu->a ^ result) & 0x80) {
     cpu->setFlags(STATUS_OVERFLOW);
   }
 

diff --git a/tests/instructions/arith/adc-1-7f.cpp b/tests/instructions/arith/adc-1-7f.cpp
@@ -11,5 +11,8 @@ TEST {
   cpu->executeStep();
 
   assert(cpu->a == 0x80);
-  STATUS_ASSERT(STATUS_NEGATIVE | STATUS_OVERFLOW);
+  assert(FLAG_IS_SET(cpu->status, STATUS_NEGATIVE));
+  assert(FLAG_IS_SET(cpu->status, STATUS_OVERFLOW));
+  assert(FLAG_IS_CLEAR(cpu->status, STATUS_ZERO));
+  assert(FLAG_IS_CLEAR(cpu->status, STATUS_CARRY));
 }