2 files changed, 54 insertions, 1 deletions
diff --git a/src/main/kotlin/cpu/Registers.kt b/src/main/kotlin/cpu/Registers.kt
index 70c7f06..54539a8 100644
--- a/src/main/kotlin/cpu/Registers.kt
+++ b/src/main/kotlin/cpu/Registers.kt
@@ -131,4 +131,36 @@ class Registers {
     return currentSP
   }
 
+  fun getFlag(flag: Flag): Int {
+    return (F shr flag.bitPosition) and 0x01
+  }
+
+  fun setFlag(flag: Flag) {
+    F = F or (1 shl flag.bitPosition)
+  }
+
+  fun clearFlag(flag: Flag) {
+    F = F and (1 shl flag.bitPosition).inv()
+  }
+
+  fun setFlagFromBool(flag: Flag, isSet: Boolean) {
+    if(isSet) {
+      setFlag(flag)
+    }
+    else {
+      clearFlag(flag)
+    }
+  }
+
+  fun clearFlags() {
+    F = 0x00
+  }
+
+  enum class Flag(val bitPosition: Int) {
+    ZERO(7),
+    SUBTRACT(6),
+    HALF_CARRY(5),
+    CARRY(4),
+  }
+
 }
 \ No newline at end of file
diff --git a/src/main/kotlin/cpu/opcodes/Loads16Bit.kt b/src/main/kotlin/cpu/opcodes/Loads16Bit.kt
index 671a02d..830f6ab 100644
--- a/src/main/kotlin/cpu/opcodes/Loads16Bit.kt
+++ b/src/main/kotlin/cpu/opcodes/Loads16Bit.kt
@@ -2,6 +2,7 @@ package cpu.opcodes
 
 import cpu.Operation
 import cpu.Registers
+import cpu.Registers.Flag
 import ram.Ram
 import BitManipulation as bm
 
@@ -15,7 +16,27 @@ var loads16Bit = mapOf(
 
   0xF9 to Operation("LD SP,HL", 0, 8, {r, _, _ -> r.SP = r.HL}),
 
-  0xF8 to Operation("LDHL SP,n", 1, 12, {r, _, a -> r.HL = (r.SP + a[0])}), // TODO - not sure what the flags should be here
+  0xF8 to Operation("LDHL SP,n", 1, 12, {r, _, a ->
+
+    // https://stackoverflow.com/questions/5159603/gbz80-how-does-ld-hl-spe-affect-h-and-c-flags
+    // the carry flag is set if there's an overflow from the 7th to 8th bit.
+    // the half carry flag is set if there's an overflow from the 3rd into the 4th bit.
+    r.clearFlags()
+
+    val absoluteValue = bm.getAbsoluteValue(a[0])
+
+    if (bm.isSignedBitNegative(a[0])) {
+      r.setFlagFromBool(Flag.HALF_CARRY, r.SP and 0x0F < absoluteValue and 0x0F)
+      r.setFlagFromBool(Flag.CARRY,r.SP and 0xFF < absoluteValue)
+      r.HL = r.SP - absoluteValue
+
+    } else {
+      r.setFlagFromBool(Flag.HALF_CARRY,(r.SP and 0x0F) + (absoluteValue and 0x0F) > 0x0F)
+      r.setFlagFromBool(Flag.CARRY,(r.SP and 0xFF) + absoluteValue > 0xFF)
+      r.HL = r.SP + absoluteValue
+    }
+
+  }),
   0x08 to Operation("LD (nn),SP", 2, 20, {r, m, a ->
     m.writeByte(bm.argsToWord(a), bm.getLsb(r.SP))
     m.writeByte(bm.argsToWord(a) + 1, bm.getMsb(r.SP))