From 50cac936b9b7ee2d02deb82cdbcd440ce712fc39 Mon Sep 17 00:00:00 2001
From: MadMaurice <madmaurice@zom.bi>
Date: Tue, 29 Aug 2023 21:00:37 +0200
Subject: [PATCH] cpu/decoder - Implement PREFIX (bit shift & bit ops)

---
 cpu/decoder.cpp | 98 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 98 insertions(+)

diff --git a/cpu/decoder.cpp b/cpu/decoder.cpp
index 9898da0..b9b84b3 100644
--- a/cpu/decoder.cpp
+++ b/cpu/decoder.cpp
@@ -181,6 +181,104 @@ void Cpu::executeInstruction()
       u16 rst_addr = op & 0x38;
       doCall(rst_addr);
     }
+  else if(op == 0xCB) // PREFIX
+    {
+      currentpc = state.PC;
+      opcode_t prefix_op = readPC8();
+
+#if 0
+      printf("@0x%04x: CB opcode %02X\n", currentpc, prefix_op);
+#endif
+
+      u8 reg = prefix_op & 0x7;
+
+      u8 data;
+      switch(reg)
+        {
+        case 0x6: data = bus->read8(state.HL); mcycles = 3; break;
+        default: data = state.reg8(reg); mcycles = 2; break;
+        }
+
+      // For BIT, RES, SET
+      u8 bit = (prefix_op >> 3) & 0x7;
+
+      switch(prefix_op & 0xC0)
+        {
+        case 0x00:
+          switch(prefix_op & 0xF1)
+            {
+            case 0x00: // RLC
+              {
+                bool msb_set = (data & 0x80);
+                data = (data << 1) | (msb_set ? 0x1 : 0x0);
+                state.carry = msb_set;
+              }
+              break;
+            case 0x08: // RRC
+              {
+                bool lsb_set = (data & 0x01);
+                data = (data >> 1) | (lsb_set ? 0x80 : 0x00);
+                state.carry = lsb_set;
+              }
+              break;
+            case 0x10: // RL
+              {
+                bool msb_set = (data & 0x80);
+                data = (data << 1) | (state.carry ? 0x1 : 0x0);
+                state.carry = msb_set;
+              }
+              break;
+            case 0x18: // RR
+              {
+                bool lsb_set = (data & 0x01) != 0;
+                data = (data >> 1) | (state.carry ? 0x80 : 0x00);
+                state.carry = lsb_set;
+              }
+              break;
+            case 0x20: // SLA
+              state.carry = (data & 0x80);
+              data = (data << 1);
+              break;
+            case 0x28: // SRA
+              state.carry = (data & 0x01);
+              data = (data >> 1) | (data & 0x80);
+              break;
+            case 0x30: // SWAP
+              data = ((data >> 4) & 0x0F) | ((data << 4) & 0xF0);
+              break;
+            case 0x38: // SRL
+              state.carry = (data & 0x01);
+              data = (data >> 1);
+              break;
+            }
+          state.halfcarry = false;
+          state.subtract = false;
+          state.zero = (data == 0);
+          break;
+        case 0x40: // BIT
+          {
+            state.zero = (data & (1 << bit)) == 0;
+            state.subtract = false;
+            state.halfcarry = true;
+          }
+          break;
+        case 0x80: // RES
+          data &= ~(1 << bit);
+          break;
+        case 0xC0: // SET
+          data |= (1 << bit);
+          break;
+        }
+      // All ops except for BIT write the data back to where it came from
+      if ((prefix_op & 0xC0) != 0x40)
+        {
+          switch(reg)
+            {
+            case 0x6: bus->write8(state.HL, data); mcycles = 4; break;
+            default: state.reg8(reg) = data; break;
+            }
+        }
+    }
   else
     {
       switch(op)