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diff --git a/Pico/cd/cell_map.c b/Pico/cd/cell_map.c
new file mode 100644
index 0000000..5358a09
--- /dev/null
+++ b/Pico/cd/cell_map.c
@@ -0,0 +1,38 @@
+
+// 64 x32 x16 x8 x4 x4
+static unsigned int cell_map(int celln)
+{
+  int col, row;
+
+  switch ((celln >> 12) & 7) { // 0-0x8000
+    case 0: // x32 cells
+    case 1:
+    case 2:
+    case 3:
+      col = celln >> 8;
+      row = celln & 0xff;
+      break;
+    case 4: // x16
+    case 5:
+      col  = celln >> 7;
+      row  = celln & 0x7f;
+      row |= 0x10000 >> 8;
+      break;
+    case 6: // x8
+      col  = celln >> 6;
+      row  = celln & 0x3f;
+      row |= 0x18000 >> 8;
+      break;
+    case 7: // x4
+      col  = celln >> 5;
+      row  = celln & 0x1f;
+      row |= (celln & 0x7800) >> 6;
+      break;
+    default: // just to make compiler happy
+      col = row = 0;
+      break;
+  }
+
+  return (col & 0x3f) + row*64;
+}
+