Pu region sizing/bounds fix (melonDS-emu#2024)

* fix the pu region's end point overflowing

According to gericom it cannot overflow at all

* set a minimum and a better maximum for the pu region size

* fix pu logging

* PU regions with a size of 31 always take up the entire address space

also tweak some logging a little more

* start is actually force aligned by size, oops

* small tweaks

* hopefully more clear code

* math is for nerds

src/CP15.cpp

@@ -186,10 +186,14 @@ void ARMv5::UpdatePURegion(u32 n)
         return;
     }
 
-    u32 start = rgn >> 12;
-    u32 sz = 2 << ((rgn >> 1) & 0x1F);
-    u32 end = start + (sz >> 12);
-    // TODO: check alignment of start
+    // notes:
+    // * min size of a pu region is 4KiB (12 bits)
+    // * size is calculated as size + 1, but the 12 lsb of address space are ignored, therefore we need it as size + 1 - 12, or size - 11
+    // * pu regions are aligned based on their size
+    u32 size = std::max((int)((rgn>>1) & 0x1F) - 11, 0); // obtain the size, subtract 11 and clamp to a min of 0.
+    u32 start = ((rgn >> 12) >> size) << size; // determine the start offset, and use shifts to force alignment with a multiple of the size.
+    u32 end = start + (1<<size); // add 1 left shifted by size to start to determine end point
+    // dont need to bounds check the end point because the force alignment inherently prevents it from breaking
 
     u8 usermask = 0;
     u8 privmask = 0;
@@ -239,7 +243,7 @@ void ARMv5::UpdatePURegion(u32 n)
         "PU region %d: %08X-%08X, user=%02X priv=%02X, %08X/%08X\n",
         n,
         start << 12,
-        end << 12,
+        (end << 12) - 1,
         usermask,
         privmask,
         PU_DataRW,
@@ -579,12 +583,12 @@ void ARMv5::CP15Write(u32 id, u32 val)
 
         std::snprintf(log_output,
                  sizeof(log_output),
-                 "PU: region %d = %08X : %s, %08X-%08X\n",
+                 "PU: region %d = %08X : %s, start: %08X size: %02X\n",
                  (id >> 4) & 0xF,
                  val,
                  val & 1 ? "enabled" : "disabled",
                  val & 0xFFFFF000,
-                 (val & 0xFFFFF000) + (2 << ((val & 0x3E) >> 1))
+                 (val & 0x3E) >> 1
         );
         Log(LogLevel::Debug, "%s", log_output);
         // Some implementations of Log imply a newline, so we build up the line before printing it