Add Loongson document, compared with MIPS III and MIPS 64

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+Differences between Loongson 2, MIPS III and MIPS 64
+
+Last modified: 2007-09-17 by Songmao Tian <[email protected]>
+
+The following material is translated from Loongson 2F user manual,
+but holds true to Loongson 2E.
+
+1. CP0 $22 Diagnostic
+===========================
+Loongson:
+  tblwi/tlbwr doesn't affect ITLB, a write to first bit of $22 is
+needed to flush ITLB.
+MIPS III:
+  Use tlbwi/tblwr directly
+MIPS 64:
+  The same as MIPS III
+
+2. CP0 $24 $25
+===========================
+Loongson:
+  They are used for performance counter.
+MIPS III:
+  They are reserved in MIPS R4000, and $24 is reserved in R10000.
+MIPS 64:
+  $24 is defined as DEPC, used for EJTAG.
+
+3. CP0 $27
+===========================
+Loongson:
+  CacheErr($27) is reserved in Loonson. When ERL is set, ErrEPC doesn't
+stored return address and the first 512MB user space won't change to
+unmapped, uncached.
+MIPS III:
+  CacheErr Exception is supported both in R4k and R10000.
+MIPS 64:
+  CPU varient dependent.
+
+4. TLB Entry
+===========================
+Loongson:
+  Every TLB Entry maps to same page size.
+MIPS III:
+  R4k and R10000 support different tlb entry maps to different page size.
+MIPS 64:
+  The same as MIPS III.
+
+5. Address Error Exception.
+===========================
+Loongson:
+  Loongson disables address error exception when loading data into $0,
+facilitating instruction prefetch when compiling.
+MIPS III:
+  If address error occur or translation failed when loading data into $0,
+address error exception will occur.
+MIPS 64:
+  The same as MIP III.
+
+6. Floating point
+===========================
+Loongson:
+  In floating point computations, invalid operation causes FPU to supply 0x80000000
+or 0x80000000 00000000, when corresponding enable bit is cleared.
+MIPS III:
+  FPU supplies 0x7fffffff or 0x7fffffff ffffffff.
+MIPS 64:
+  The same as MIPS III.
+
+7. Floating CSR
+===========================
+Loongson:
+  [CC2:CC7] bits in FSCR register are read-only, and always get zero when read.
+MIPS III:
+  The same as Loongson.
+MIPS 64:
+  These bits are available.
+
+8. TLB exception
+===========================
+Loongson:
+  Loongson doesn't support KX, UX, SX. Exception entry of XTLB and TLB refill is the same.
+MIPS III:
+  KX, UX, and SX are used to determine wheather 64-bit address is used, and entry of TLB and
+XTLB are different.
+MIPS 64:
+  The same as MIPS III.
+