mfspr r0,638. Why 638 ?

[zengshuai at sogou.com]

We know "mfspr r0,638" can get the IMMR.
But why?
I checked "MPC82xx Reference Manual","G2 PowerPC™ Core Reference Manual","Programming Environments Manual",
but I didn't find any word about that.How did the person who first known know?

"G2 PowerPC™ Core Reference Manual"  where is the "SPR 638"?
Table 3-33. Implementation-Specific SPR Encodings (mfspr)
Decimal spr[5?9] spr[0?4] RegisterName Access
58 00001 11010 CSRR0  Supervisor
59 00001 11011 CSRR1  Supervisor
276 01000 10100 SPRG4  Supervisor
277 01000 10101 SPRG5  Supervisor
278 01000 10110 SPRG6  Supervisor
279 01000 10111 SPRG7  Supervisor
286 01000 11110 SVR  Supervisor
309 01001 10101 IBCR  Supervisor
310 01001 10110 DBCR  Supervisor
311 01001 10111 MBAR  Supervisor
317 01001 11101 DABR2  Supervisor
560 10001 10000 IBAT4U  Supervisor
561 10001 10001 IBAT4L  Supervisor
562 10001 10010 IBAT5U  Supervisor
563 10001 10011 IBAT5L  Supervisor
564 10001 10100 IBAT6U  Supervisor
565 10001 10101 IBAT6L  Supervisor
566 10001 10110 IBAT7U  Supervisor
567 10001 10111 IBAT7L  Supervisor
568 10001 11000 DBAT4U  Supervisor
569 10001 11001 DBAT4L  Supervisor
570 10001 11010 DBAT5U  Supervisor
571 10001 11011 DBAT5L  Supervisor
572 10001 11100 DBAT6U  Supervisor
573 10001 11101 DBAT6L Supervisor
574 10001 11110 DBAT7U  Supervisor
575 10001 11111 DBAT7L  Supervisor
976 11110 10000 DMISS Supervisor
977 11110 10001 DCMP Supervisor
978 11110 10010 HASH1 Supervisor
979 11110 10011 HASH2 Supervisor
980 11110 10100 IMISS Supervisor
981 11110 10101 ICMP Supervisor
982 11110 10110 RPA Supervisor
1008 11111 10000 HID0 Supervisor
1009 11111 10001 HID1 Supervisor
1010 11111 10010 IABR Supervisor
1011 11111 10011 HID2 Supervisor
1013 11111 10101 DABR  Supervisor
1018 11111 11010 IABR2  Supervisor

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