Add documents about ITS paging.

its.paging by John Wilson.
ITS Hardware Memo 2 by Jack Holloway, from Michael Thompson.

doc/sysdoc/its.paging

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+09/06/93	JMBW	Created (from KSDEFS, ucode, and Alan's email)
+
+Page tables are pointed to by DBR1-DBR4:  ("descriptor base registers")
+
+DBR1	user space low 128K
+DBR2	user space high 128K
+DBR3	exec space high 128K
+DBR4	exec space low 128K (not present on all KA10 paging boxes)
+	(identity mapped in ITS anyway)
+
+Each DBR contains the physical address of a 64-word block:
+
+           LH   RH
+	+-----+-----+
+	|  0  |  1  |
+	+-----+-----+
+	|  2  |  3  |
+	  .   .   .
+	  .   .   .
+	| 174 | 175 |
+	+-----+-----+
+	| 176 | 177 |
+	+-----+-----+
+
+where halfwords 0-177 contain the entries for pages 0-177 (1K each) in the
+segment addressed by that DBR.
+
+Note that the EPT and UPT have nothing to do with paging.
+
+Page table entry (halfword):
+
+	                     1 1 1 1 1 1 1 1
+	 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7
+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+	|ACC|///|A|C|///| PHYSICAL PAGE NO. |
+	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ACC (access)
+	00	no access allowed
+	01	read only
+	10	read/write/first (=01, for software "dirty" bit)
+	11	read/write
+
+A (age bit)
+	cleared on first access (was 3-bit field on KA10, set to 000)
+
+C (cacheable)
+	if 1, data from this page may be put in cache
+
+The physical page # is the ITS page # (not DEC), i.e. shift the page number
+left ten bits to get the absolute base address of the 1K page.
+
+The DBRs are loaded by the LDBR1-LDBR4 instructions (which usurp the opcodes
+for WRSPB, WRCSB, WRPUR, and WRCSTM), which also flush the cache and both
+halves of the TLB (including the half not mapped by the DBR being loaded).
+These instructions use the immediate operand as the physical address of the
+page tables, which means they have to be in the first 256K (Alan says in fact
+they are always in the first 128K).  The user DBRs can also be loaded by LPMR
+(702^9 17,), which also loads the quantum timer (and the JPCs, if supported by
+this microcode).
+
+The DBRs may be retrieved by SDBR1-SDBR4 (which usurp RDSPB, RDCSB, RDPUR,
+RDCSTM), which each store the DBR at the address given by the EA.  Similarly,
+SPM (702^9 7,) is the reverse of LPMR.