Digi NS9215 manual First-level fetch concatenation and address, First-level descriptor

WO R K I N G W I T H T H E C P U

MemoryManagement Unit (MMU)

First-level fetch concatenation and address

Modified virtual address

31 20 19 0

Table index

Translation table base
31	14 13		0
Translation base
31	14 13		2 1 0
Translation base		Table index		0	0
31			0
First-level descriptor

First-level descriptor

This address selects a 4-byte translation table entry. This is a first-level descriptor for either a section or a page.

The first-level descriptor returned is a section description, a coarse page table descriptor, a fine page table descriptor, or is invalid. This is the format of a first- level descriptor.

31	20	19	12	11	10	9	8	5	4	3	2	1	0
												0	0	Fault
														Coarse page table
	Coarse page table base address						Domain		1			0	1
														Section
	Section base address			AP			Domain		1	C	B	1	0
														Fine page table
	Fine page table base address						Domain		1			1	1

Page table descriptors

A section descriptor provides the base address of a 1 MB block of memory.

The page table descriptors provide the base address of a page table that contains second-level descriptors. There are two page-table sizes:

Coarse page tables, which have 256 entries and split the 1 MB that the table describes into 4 KB blocks.

Fine page tables, which have 1024 entries and split the 1 MB that the table describes into 1 KB blocks.

110Hardware Reference NS9215