M68040 datasheet(76/442 Pages) FREESCALE

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MOTOROLA

M68040 USER'S MANUAL

3- 27

Each ATC entry consists of a physical address, attribute information from a corresponding

page descriptor, and a tag that contains a logical address and status information. Figure

3-21, which illustrates the entry and tag fields, is followed by field definitions listed in

alphabetical order.

PHYSICAL ADDRESS*

ENTRY

FC2

LOGICAL ADDRESS*

TAG

* For 4-Kbyte page sizes this field uses address bits 31–12; for 8-Kbyte page sizes, bits 31–13.

Figure 3-21. ATC Entry and Tag Fields

CM—Cache Mode

This field selects the cache mode and accesses serialization as follows:

00 = Cachable, Write-through

01 = Cachable, Copyback

10 = Noncachable, Serialized

11 = Noncachable

Section 4 Instruction and Data Caches provides detailed information on caching

modes, and Section 7 Bus Operation provides information on serialization.

FC2—Function Code Bit 2 (Supervisor/User)

This bit contains the function code corresponding to the logical address in this entry.

FC2 is set for supervisor mode accesses and cleared for user mode accesses.

G—Global

When set, this bit indicates the entry is global. Global entries are not invalidated by the

PFLUSH instruction variants that specify nonglobal entries, even when all other

selection criteria are satisfied.

Logical Address

This 13-bit field contains the most significant logical address bits for this entry. All 16

bits of this field are used in the comparison of this entry to an incoming logical address

when the page size is 4 Kbytes. For 8-Kbytes pages, the least significant bit of this field

is ignored.

M—Modified

The modified bit is set when a valid write access to the logical address corresponding to

the entry occurs. If the M-bit is clear and a write access to this logical address is

attempted, the M68040 suspends the access, initiates a table search to set the M-bit in

the page descriptor, and writes over the old ATC entry with the current page descriptor

information. The MMU then allows the original write access to be performed. This

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