Page Identification; Virtual Address Formation; Address Space Identifier Convention - IBM PPC440X5 CPU Core User Manual

User's Manual
PPC440x5 CPU Core
Table 5-1. TLB Entry Fields (continued)
TLB
Bit Field
Word
2 31 SR
5.3 Page Identification
The Valid (V), Effective Page Number (EPN), Translation Space Identifier (TS), Page Size (SIZE), and Trans-
lation ID (TID) fields of a particular TLB entry identify the page associated with that TLB entry. Except as
noted, all comparisons must succeed to validate this entry for subsequent translation and access control
processing. Failure to locate a matching TLB entry based on this criteria for instruction fetches will result in a
TLB Miss exception type Instruction TLB Error interrupt. Failure to locate a matching TLB entry based on this
criteria for data storage accesses will result in a TLB Miss exception which may result in a Data TLB Error
interrupt, depending on the type of data storage access (certain cache management instructions do not result
in an interrupt if they cause an exception; they simply no-op).

5.3.1 Virtual Address Formation

The first step in page identification is the expansion of the effective address into a virtual address. Again, the
effective address is the 32-bit address calculated by a load, store, or cache management instruction, or as
part of an instruction fetch. The virtual address is formed by prepending the effective address with a 1-bit
address space identifier and an 8-bit process identifier. The process identifier is contained in the Process ID
(PID) register. The address space identifier is provided by MSR[IS] for instruction fetches, and by MSR[DS]
for data storage accesses and cache management operations, including instruction cache management
operations. The resulting 41-bit value forms the virtual address, which is then compared to the virtual
addresses contained in the TLB entries.
Note that the tlbsx[.] instruction also forms a virtual address, for software controlled search of the TLB. This
instruction calculates the effective address in the same manner as a data access instruction, but the process
identifier and address space identifier are provided by fields in the MMUCR, rather than by the PID and MSR,
respectively (see TLB Search Instruction (tlbsx[.]) on page 153).
5.3.2 Address Space Identifier Convention
The address space identifier differentiates between two distinct virtual address spaces, one generally associ-
ated with interrupt-handling and other system-level code and/or data, and the other generally associated with
application-level code and/or data.
Typically, user mode programs will run with MSR[IS,DS] both set to 1, allowing access to application-level
code and data memory pages. Then, on an interrupt, MSR[IS,DS] are both automatically cleared to 0, so that
the interrupt handler code and data areas may be accessed using system-level TLB entries (that is, TLB
entries with the TS field = 0). It is also possible that an operating system could set up certain system-level
code and data areas (and corresponding TLB entries with the TS field = 1) in the application-level address
Page 138 of 589
Supervisor State Read Enable (1 bit) See Read Access on page 143.
Load operations and the dcbt, dcbtst, dcbst, dcbf, icbt, and icbi instructions are not per-
mitted from this page when MSR[PR]=0 and will cause a Read Access Control exception.
0
Except for the dcbt, dcbtst, and icbt instructions, a Data Storage interrupt will occur (see
Table 5-4 on page 144).
Load operations and the dcbt, dcbtst, dcbst, dcbf, icbt, and icbi instructions are permit-
1
ted from this page when MSR[PR]=0.
Description
Preliminary
mmu.fm.
September 12, 2002