Altera Nios II User Manual page 58

3–18
■ The tag portion of pteaddr.VPN
■
tlbmisc.PID
The tlbacc register
■
The TLB entry to be read is specified by the following values:
■ the line portion of pteaddr.VPN
■
tlbmisc.WAY
When system software changes the fields that specify the TLB entry, there is no
immediate effect on pteaddr.VPN, tlbmisc.PID, or the tlbacc register. The registers
retain their previous values until the next TLB read operation is initiated. For
example, when the operating system sets pteaddr.VPN to a new value, the contents of
tlbacc continues to reflect the previous TLB entry. tlbacc does not contain the new
TLB entry until after an explicit TLB read.
The WE Flag
When WE = 1, a write to tlbacc writes the tlbacc register and a TLB entry. When WE =
0, a write to tlbacc only writes the tlbacc register.
Hardware sets the WE flag to one on a TLB permission violation exception, and on a
TLB miss exception when status.EH = 0. When a TLB write operation writes the
tlbacc register, the write operation also writes to a TLB entry when WE = 1.
The WAY Field
The WAY field controls the mapping from the VPN to a particular TLB entry. WAY
specifies the set to be written to in the TLB. The MMU increments WAY when system
software performs a TLB write operation. Unused upper bits in WAY must be written as
zero.
1
The number of ways (sets) is configurable in Qsys at generation time, up to a
maximum of 16.
The PID Field
PID is a unique identifier for the current process that effectively extends the virtual
address. The process identifier can be less than 14 bits. Any unused upper bits must
be zero.
tlbmisc.PID contains the PID field from a TLB tag. The operating system must set the
PID field when switching processes, and before each TLB write operation.
1
Use of the process identifier is optional. To implement memory management without
process identifiers, clear tlbmisc.PID to zero. Without a process identifier, all
processes share the same virtual address space.
The MMU sets tlbmisc.PID on a TLB read operation. When the software triggers a
TLB read, by setting tlbmisc.RD to one with the wrctl instruction, the PID value read
from the TLB has priority over the value written by the wrctl instruction.
The size of the PID field is configured in Qsys at system generation, and can be from 8
to 14 bits. If system software defines a process identifier smaller than the PID field,
unused upper bits must be written as zero.
Nios II Processor Reference Handbook
Chapter 3: Programming Model
Registers
February 2014 Altera Corporation