Xilinx MicroBlaze Reference Manual page 62

TLB Access-Protection Controls
Each TLB entry controls three types of access:
• Process—Processes are protected from unauthorized access by assigning a unique
process ID (PID) to each process. When system software starts a user-mode application,
it loads the PID for that application into the PID register. As the application executes,
memory addresses are translated using only TLB entries with a TID field in Translation
Look-Aside Buffer High (TLBHI) that matches the PID. This enables system software to
restrict accesses for an application to a specific area in virtual memory.
A TLB entry with TID=0x00 represents a process-independent translation. Pages that
are accessed globally by all processes should be assigned a TID value of 0x00.
• Execution—The processor executes instructions only if they are fetched from a virtual
page marked as executable (TLBLO[EX]=1). Clearing TLBLO[EX] to 0 prevents execution
of instructions fetched from a page, instead causing an instruction-storage interrupt
(ISI) to occur. The ISI does not occur when the instruction is fetched, but instead occurs
when the instruction is executed. This prevents speculatively fetched instructions that
are later discarded (rather than executed) from causing an ISI.
The zone-protection register can override execution protection.
• Read/Write—Data is written only to virtual pages marked as writable (TLBLO[WR]=1).
Clearing TLBLO[WR] to 0 marks a page as read-only. An attempt to write to a read-only
page causes a data-storage interrupt (DSI) to occur.
The zone-protection register can override write protection.
TLB entries cannot be used to prevent programs from reading pages. In virtual mode, zone
protection is used to read-protect pages. This is done by defining a no-access-allowed zone
(ZPR[Zn] = 00) and using it to override the TLB-entry access protection. Only programs
running in user mode can be prevented from reading a page. Privileged programs always
have read access to a page.
Zone Protection
Zone protection is used to override the access protection specified in a TLB entry. Zones are
an arbitrary grouping of virtual pages with common access protection. Zones can contain
any number of pages specifying any combination of page sizes. There is no requirement for
a zone to contain adjacent pages.
The zone-protection register (ZPR) is a 32-bit register used to specify the type of protection
override applied to each of 16 possible zones. The protection override for a zone is encoded
in the ZPR as a 2-bit field. The 4-bit zone-select field in a TLB entry (TLBLO[ZSEL]) selects
one of the 16 zone fields from the ZPR (Z0–Z15). For example, zone Z5 is selected when
ZSEL = 0101.
MicroBlaze Processor Reference Guide
UG984 (v2016.2) June 8, 2016
UG984 (v2016.1) April 6, 2016
www.xilinx.com
Chapter 2: MicroBlaze Architecture
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