VR4102 datasheet(158/701 Pages) NEC

158 / 701 page

CHAPTER 6 EXCEPTION PROCESSING

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(2) Operating mode

The operating mode is specified by KSU bit in the Status register when both the exception level and error level

are normal (0).

(3) Exception/error levels

The operation enters Kernel mode when either EXL bit or ERL bit in the Status register is set to 1. Returning

from an exception resets the exception level to normal (0) (for details, see Chapter 27).

The registers that retain address, cause, and status information during exception processing are described in 6.3

EXCEPTION PROCESSING REGISTERS. For a description of the exception process, see 6.4 DETAILS OF

EXCEPTIONS.

6.2 PRECISION OF EXCEPTIONS

VR4102 exceptions are logically precise; the instruction that causes an exception and all those that follow it are

aborted and can be re-executed after servicing the exception. When succeeding instructions are killed, exceptions

associated with those instructions are also killed. Exceptions are not taken in the order detected, but in instruction

fetch order.

There is a special case in which the VR4102 processor may not be able to restart easily after servicing an

exception. When a cache data parity error exception occurs on a load with a cache hit, the VR4102 processor does

not prevent the cache data (with erroneous parity) from being written back into the register file during the WB stage.

The exception is still precise, since both the EPC and CacheError registers are updated with the correct virtual

address pointing to the offending load instruction, and the exception handler can still determine the cause of

exception and its origin. The program can be restarted by rewriting the destination register - not automatically,

however, as in the case of all the other precise exceptions where no status change occurs.

VR4102 Scheda tecnica(PDF) 158 Page - NEC