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8–6 Error Detection and Error Handling
Alpha 21264/EV67 Hardware Reference Manual
Bcache Data Single-Bit Correctable ECC Error
8.8.2 Dcache Fill from Bcache
If the quadword in error is not used to satisfy a load instruction, a hardware recovery
flow is not invoked. The quadword in error, and its associated check bits, are written
into the Dcache. However, status is logged as shown in the bulleted list below, and a
corrected read data (CRD) error interrupt is posted, when enabled. PALcode may elect
to correct the error by scrubbing the block. If the error is not corrected by PALcode
when it occurs, the error will be detected and corrected by a later load/victim operation.
If the quadword in error is used to satisfy a load instruction, then the flow is very simi-
lar to that used for a Dcache ECC error. The LSD ECC checker detects the error and the
21264/EV67 performs the following actions:
• The load instruction’s destination register is written with incorrect data; however,
the load queue will retain the state associated with the load instruction.
• A consumer of the load instruction’s data may be issued before the error is
recognized. The Ibox will invoke a replay trap at an instruction that is older than (or
equal to) any instruction that consumes the load instruction’s data. The 21264/
EV67 then stalls the replayed Istream in the map stage of the pipeline, until the
error is corrected.
• With a READ_ERR read type from the Mbox for the load instruction in error, the
Cbox scrubs the block in the Dcache by evicting the block into the victim buffer
and writing it back into the Dcache.
• C_STAT[DSTREAM_BC_ERR] is set.
• C_ADDR contains bits [42:6] of the Bcache fill address of the block that contains
the error.
• C_SYNDROME_0[7:0] and C_SYNDROME_1[7:0] contain the syndrome of
quadword 0 and 1, respectively, of the octaword subblock that contains the error.
• The load queue retries the load instruction and rewrites the register.
• DC_STAT[ECC_ERR_LD] is set.
• A corrected read data (CRD) error interrupt is posted, when enabled.
Note: Errors in speculative load instructions cause a CRD error to be posted but
the data is not scrubbed by hardware. The PALcode cannot perform a scrub
operation because C_STAT is zero and C_ADDR does not contain the
address of the block in error.
8.8.3 Bcache Victim Read
A victim from the Bcache is written directly to the system port, without correction. The
ECC parity checker on the LSD detects the error and posts a corrected read data (CRD)
error interrupt. The Cbox error register is not updated.
8.8.3.1 Bcache Victim Read During a Dcache/Bcache Miss
While the Bcache is servicing a Dcache miss and that Bcache access is also a miss, and
an error occurs during that Bcache data access, the Cbox does not latch the error infor-
mation. However, the Mbox correction state machine is activated and it invokes a CRD
error despite the fact that no correction is performed.