Vax 613 0S Instruction Manual download pdf (Page 74)

4.2 Victim Address Register and Counter Register (VAR/VACR)

The operating system can use the victim address register (VAR) and victim

address counter register (VACR) to identify blocks in the cache that are

thrashing. Those blocks can be remapped to reduce the thrashing and improve

cache performance.

The VAR captures every nth Bcache Victim Address. The N is a programmable

number from 0 to 511 that indicates the maximum count of the victim address

counter register (VACR) before it must reset itself and load a new victim address.

A victim is a dirty cache block which is being swapped out to main memory. The

VAR is readable through I/O space. (I/O space is a subset of the non-memory

region.)

The VACR increments every time a victim is written to main memory. When it

reaches N, it resets itself to zero and loads a new value into the VAR. The N is

writeable through I/O space.

4.2.1 Initialization

Both the VAR and VACR are loaded with undeﬁned values during system startup.

No initialization by the console or operating system is needed to ensure correct

system operation. You need access these registers only if some algorithm is being

used to reduce cache thrashing.

4.2.2 Writing the VACR

The value of the address for a write to the VACR is:

05 04 0014 1320 19 183133 32

MR−0077−93RAGS

0 1 C22 XXXXX

VALUE TO WRITE TO N

<08:00>

XXXXX

4.2.3 Reading the VAR

The VAR contains the Bcache index of the victim address that was loaded the

last time the counter reset. The index is address bits<20:5>. The address ASIC

supports cache sizes of 128 KB, 512 KB, and 2 MB. For a cache size of 128

KB, VAR<20:17> is not deﬁned. For a cache size of 512 KB, VAR<20:19> is not

deﬁned.

The value of the address for a read operation from the VAR is:

07 06 05 04 03 02 01 0011 10 09 0815 14 13 1221 20 19 1829 17 1628 27 26 25 24 23 2231 3033 32

0 1 C22 XXXX

MR−0170−93RAGS

XXXXXXXXXXXXXXX1

Bits on the data bus are ignored on an I/O write operation, since the data resides in the

address bits.

4–6 Address ASIC Registers (400/500/600/700/800/900 Models)

1 2 ... 69 70 71 72 73 74 75 76 77 78 79 ... 343 344

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Vax 613 0S Instruction Manual Page 74