Buffer Address Format - IBM System/370 Manual

Page of Ge2D-17 30-0
Revised 7/14/70
By TNL GN2D-2227
Processor Storage Address Bits
Bits
8-19,20
20-26
21-26
27-28
29-31
Used for address compare (16K, 8K buffer)
Used to reference 16K buffer columns
Used to reference SK buffer columns
Used to reference doubleword within a block
Used to reference byte within doubleword
Figure 10.15.5. Processor storage address format for buffer reference
Buffer Reference Bits
Bits
0-1 Generated as result of address compare; this two-bit encoded
field represents block in buffer column containing desired
data
2-S
3-8
8-10
Used to reference column for 16K buffer (processor storage
address bits 20-26)
Used to reference column for SK buffer (processor storage
address bits 21-26)
Used to reference doubleword within a block (processor
storage address bits 27-2S)
Figure 10.15.6. Buffer address format
Buffer contents and buffer block assignment are controlled by an
address array, shown in Figure 10.15.4, and a special replacement
array. The address array, like the SK buffer, is divided into 64
columns conSisting of four block address registers each"such that there
is a one-for-one correspondence between address array registers and
blocks in the buffer. An address array block register contains the
13-bit processor storage block address from bits 8-20 of the processor
storage address of the data contained in its corresponding buffer
block. When a CPU to processor storage reference is made, the four
appropriate address array column registers (13-bit block addresses)
are interrogated to determine whether the requested data is currently
in the buffer.
The replacement array is used to maintain knowledge of the activity
of the data blocks within each of the buffer columns. When an 8K
buffer is present, the array consists of 64 logic-controlled activity
lists, one list for each column in the buffer. A list contains four
entries, one for each buffer block in its column. A block's entry
is
plac'~d
at the top of the list for its column when the buffer block
is referenced during a CPU fetch operation. This approach insures
that the block used longest ago within a given column is at the bottom
of the list. When a block within a buffer column has to be assigned
and loaded, because the data requested by the CPU is not in the buffer,
the buffer block at the bottom of a column activity list is allocated.
Thus, the more active data is maintained in the buffer.
The SK buffer operates as follows. When the CPU requests data,
bits 21-26 of the data's processor storage address are used to obtain
a buffer column address. The 13 high-order bits of the processor
storage address are then compared to the address in each of the four
block address registers in that buffer column in the address array.
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