Ccu To/From Storage; High-Speed Buffer (Hsb Or Cache) - IBM 3745 Maintenance Information Reference

ccu
to/from Storage
High-Speed Buffer (HSB
or
CACHE)
The HSB, packaged on the TCM or the PUC card together with the CCU, pro-
vides the CCU with instructions and data at cycle rate.
If the instruction or data is not in the HSB when it is fetched, the 16 contiguous
bytes are automatically transferred into the HSB. The probability is high that
the next instruction(s) or data will be in these 16 bytes, then, as long as
instructions and data are found in the HSB, the CCU is not slowed down by
main storage and can run at full speed.
High-Speed Buffer (HSB) Organization
1
row - .
(2 lines)
512
1
i nes (TCM)
2048
lines
(PUC)
of 16 bytes
111111111111111
512 lines (TCM)
2048 1 i
nes
(PUC)
of 16 bytes
111111111111111
1===============1====1===============1
I I I I
On CCU request, the HSB can either write one byte, one halfword, or one
fullword. However, the HSB is always loaded from storage with 16 contiguous
bytes forming a line.
For the TCM the 16KB HSB is made of 1024 x 16-byte lines and its directory is
two-set associative. The HSB organization is thus: 512 rows x 2 sets x 16
bytes.
For the PUC the 64KB HSB is made of 4096 x 16-byte lines and its directory is
two-set associative. The HSB organization is thus: 2048 rows x 2 sets x 16
bytes.
The HSB is controlled by means of an X'74' out instruction. The following table
summarizes the various HSB data path functions:
Chapter 2. Central Control Unit (CCU)
2-19