Prefetch Latch - Advanced Micro Computers Am96/4116A User Manual

The Element Pointer field definitions vary depending on the state of
the Group Pointer. When one of the Counter Groups is specified, the
Element field selects one of the three 16-bit registers within the
group for access. The element Pointer also selects a type of
sequencing that may be used.
When control bit MM14
=
1, automatic sequencing of the Data Pointer
register is disabled and the contents of the Group and Element fields
will change only when the Data Pointer is loaded by command. This
allows the host processor to repetitively access any given internal
data register without intervening commands.
When control bit MM14
=
0, the Data Pointer automatically sequences
through new pointer values as information is transferred into or out of
the data port. Several types of sequencing operations are available
depending on the data bus width being used and the initial Data Pointer
val
UE~
entered by command. When E1
=
0 or E2
=
0, and MrU4
=
0, the
Element field is automatically sequenced through three values: 00,01
and 10. When the transition from 10 to 00 occurs, the Group Field is
also incremented by one. Note that the Element field in this case does
not sequence to a value of 11. When E1
=
1 and E2
=
1, only the Group
field is sequenced. This allows the Hold registers to be sequentially
accessed while bypassing the Mode and Load registers.
Once the Am9513 is operating, the two illegal codes shown in figure
3-16 for the Group Pointer field will not occur because:
• no commands wi
11
load the ill ega
1
va
1
ues into the Group Poi nter ,
and
• no automatic sequencing will change the Group Pointer to an
illegal value.
The illegal codes can occur following power-up and the Data Pointer
should always be loaded with some appropriate value during
i nit
-j
ali za t ion.
~Jhen the Control Group is specified (G4, G2, G1
=
111), automatic
increment of the Gr'oup field is d"isabled and the Element field will
circulate through the first three values (assuming MM14
=
0). When one
of the Counter Groups is
specified~
the G field can increment through
the fi ve Counte r Group val ue s; it wi
11
not sequence into the ill ega
1
codes or into the Control Group.
PREFETCH LA TCH
A Prefetch Latch is used for all read operations through the data port
to reduce the read access time to internal Am9513 registers. Following
each read or wri te operati on throu1gh the Data Port, the Data Poi nter
regi ster is updated to poi nt to the next regi ster to be acces sed.
Immediately following this update the new register data is transferred
to a special Prefetch Latch at the interface pad logic. When the user
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