12. Register Descriptions
291
PowerSpan II User Manual
80A1010_MA001_09
Integrated Device Technology
www.idt.com
The MODE bit and the MEM_IO bit work together to control the size of the transaction (see
).
PRKEEP:
Prefetch Read Keep stores prefetch data beyond an initial read. When set, subsequent read
requests to the same image at the next address retrieves the read data directly from the switching fabric
instead of causing either PCI bus to fetch more data. The read data is invalidated when a read with a
non-matching address occurs.
RD_AMT[2:0]:
The Read Amount setting determines different values to prefetch from PCI. If
PRKEEP is not set, it is recommended limiting the RD_AMT to 32-bytes.
If the slave image is programmed to be in IO mode (the MODE bit in the
x Control Register” on page 287
set to 1 then the RD_AMT is not used and a maximum of 4 bytes will
be read from the PCI bus.
Table 74: Setting for MODE and MEM_IO Bits
MODE Setting
MEM_IO setting
Transaction size
0
X
a
a.
X means either 0 or 1.
Memory cycle (minimum
8 byte memory read)
1
0
I/O cycle
1
1
Memory cycle
(1,2,3, or 4 byte memory
reads on the PCI bus(es))
The ARTRY_EN bit must be set to 1 in order for the PowerSpan II Prefetch
Keep feature to keep prefetched data. The ARTRY_EN bit is in the
Bus Miscellaneous Control and Status Register” on page 304
.
Table 75: Read Amount
RD_AMT[2:0]
Data Fetched
000
8 bytes
001
16 bytes
010
32 bytes
011
64 bytes
100
128 bytes
101-111
Reserved
The EEPROM load capability allows a processor on the Processor Bus to boot
directly from a device on PCI. Only the control registers for Processor Bus Slave
Image 0 are loaded from EEPROM.