4-15
Themis Computer
4—Universe II Description
Slave Image Programming
4.3.2.4 Special PCI Target Image
A special PCI target image is provided to expedite A16 and A24 transactions. The
other eight, standard, PCI target images are typically programmed to access A32
space. The special PCI target image is a 64 MB space, located either within memory
or I/O space, that is decoded using PCI address lines [31:26]. Its base address is
aligned on 64-MB boundaries and no offsets are provided. Therefore, PCI address
information is mapped directly to the VMEbus. The special PCI target image has a
lower priority than any other PCI target image.
The 64-MB space is divided into four (4), 16-MB spaces that are selected using
AD[25:16]. For each region, the upper 64 KB map to VMEbus A16 space, while the
remaining portion map to VMEbus A24 space. The addressing of this slave image is
depicted in
Figure 4-2
on page 4-11.
The special PCI target image register is described
Table 4-13
.
Table 4-11.
PCI Bus Fields for Special PCI Bus Target Image
Field
Register Bits
Description
base
BS[05]
64 MB aligned base address
for the image
address space
LAS
Places Image in Memory or I/O
maximum data width
VDW
separately set each region for
16 or 32 bits
mode
SUPER
separately set each region for
supervisor or non-privileged
type
PGM
Program or data
cycle
VCT
Separately sets each region as
program or data
Table 4-12.
Control Fields for Special PCI Bus Target Image
Field
Register Bits
Description
image enable
EN
enable bit
posted write
PWEN
posted write enable bit
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