User`s guide

The XMI
2.2.1 XMI Memory Space
Memory address space is the lower part of the address space no matter
which address mode, 30-bit or 32-bit, is used. The maximum amount of
I/O space available is 512 Mbytes in either mode. Selection of memory
space for a system using a 30-bit address space is dependent upon the
state of bit <29>, the most significant bit in the 30-bit address. If bit <29>
is clear, memory is addressed. If bit <29> is set, I/O space is addressed.
Selection of memory space for a system using a 32-bit address space is
dependent upon the state of bits <31:29>. If any of these bits are clear,
memory is addressed. If all are set, I/O space is addressed.
Figure 2–4 shows how the address corresponds to fields on the XMI D
lines. In the 30-bit case, address bits <29:0> correspond to XMI D lines
<29:0>. In the 32-bit case, XMI D<29> NO LONGER HAS ANYTHING
TO DO WITH COUNTING, instead it is interpreted solely as an I/O bit.
It is set only when bits <31:29> of the address are set. When the I/O bit is
set, only bits <28:0> of the address are relevant and the rest of the address
is ignored. This scheme causes the size of the I/O space to be 512 Mbytes.
Figure 2–4 Address Association
2
9MSB 0 LSB
30−Bit Address
5
0
4
9
4
8
3
0
2
8
//
//
XMI D Lines
32−Bit Address
3
1
msb−p167−89
2
9 0
3
0
XMI D <29> sets only if
Address bits <31:29> set
2
9
02
8
2–13