Xycom XVME-976/202, Manual

Page: 70 / 87

XVME-976 Adapter Module
4- 4-22
Selecting Portions of Large Devices
The memory window is 64 Kbytes in size. If the memory devices are the
same size as the window, their entire contents appear in the window when
that device is selected. If the device is smaller than the window, its contents
are mirrored in the window. For example, a 32 Kbyte memory will appear
twice in a 64 Kbyte window, once at D0000h, and again at E0000h.
If the memory device is larger than the window, only part of its contents can
appear in the window. The part of the device to appear is selected via the
PS0-3 bits in the control registers. These bits are used as high order address
lines, A16 through A19, by the memory devices. They are set by writing to
the control registers with I/O instructions.
A 128 Kbyte device can appear as two 64 Kbyte "pages" so only PS0 is re-
quired to select the upper
(PS0 = 1) or lower 64K (PS0 = 0) 64 Kbytes. All four bits select up to 16
(64 Kbyte) pages of a 1 Mbyte memory. Figure 4-6 shows a 64 Kbyte por-
tion of a 1 Mbyte memory being selected. PS0 is the LSB (A16), and PS3 is
the MSB (A19).
Figure 4-6. Selecting Pages within Large Devices (PS0-3)