Chapter 5 - Memory Expansion Module (XVME-956/10.1)
1
1 1 1
1
- - - - -
1 1 --
0-
1 1 0 1
LARGE
MEMORY
DEVICE
0 0 0 1
0 0 0 0
5.8.4 S e l e c t i n g Portions of Large Devices
The memory window is 64 Kbytes in size. I f the memory devices are the same size as the window, their
entire contents appear in the window when that device is selected. I f the device is smaller than the
window, its contents are mirrored in the window. F o r 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. T h e
part of the device to appear is selected via the PSO-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 PSO is required to select the upper
(PSO = 1) or lower 64K (PSO = 0) 64 Kbytes. A l l four bits select up to 16 (64 Kbyte) pages of a 1 Mbyte
memory. Figure 5-6 shows a 64 Kbyte portion of a 1 Mbyte memory being selected. PSO is the LSB
(A16), and PS3 is the MSB (A19).
CONTROL REGISTER 2
CONTROL REGISTER 1
...... --
..- . . . . .
64KBYTE PAGE
OF LARGE DEVICE-
APPEARS IN
64 KBYTE WINDOW
Figure 5-6. Selecting Pages within Large Devices (PSO-3)
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