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LOCAL BUS INTERFACE
NA'ILl NA,
(PIN
'13)
I
(INTERNAL)
8516#
8 5 1 6 # )
(PIN C14)
1
(INTERNAL)
80386 CHIP
..-
-
231630-22
Figure 3-12. Internal NA# and
Bs
16# Logic
Figure 3-13 compares the signals for 32-bit and 16-bit bus cycles.
3.1.10 16-Bit Byte Enables and Operand Alignment
For a I6-bit data bus, the 80386 views memory and I/O as sequences of I6-bit words. For
this configuration, the Bus High Enable (BHE#), AO or Bus Low Enable (BLE#), and Al
signals are needed. BHE# and BLE# are byte enables that correspond to two banks of memory
in the same way that BE3#-BEO# correspond to four banks. Al is added to A3I-A2 to
generate the addresses of 2-byte locations instead of 4-byte locations. Figure 3-14 compares
the addressing configurations of 32-bit and 16-bit data buses.
The BHE#, BLE#, and Al signals can be generated from BE3#, BE2#, BE1#, and BEO#
using just four external logic gates. Table 3-S shows the truth table for this conversion. Note
that certain combinations of BE3#-BEO#are never generated.
When BSI6# is sampled active, the states of BE3#-BEO# determine how the 80386 responds:
• BSI6# has no effect if activated for a bus cycle in which BE3# and BE2# are inactive.
• If
BEO# and BEI# are both inactive during a BS16 cycle, and either BE2# or BE3# is
active,
-For a write cycle, data on D3I-DI6 is duplicated on DIS-DO, regardless of the state
of BSI6#. (This duplication occurs because BS16# is sampled late in the cycle but
data must be available early).
-For a read cycle, data that would normally be read on D31-D24 is read on DIS-D8,
and data that would normally be read on D23-D16 is read on D7-DO.
• If
BEO# or BE1# is active, and BE2# or BE3# is active, two bus cycles are required. The
two cycles are identical except BEO# and BE1# are inactive in the second cycle and
-For a write cycle, the data that was on D3I-DI6 in the first cycle is copied onto
DIS-DO.
-For a read cycle, data that would normally be read on D3I-D24 is read on D1S-D8,
and data that would normally be read on D23-D 16 is read on D7 -DO.
3-20
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