16-Bit Byte Enables And Operand Alignment; Internal Na# And Bs16# Logic - Intel 80386 Reference Manual

LOCAL BUS INTERFACE
NA'ILl . NA,
(PIN 013)
I
(INTERNAl)
8S161 8516,
(PIN C14)
T
(INTERNAL).
80386 CHIP ,
231630-22
Figure 3-12. Internal NA# and BS16# Logic
Figure 3-13 compares the signals for 32-bit and l6-bit bus cycles.
3.1.10 16-Bit Byte Enables and Operand Alignment
For a l6-bit data bus, the 80386 views memory and I/O as sequences of l6-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 A3l-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 l6-bit data buses.
The BHE#, BLE#, and Al signals can be generated from BE3#, BE2#, BEl#, and BEO#
using just four external logic gates. Table 3-5 shows .the truth table for this conversion. Note
that certain combinations of BE3#-BEO# are never generated.
When BS16# is sampled active, the states of BE3#-BEO# determine how the 80386 responds:
• BS16# has no effect if activated for a bus cyCle in which BE3# and BE2# are inactive.
• If BEO# and BEl# are both inactive, during a BS16 cycle, and either BE2# or BE3# is
active,
-For a write cycle, data on D3l-D16 is duplicated on D15-DO, regardless of the state
of BS16#. (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 D3l-D24 is read on D15-D8,
.. and data that would normally be read on D23-D16 is read on D7-DO. .
• If BEO# or BEl#is active, and BE2# or BE3# is active, two bus cycles are required. The
two cycles are identical except BEO# and BEl# are inactive in the second cycle and
-For a write cycle, the data that was on D3l-D16 in the first cycle is copied onto
D15-DO.
-Fora read cycle, data that would normally be read on D3l-D24 is read on D15-D8,
and data that would normally be read on D23-D16 is read on D7-DO.
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