
Intel386
TM
DX MICROPROCESSOR
Bus States:
T1Dfirst clock of a non-pipelined bus cycle (Intel386 DX drives new ad-
dress and asserts ADS
Y
).
T2Dsubsequent clocks of a bus cycle when NA
Y
has not been sampled
asserted in the current bus cycle.
T2IDsubsequent clocks of a bus cycle when NA
Y
has been sampled as-
serted in the current bus cycle but there is not yet an internal bus request
pending (Intel386 DX will not drive new address or assert ADS
Y
).
T2PDsubsequent clocks of a bus cycle when NA
Y
has been sampled
asserted in the current bus cycle and there is an internal bus request pend-
ing (Intel386 DX drives new address and asserts ADS
Y
).
T1PDfirst clock of a pipelined bus cycle.
TiDidle state.
ThDhold acknowledge state (Intel386 DX asserts HLDA).
Asserting NA
Y
for pipelined address gives access to three more bus
states: T2I, T2P and T1P.
Using pipelined address, the fastest bus cycle consists of T1P and T2P.
231630–24
Figure 5-20. Intel386
TM
DX Complete Bus States (including pipelined address)
Realistically, address pipelining is almost always
maintained as long as NA
Y
is sampled asserted.
This is so because in the absence of any other re-
quest, a code prefetch request is always internally
pending until the instruction decoder and code pre-
fetch queue are completely full. Therefore address
pipelining is maintained for long bursts of bus cycles,
if the bus is available (i.e., HOLD negated) and NA
Y
is sampled asserted in each of the bus cycles.
5.4.3.6 PIPELINED ADDRESS WITH DYNAMIC
DATA BUS SIZING
The BS16
Y
feature allows easy interface to 16-bit
data buses. When asserted, the Intel386 DX bus
interface hardware performs appropriate action to
make the transfer using a 16-bit data bus connected
on D0–D15.
There is a degree of interaction, however, between
the use of Address Pipelining and the use of Bus
Size 16. The interaction results from the multiple bus
cycles required when transferring 32-bit operands
over a 16-bit bus. If the operand requires both 16-bit
halves of the 32-bit bus, the appropriate Intel386 DX
action is a second bus cycle to complete the oper-
and’s transfer. It is this necessity that conflicts with
NA
Y
usage.
When NA
Y
is sampled asserted, the Intel386 DX
commits
itself
to
perform
the
next
inter-
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