8086-2 datasheet(13/30 Pages) INTEL

13 / 30 page

8086

lines D7–D0 as supplied by the inerrupt system logic

(i.e., 8259A Priority Interrupt Controller). This byte

identifies the source (type) of the interrupt. It is multi-

plied by four and used as a pointer into an interrupt

vector lookup table, as described earlier.

BUS TIMING–MEDIUM SIZE SYSTEMS

For medium size systems the MN/MX pin is con-

nected to VSS and the 8288 Bus Controller is added

to the system as well as a latch for latching the sys-

tem address, and a transceiver to allow for bus load-

ing greater than the 8086 is capable of handling.

Signals ALE, DEN, and DT/R are generated by the

8288 instead of the processor in this configuration

although their timing remains relatively the same.

The 8086 status outputs (S2, S1, and S0) provide

type-of-cycle information and become 8288 inputs.

This bus cycle information specifies read (code,

data,

I/O),

write

(data

I/O),

interrupt

acknowledge, or software halt. The 8288 thus issues

control signals specifying memory read or write, I/O

read or write, or interrupt acknowledge. The 8288

provides two types of write strobes, normal and ad-

vanced, to be applied as required. The normal write

strobes have data valid at the leading edge of write.

The advanced write strobes have the same timing

as read strobes, and hence data isn’t valid at the

leading edge of write. The transceiver receives the

usual DIR and G inputs from the 8288’s DT/R and

DEN.

The pointer into the interrupt vector table, which is

passed during the second INTA cycle, can derive

from an 8259A located on either the local bus or the

system bus. If the master 8259A Priority Interrupt

Controller is positioned on the local bus, a TTL gate

is required to disable the transceiver when reading

from the master 8259A during the interrupt acknowl-

edge sequence and software ‘‘poll’’.

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