8088
The basic difference between the interrupt acknowl-
edge cycle and a read cycle is that the interrupt ac-
knowledge (INTA) signal is asserted in place of the
read (RD) signal and the address bus is floated.
(See Figure 9) In the second of two successive INTA
cycles, a byte of information is read from the data
bus, as supplied by the interrupt system logic (i.e.
8259A priority interrupt controller). This byte identi-
fies the source (type) of the interrupt. It is multiplied
by four and used as a pointer into the interrupt vec-
tor lookup table, as described earlier.
the same way the 8086 does with the distinction of
handling only 8 bits at a time. Sixteen-bit operands
are fetched or written in two consecutive bus cycles.
Both processors will appear identical to the software
engineer, with the exception of execution time. The
internal register structure is identical and all instruc-
tions have the same end result. The differences be-
tween the 8088 and 8086 are outlined below. The
engineer who is unfamiliar with the 8086 is referred
to the iAPX 86, 88 User’s Manual, Chapters 2 and 4,
for function description and instruction set informa-
tion. Internally, there are three differences between
the 8088 and the 8086. All changes are related to
the 8-bit bus interface.
Bus TimingÐMedium Complexity
Systems
The queue length is 4 bytes in the 8088, whereas
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the 8086 queue contains 6 bytes, or three words.
The queue was shortened to prevent overuse of
the bus by the BIU when prefetching instructions.
This was required because of the additional time
necessary to fetch instructions 8 bits at a time.
(See Figure 10)
For medium complexity systems, the MN/MX pin is
connected to GND and the 8288 bus controller is
added to the system, as well as a latch for latching
the system address, and a transceiver to allow for
bus loading greater than the 8088 is capable of han-
dling. Signals ALE, DEN, and DT/R are generated
by the 8288 instead of the processor in this configu-
ration, although their timing remains relatively the
same. The 8088 status outputs (S2, S1, and S0) pro-
vide type of cycle information and become 8288 in-
puts. This bus cycle information specifies read
(code, data, or I/O), write (data or I/O), interrupt ac-
knowledge, 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 is not valid at the
leading edge of write. The transceiver receives the
usual T and OE inputs from the 8288’s DT/R and
DEN outputs.
To further optimize the queue, the prefetching al-
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gorithm was changed. The 8088 BIU will fetch a
new instruction to load into the queue each time
there is a 1 byte hole (space available) in the
queue. The 8086 waits until a 2-byte space is
available.
The internal execution time of the instruction set
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is affected by the 8-bit interface. All 16-bit fetches
and writes from/to memory take an additional
four clock cycles. The CPU is also limited by the
speed of instruction fetches. This latter problem
only occurs when a series of simple operations
occur. When the more sophisticated instructions
of the 8088 are being used, the queue has time to
fill and the execution proceeds as fast as the exe-
cution unit will allow.
The 8088 and 8086 are completely software com-
patible by virtue of their identical execution units.
Software that is system dependent may not be com-
pletely transferable, but software that is not system
dependent will operate equally as well on an 8088
and an 8086.
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 8289A 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’’.
The hardware interface of the 8088 contains the ma-
jor differences between the two CPUs. The pin as-
signments are nearly identical, however, with the fol-
lowing functional changes:
A8–A15ÐThese pins are only address outputs
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on the 8088. These address lines are latched in-
ternally and remain valid throughout a bus cycle
in a manner similar to the 8085 upper address
lines.
The 8088 Compared to the 8086
The 8088 CPU is an 8-bit processor designed
around the 8086 internal structure. Most internal
functions of the 8088 are identical to the equivalent
8086 functions. The 8088 handles the external bus
BHE has no meaning on the 8088 and has been
eliminated.
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14
INTEL [ INTEL ]
