Tandy 1000 HX Technical Reference Manual page 94

8088
Figure 9. Interrupt Acknowledge Sequence
External Synchronization via TEST
As an alternative to interrupts, the 8088 provides a
single software-testable input pin (TEST). This input
is utilized by executing a WAIT instruction. The sin-
gle WAIT instruction is repeatedly executed until the
TEST input goes active (LOW). The execution of
WAIT does not consume bus cycles once the queue
is full.
If a local bus request occurs during WAIT execution,
the 8088 3-states all output drivers. If interrupts are
enabled, the 8088 will recognize interrupts and pro-
cess them. The WAIT instruction is then refetched,
and reexecuted.
Basic System Timing
In minimum mode, the MN/MX pin is strapped to
VQC and the processor emits bus control signals
compatible with the 8085 bus structure. In maximum
mode, the MN/MX pin is strapped to GND and the
processor emits coded status information which the
8288 bus controller uses to generate MULTIBUS
compatible bus control signals.
System Timing—Minimum System
(See Figure 8)
The read cycle begins in T1 with the assertion of the
address latch enable (ALE) signal. The trailing (low
going) edge of this signal is used to latch the ad-
dress information, which is valid on the address/
data bus (AD0-AD7) at this time, into the
8282/8283 latch. Address lines A8 through A15 do
not need to be latched because they remain valid
throughout the bus cycle. From T1 to T4 the IO/M
signal indicates a memory or I/O operation. At T2
the address is removed from the address/data bus
and the bus goes to a high impedance state. The
read control signal is also asserted at T2. The read
(RD) signal causes the addressed device to enable
its data bus drivers to the local bus. Some time later,
valid data will be available on the bus and the ad-
dressed device will drive the READY line HIGH.
When the processor returns the read signal to a
HIGH level, the addressed device will again 3-state
its bus drivers. If a transceiver ]s required to buffer
the 8088 local bus, signals DT/R and DEN are pro-
vided by the 8088.
A write cycle also begins with the assertion of ALE
and the emission of the address. The IO/M signal is
again asserted to indicate a memory or I/O write
operation. In T2, immediately following the address
emission, the processor emits the data to be written
into the addressed location. This data remains valid
until at least the middle of T4. During T2, T3, and
Tw, the processor asserts the write control signal.
The write (WR) signal becomes active at the begin-
ning of T2, as opposed to the read, which is delayed
somewhat into T2 to provide time for the bus to
float.
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