80C186EC/80C188EC Microprocessor User's Manual

BUS INTERFACE UNIT
In general, prefix bytes (such as LOCK) are considered extensions of the instructions they precede.
Interrupts, DMA requests and refresh requests that occur during execution of the prefix are
not acknowledged until the instruction following the prefix completes (except for instructions
that are servicing interrupts during their execution, such as HALT, WAIT and repeated string
primitives). Note that multiple prefix bytes can precede an instruction.
Another example is a string primitive preceded by the repetition prefix (REP), which can be interrupted
after each execution of the string primitive, even if the REP prefix is combined with the
LOCK prefix. This prevents interrupts from being locked out during a block move or other repeated
string operations. However, bus hold, DMA and refresh requests remain locked out until
LOCK is removed (either when the block operation completes or after an interrupt occurs.
3.7 MULTI-MASTER BUS SYSTEM DESIGNS
The BIU supports protocols for transferring control of the local bus between itself and other devices
capable of acting as bus masters. To support such a protocol, the BIU uses a hold request
input (HOLD) and a hold acknowledge output (HLDA) as bus transfer handshake signals. To
gain control of the bus, a device asserts the HOLD input, then waits until the HLDA output goes
active before driving the bus. After HLDA goes active, the requesting device can take control of
the local bus and remains in control of the bus until HOLD is removed.
3.7.1 Entering Bus HOLD
In responding to the hold request input, the BIU floats the entire address and data bus, and many
of the control signals. Figure 3-34 illustrates the timing sequence when acknowledging the hold
request. Table 3-7 lists the states of the BIU pins when HLDA is asserted. All device pins not
mentioned in Table 3-7 or shown in Figure 3-34 remain either active (e.g., CLKOUT and
T1OUT) or inactive (e.g., UCS and INTA). Refer to the data sheet for specific details of pin functions
during a bus hold.
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