Intel 80C186XL, User Manual

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PERIPHERAL CONTROL BLOCK
4-4
4.3 RESERVED LOCATIONS
Many locations within the Peripheral Control Block are not assigned to any peripheral. Unused
locations are reserved. Reading from these locations yields an undefined result. If reserved reg-
isters are written (for example, during a block MOV instruction) they must be set to 0H.
NOTE
Failure to follow this guideline could result in incompatibilities with future
80C186 Modular Core family products.
4.4 ACCESSING THE PERIPHERAL CONTROL BLOCK
All communication between integrated peripherals and the Modular CPU Core occurs over a spe-
cial bus, called the F-Bus, which always carries 16-bit data. The Peripheral Control Block, like
all integrated peripherals, is always accessed 16 bits at a time.
4.4.1 Bus Cycles
The processor runs an external bus cycle for any memory or I/O cycle accessing a location within
the Peripheral Control Block. Address, data and control information is driven on the external pins
as with an ordinary bus cycle. Information returned by an external device is ignored, even if the
access does not correspond to the location of an integrated peripheral control register. This is also
true for the 80C188 Modular Core family, except that word accesses made to integrated registers
are performed in two bus cycles.
4.4.2 READY Signals and Wait States
The processor generates an internal READY signal whenever an integrated peripheral is access-
ed. External READY is ignored. READY is also generated if an access is made to a location with-
in the Peripheral Control Block that does not correspond to an integrated peripheral control
register. For accesses to timer control and counting registers, the processor inserts one wait state.
This is required to properly multiplex processor and counter element accesses to the timer control
registers. For accesses to the remaining locations in the Peripheral Control Block, the processor
does not insert wait states.