Intel 80C186XL, User Manual

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3-1
CHAPTER 3
BUS INTERFACE UNIT
The Bus Interface Unit (BIU) generates bus cycles that prefetch instructions from memory, pass
data to and from the execution unit, and pass data to and from the integrated peripheral units.
The BIU drives address, data, status and control information to define a bus cycle. The start of a
bus cycle presents the address of a memory or I/O location and status information defining the
type of bus cycle. Read or write control signals follow the address and define the direction of data
flow. A read cycle requires data to flow from the selected memory or I/O device to the BIU. In a
write cycle, the data flows from the BIU to the selected memory or I/O device. Upon termination
of the bus cycle, the BIU latches read data or removes write data.
3.1 MULTIPLEXED ADDRESS AND DATA BUS
The BIU has a combined address and data bus, commonly referred to as a time-multiplexed bus.
Time multiplexing address and data information makes the most efficient use of device package
pins. A system with address latching provided within the memory and I/O devices can directly
connect to the address/data bus (or local bus). The local bus can be demultiplexed with a single
set of address latches to provide non-multiplexed address and data information to the system.
3.2 ADDRESS AND DATA BUS CONCEPTS
The programmer views the memory or I/O address space as a sequence of bytes. Memory space
consists of 1 Mbyte, while I/O space consists of 64 Kbytes. Any byte can contain an 8-bit data
element, and any two consecutive bytes can contain a 16-bit data element (identified as a word).
The discussions in this section apply to both memory and I/O bus cycles. For brevity, memory
bus cycles are used for examples and illustration.
3.2.1 16-Bit Data Bus
The memory address space on a 16-bit data bus is physically implemented by dividing the address
space into two banks of up to 512 Kbytes each (see Figure 3-1). One bank connects to the lower
half of the data bus and contains even-addressed bytes (A0=0). The other bank connects to the
upper half of the data bus and contains odd-addressed bytes (A0=1). Address lines A19:1 select
a specific byte within each bank. A0 and Byte High Enable (BHE) determine whether one bank
or both banks participate in the data transfer.