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6-5
BUS INTERFACE UNIT
6.2 BUS OPERATION
The processor generates eight different types of bus operations:
• Memory data read (data fetch)
• Memory data write
• Memory code read (instruction fetch)
• I/O data read (data fetch)
• I/O data write
• Halt or shutdown
• Refresh
• Interrupt acknowledge
These operations are defined by the states of four bus status pins: M/IO#, D/C#, W/R# and RE-
FRESH#. Table 6-2 lists the various combinations and their definitions.
Table 6-2. Bus Status Definitions
M/IO# D/C# W/R# REFRESH# Bus Operation
0 0 0 1 interrupt acknowledge cycle
0 0 1 1 never occurs
0 1 0 1 I/O data read
0 1 1 1 I/O data write
1 0 0 1 memory code read
1 0 1 1 halt or shutdown cycle*
1 1 0 0 refresh cycle
1 1 0 1 memory data read
1 1 1 1 memory data write
*The byte address is 2 for a halt and 0 for a shutdown. For both conditions, BHE# is high and BLE# is low.