BUS INTERFACE UNIT

Signals From CPU

 

 

A19:16

4

I

 

3

O

S2:0

I

 

 

 

 

STB

O

 

 

OE

 

AD15:8

8

I

 

 

 

 

 

STB

O

 

 

OE

 

AD7:0

8

I

 

 

 

ALE

 

STB

O

 

 

OE

 

Latched

Address Signals

4 LA19:16

3 LS2:0

8

LA15:8

8

LA7:0

A1102-0A

Figure 3-11. Demultiplexing Address Information

Table 3-1. Bus Cycle Types

 

Status Bit

 

Operation

 

 

 

 

S2

S1

 

S0

 

 

 

 

 

 

 

0

0

 

0

Interrupt Acknowledge

 

 

 

 

 

0

0

 

1

I/O Read

 

 

 

 

 

0

1

 

0

I/O Write

 

 

 

 

 

0

1

 

1

Halt

 

 

 

 

 

1

0

 

0

Instruction Prefetch

 

 

 

 

 

1

0

 

1

Memory Read

 

 

 

 

 

1

1

 

0

Memory Write

 

 

 

 

 

1

1

 

1

Idle (passive)

 

 

 

 

 

3-12

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