Motorola MB68k-100 User Manual page 10

68000 Motherboard User's Manual Rev. A
Figure 2: Data Flow Diagrams, Read
The mechanics of the Write operation, shown in Figure 3 below, closely follow the Read.
The target device is specified by its address. But the Read/Write line here indicates a
Write operation is to be executed. The Data Bus this time is driven by the processor with
the data to be written to target. The target device is selected by the decoding logic based
on its address, and it reads the data from the processor. The cycle is then closed with the
processor receiving the acknowledge signal that the Write operation is complete. In this
example, the light bar output driver is updated by the processor. Through these steps, the
input data is transferred to the light bar output.
Figure 3: Data Flow Diagrams, Write
The devices within the computer are organized under the control of the microprocessor.
The processor addresses the device required for the current operation, completes the data
transaction with that device and then continues to the next operation. The sequence of
operations that the microprocessor follows is the software program. The program is
comprised of a series of steps that coordinate the computer's operation. These steps are
selected from among the set of basic functions that are directly executed by the processor.
Examples of these fundamental operations include reading data from memory or an input
device, doing arithmetic operations on data, making decisions on which part of the
program to execute next, and writing data to memory or output devices, to name a few.
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