Data Subsystem

A data subsystem for the incdec example is shown in Figure 91.

**Figure 91:** `incdec` data subsystem.
$\begin{figure} \begin{center} \epsfig{file=images/rtl-incdec-4.eps}\end{center}\end{figure}$

This data subsystem consists of a number of functional modules together with input, output and control signals. These facilitate the actions in the algorithm.

reg x, reg y - registers to store the x and y values. These registers are loaded on assertion of ldX, ldY respectively.
left2 - multiply by 4 (by shift left twice)
inc - to increment by 1
comp - compares values, so k=1 if x < y; zero otherwise.
abs - absolute value
dec - to decrement by 1
mux1 - a 2-input multiplexer
mux2 - a 3-input multiplexer

The data subsystem is controlled by the following control points:

`ldX`	load register x
`ldY`	load register y
`selX`	select input to x (1 bit)
`selY`	select input to y (2 bit)

These control points are outputs of the controller subsystem.

The data subsystem provides the following condition for the control subsystem:

`k`	k=1 if x < y; zero otherwise

This condition signal is an input to the control subsystem. The external environment provides the following control input for the control subsystem:

start external signal to start execution

The modules used in the data subsystem may come from a standard parts library, or designed specifically.

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