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CHAPTER 5 DEVICES
5
5.3 Internal System Devices
5.3.1 Function devices (FX, FY, FD)
(c) Function register (FD)
The function register is used for data writing or reading between a subroutine program and a calling program.
The CPU module auto-detects the input or output conditions of the function register. Source data are input data
of the subroutine program. Destination data are output data of the subroutine program. The function register of
one point can occupy up to four words. Note that, however, the number of words used differs depending on the
instruction in the subroutine program.
• A one-word instruction uses one word only.
• A two-word instruction uses two words.
• At a destination using 32-bit multiplication or division, four words are used.
• Word devices of the CPU module can be used.
In subroutine programs with argument passing, do not use any devices that are used by the function register.
If this occurs, function register values will not be normally passed to the calling program.
Data are stored in D0 (1 point).
Data are stored in D0 and D1 (2 points).
Data are stored in D0 to D3 (4 points).
Since D0 to D3 are used for FD0, D3 cannot be
used in the subroutine program.
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