184
A subroutine can be executed by placing SBS(91) in the main program at the
point where the subroutine is desired. The subroutine number used in
SBS(91) indicates the desired subroutine. When SBS(91) is executed (i.e.,
when the execution condition for it is ON), the instructions between the
SBN(92) with the same subroutine number and the first RET(93) after it are
executed before execution returns to the instruction following the SBS(91)
that made the call.
SBS(91)
00
SBN(92)
00
RET(93)
END(01)
Main program
Subroutine
Main program
SBS(91) may be used as many times as desired in the program, i.e., the
same subroutine may be called from different places in the program).
SBS(91) may also be placed into a subroutine to shift program execution
from one subroutine to another, i.e., subroutines may be nested. When the
second subroutine has been completed (i.e., RET(93) has been reached),
program execution returns to the original subroutine which is then completed
before returning to the main program. Nesting is possible to up to sixteen
levels. A subroutine cannot call itself, (e.g., SBS(91) 00 cannot be pro-
grammed within the subroutine defined with SBN(92) 00). The following dia-
gram illustrates two levels of nesting.
SBN(92) 10
SBN(92) 11
SBN(92) 12
SBS(91) 11
RET(93)
SBS(91) 10
SBS(91) 12
RET(93)
RET(93)
Although subroutines 00 through 31 can be called by using SBS(91), they
are also activated by interrupt signals from Interrupt Input Units. Subroutine
99, which can also be called using SBS(91), is used for the scheduled inter-
rupt. (Refer to the next subsection for details.)
Description
Subroutines and Interrupt Control
Section 5-20
