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   N=0
1  WHILE N LT 10
   N=N+1
   PRINT"N = ";N
   GOTO 1
   WEND \SKIPS OUT WHEN N = 10
   PRINT"THAT'S ALL, FOLKS!"
   END

WHILE/​WEND loops can be nested in the same way as FOR/​NEXT loops.

‘Subroutines’

The use of GOSUB…RETURN as a form of ‘subroutine’ in BASIC has been the subject of much hot air and plonking arguments, culminating in the DEFPROC/​PROC/​ENDPROC to be found in BBC BASIC. If used properly, there is nothing wrong with GOSUB, whether straight or computed. CBASIC has an additional feature over MBASIC and this is the multiple-line function. This clever device enables those who are allergic to the use of GOSUB to define the ‘subroutine’ function as follows: (note the full stop between the FN and the dummy argument)

DEF FN.CROSS.SECTION(AREA)

In this case, the function name {in this case, CROSS.SECTION) is followed by a dummy argument, AREA, which is a real numeric variable used within the function and Cross Section is treated as a variable which may have values assigned to it within the function itself — in this way, values of variables needed within the ‘subroutine’ can be input without the need to define them in assignments. Constants or expressions cannot be used as dummy arguments — only variables can be used in this way. The variable can be a string, integer or real type and is treated as local to the function, and only able to be modified within the function. The function must be terminated with a RETURN, followed by FEND; FEND is not executed under normal circumstances but if RETURN is missed out, then CRUN will send an error message and return to CP/M.

Initially, the use of multiple line functions seems a little difficult but with experience, the advantages of the facility become apparent. Essentially, the CBASIC function follows the Fortran method. The User Guide is reasonably clear on the use of this type of function, and provides some examples of its application.

Overlaying and Chaining

In spite of the suggestive names, these have nothing at all to do with S★X, kinky or otherwise! Overlaying is employed when a program is too big to fit into the available memory; the program is broken down into smaller units, called overlays and these are stored as separate, compiled programs. An overlay is called when the previous program reaches a CHAIN statement which explicity names the overlay in question before it completes its execution. The new overlay can end by calling yet another overlay, or the previous one or even by stopping program execution. Two important points should be borne in mind before using overlays in CBASIC.

1. An overlay overwrites the previous program but the previously used program can pass data to the overlay by means of COMMON statements which leave the data in memory; these COMMON statements must be in both the calling program and overlay.
2. Memory usage is slightly restricted by the use of overlays; full details of this are to be found in the CBASIC User Guide.

CHAINing allows COMMON variables to remain intact while the area of memory used for storage of file buffers, strings and arrays is compacted. If the %CHAIN directive is used, the system cunningly instructs the first of a series of programs to determine the maximum storage capacity of all following program overlays; this maximum figure will be used to determine how much memory will be allocated to each of the areas required by the program suite.

In its simplest form, CHAIN is used as follows:

REM FINISH PREVIOUS PROGRAM THEN
REM LOAD NEXT OVERLAY
CHAIN "NEWPROG"

An arrangement which allows an overlay to be loaded from a different disk drive follows; this assumes that the logged in drive is A and that the next program resides on drive B:

DISK.DRIVE$ = "B:"
INPUT"NAME OF NEXT PROGRAM";PROGRAM.NAME$
CHAIN DISK.DRIVE$+PROGRAM.NAME$

The COMMON statement is not found in other BASICs, and its use follows the Fortran precedent as being the first statement in a program, agreeing in type and sequence in both sending and receiving programs. Thus if variables A, B$ and C% are produced by the sending program, the receiving program must have either the same variables in the same order or different named variables of the same type in the same order, i.e. X, Y$ and Z%.

Array variables in COMMON must be followed by one parameter which specifies the number of array subscripts, and these must appear in a DIMension statement.