Loadstar 27/027.d81/t.prebase boot

Text File | 1986-01-01 | 5.8 KB | 392 lines

              PREBASE
                by
          Michael Leidel


COMMODORE POWER/PLAY MAGAZINE
August/September, 1986

**************************************
NOTE: You must copy PREBASE, PASS2,
and PREBASE BOOT to another disk
before using this program.
**************************************


THE BIG PICTURE......

  If you've been used to writing

programs in languages like FORTRAN and

Pascal, PREBASE might make writing

BASIC programs easier for you. PREBASE

is a BASIC preprocessor that allows

you to write programs that use

structured control keywords like

WHILE-ENDWHILE, LOOP-ENDLOOP, or

LOOP-UNTIL that you've known from

other high-level languages.


  PREBASE also allows the use of

English labels for branching instead

of line numbers, provides a directive

for including prewritten library

routines, and removes all REM

statements and extra spaces.


  Once you have created a PREBASE

program file, Leidlel's two programs

("PREBASE" and "PASS 2") will convert

the PREBASE source file to a normal

BASIC program.


   We have included a sample PREBASE

program, "PREBASE TEST". To see it,

run PREBASE, and enter its name as the

program you want converted.


WRITING PROGRAMS IN PREBASE

  Writing PREBASE programs is similar

to writing BASIC programs-- there are

only a few more rules to keep in mind.


-- Line numbers should only be used

for sequencing lines, never after a

THEN, GOTO, or GOSUB.  Instead,

English labels should follow a GOTO or

GOSUB; however, labels should never

follow a THEN, although other BASIC

statements may.


-- The target label should be the only

entry on the line.  It can be composed

of any combination of regular or

special characters and must be unique.

Resequencing line numbers will not

affect program control logic.


-- Nothing must come before or after a

structured control construct on a

line.  The conditional following WHILE

or UNTIL must be preceded by one

space; nothing must follow the

conditional.


-- Nesting may not exceed ten levels.

WHILE-ENDWHILE provides a condition

check at the beginning of a loop,

LOOP-UNTIL a condition check at the

end of a loop.  LOOP-ENDLOOP provides

a closed loop that must be exited by a

GOTO.


-- PREBASE will merge in other PREBASE

files during its processing cycle.  To

use this "include" directive feature,

enclose the name in quotes and then in

square brackets.  For example,


100 ["filename"]


will merge in the program file at line

100 of the program using this

directive.


  Nothing else may precede or follow

this directive on a line.  The file

used in the directive cannot itself

contain a directive of this type.


-- Any line that is not a label,

structured control construct,

"include" directive, or REM statement

must begin with a period.  This

distinguishes a regular line from a

line needing special attention.


-- A shifted space may be used instead

of a period to begin a regular line.

However, if you re-edit a line, the

shifted space will be removed.  Each

time the line is edited, the space

must be replaced.  A line beginning

with a period does not require this

attention.


-- In the translation process, PREBASE

removes all REM statements and extra

spaces, converts all structured

constructs into regular BASIC

statements, makes branch labels

correspond with line numbers, and

integrates library routines that were

merged into a normal BASIC file.  The

translated program is written to the

disk with the name of the original

PREBASE file plus a ".BAS" extention

that marks it as a file that was once

a PREBASE file.


-- PREBASE will not check normal BASIC

syntax or logic errors, but will

indicate certain errors relevant to

its special features.  If these kinds

of syntax errors are found, the errors

are listed and the final program is

not produced.


USING PREBASE

  The first of the two programs,

"PREBASE" requests the date, the name

of the PREBASE program to be

translated, its beginning line number,

and the increment for the BASIC output

file.  It then parses through both the

PREBASE source file and any "included"

library files to make a file called

SYMTBL on disk.


-- If you have the same name for two

different branch labels, you will get

a DUPLICATE LABEL error, and the

program will not execute the second

phase.  To fix, change one of the

labels.


  The second part of the program,

PASS 2, uses the information in the

SYMTBL file to construct a normal

BASIC program.  The size of the

BASIC program will depend on how many

REM statements and spaces were removed

and how many "include" directives were

used.


  If PASS 2 encounters errors, they,

along with the line at which they

occur, are displayed at the end of

processing and the BASIC program is

not created.  If it finds ten errors,

PASS 2 will stop at that point and

list them.


POSSIBLE ERRORS

-- Incorrect formation of a construct
   or label:


LOOP ERROR

ENDLOOP ERROR

INVALID LABEL

UNTIL ERROR

ENDWHILE ERROR


-- Missing part of a structured
   construct:


NO MATCHING LOOP

MISSING LOOP

NO MATCHING ENDWHILE

NO MATCHING WHILE


-- Reference to a nonexistent label:

UNDEF LABEL


  PASS 2 may crash if a line does not

begin with a REM, a structured

construct, an "include" directive, or

a period.  If this happens and you

want to examine the error list, GOTO

8020.


  Examining the SYMTBL file is also

helpful.  To do this, type in

immediate mode:


FORX=1TOLC:?LBL$(X),NUM$(X):NEXT


  The last thing PASS 2 does is create

two special REM statements as the last

two lines of the new BASIC program.

The first REM contains the name of the

program and the date.  The second REM

contains the ID string coded into

line 5 of the PASS 2 program.  These

REMs will have invalid line numbers so

they cannot be deleted from the

program.


DISK FILES USED:

PREBASE           PREBASE TEST
PASS 2

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