Andrew Kesterson
cdaefcc941
Organised the way the C128 Programmer's Reference Guide is: the language first, then each hardware area, then the reference sections. One markdown file per chapter. The verb and function references are generated from the interpreter's own dispatch table, with an assertion that every row is described, so they cannot drift out of step with what the program accepts. 98 verbs and 30 functions. Every example was run before it was written down, which caught three claims that were wrong: a whole FOR loop on one line prints nothing rather than looping once, MID and INSTR count from zero where a C128 counts from one, and a multi-line DEF returns a value the caller has to assign away. Chapter 13 is the list a BASIC 7.0 programmer needs -- roughly sixty documented differences, including the two known FOR defects and the fact that drawing does not survive a frame. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
3.6 KiB
3. The language
Variables carry their type in a suffix
This is the first thing that will catch a C128 programmer. Every variable name ends in a character that says what it holds:
| Suffix | Type | Example |
|---|---|---|
# |
integer | COUNT#, I# |
% |
floating point | RATE%, X% |
$ |
string | NAME$ |
10 COUNT# = 42
20 RATE% = 1.5
30 NAME$ = "ADA"
There is no such thing as a variable with no suffix. A bare name is a label — see
Chapter 4 — so GOTO DONE and LABEL DONE are how the two meet.
On a C128 the suffixes mean something different (% is integer, no suffix is float).
Here % is float and # is integer, following the Go implementation this was ported
from. Chapter 13 lists it with the other differences.
Variable names are case sensitive. Verb and function names are not: print,
Print and PRINT are the same word.
Numbers
Integers are 64-bit. Floats are IEEE doubles, so they print with six decimal places:
PRINT 1.5
1.500000
Literals may be written in hexadecimal with a 0x prefix. A leading zero is not
octal — 010 is ten, because a leading zero in a listing is far more often padding
than a base.
Strings
Strings are up to 255 characters and are written in double quotes. There is no escaping: a string cannot contain a double quote.
+ concatenates, and it will concatenate a string with a number:
PRINT "COUNT: " + 42
COUNT: 42
* repeats:
PRINT "-" * 20
--------------------
Arrays
DIM makes one. Subscripts start at zero and the number you give is the count, so
DIM A#(3) gives you A#(0) through A#(2):
10 DIM A#(3)
20 A#(0) = 10 : A#(1) = 20 : A#(2) = 30
30 PRINT A#(0) + A#(1) + A#(2)
Arrays can have several dimensions: DIM GRID#(10, 10). LEN(A#) gives the total
number of elements.
An array name used with no subscript means the whole array, which is what SPRSAV and
SWAP take.
Operators
In order of precedence, tightest first:
| Operators | Meaning |
|---|---|
^ |
exponentiation |
- (unary), NOT |
negation, bitwise/logical not |
* / |
multiply, divide |
+ - |
add and concatenate, subtract |
< <= > >= = == <> |
comparison |
AND OR |
bitwise, and logical |
= and ==
Both mean equality inside a condition:
10 IF A# = 5 THEN PRINT "FIVE"
20 IF A# == 5 THEN PRINT "ALSO FIVE"
Outside a condition = is assignment, which is why the distinction has to exist at
all. == works everywhere and is what the older programs in this repository use.
Truth
A comparison yields -1 for true and 0 for false, which is Commodore's convention
and the reason AND and OR double as the logical operators: -1 is every bit set.
Anything non-zero is true, so IF A# THEN ... works:
10 A# = 5
20 IF A# THEN PRINT "NON-ZERO IS TRUE"
30 IF A# = 5 AND A# > 1 THEN PRINT "AND WORKS"
40 IF NOT (A# = 9) THEN PRINT "SO DOES NOT"
AND and OR are still bitwise on ordinary numbers: PRINT 12 AND 10 gives 8.
Comments
REM comments to the end of the line.
10 REM This does nothing at all
Functions you define yourself
DEF makes a single-expression function:
10 DEF SQUARE(X#) = X# * X#
20 PRINT SQUARE(7)
A multi-line definition runs until RETURN, which is how you write a subroutine that
takes arguments:
10 DEF GREET(N$)
20 PRINT "HELLO, " + N$
30 RETURN 0
40 X# = GREET("WORLD")
RETURN carries the value back, so a multi-line DEF is a function even when you only
wanted the effect — assign the result somewhere to throw it away.