c64-basic

name

c64-basic

description

Commodore 64 BASIC V2: programming rules, the three data types (integer, floating-point, string) and their constants/variables/arrays, expressions, arithmetic/relational/logical operators and precedence, string handling, INPUT/GET, program crunching, and the full alphabetical reference of every BASIC keyword (commands, statements, functions, operators) with abbreviations. Use this skill WHEN reading, writing, or debugging C64 BASIC, or asked things like "what does FOR/NEXT do", "how do I DIM an array", "what's the abbreviation for PRINT", "why do I get ?REDIM'D ARRAY", "operator precedence in BASIC", or "is GET blocking". Pairs with c64-petscii (CHR$/screen codes), c64-keyboard (keys & editor), c64-memory-map (POKE/PEEK targets), and c64-assembly (SYS/USR).

Commodore 64 BASIC V2

C64 BASIC V2 (Microsoft-derived, in the $A000–$BFFF ROM) has a compact keyword set — commands, statements, and functions (plus a few operators). Lines are tokenized on entry; abbreviations and spaces vanish at tokenize time. The logical screen line is 80 characters (2 physical 40-column rows) — that's the hard limit on an editable program line. Two modes: DIRECT (no line number, runs on RETURN) and PROGRAM (numbered lines). Always NEW before keying a fresh program.

Data types

Three constant/variable/array types. Type is declared by a suffix on the name:

Type	Suffix	Range / form	Stored as
Floating-point	(none)	±2.93873588E-39 … ±1.70141183E+38, 9 digits shown	5 bytes
Integer	%	-32768 … +32767, whole numbers only	2 bytes
String	$	0–255 chars; "" is the null string	3 bytes + 1/char

• Arithmetic is always done in floating point; integers are converted in and the result converted back if assigned to a % name.
• Numeric constants take no commas (32000, never 32,000) — a comma gives ?SYNTAX ERROR. Scientific notation: 3E3 = 3·10³ = 3000.
• A string can hold anything except " — use CHR$(34) to embed a quote.

Variable names — the 2-character trap

Names may be any length but only the first two characters are significant. COUNT and COUNTER are the same variable. The first char must be a letter; %/$ may only be the last char. A name must not be, or contain, a keyword (e.g. FORK contains FOR) → ?SYNTAX ERROR.

Arrays

DIM A(n) makes elements 0..n (so n+1 elements). Referencing an undimensioned array auto-DIMs it to 10 (subscripts 0–10); a later explicit DIM then gives ?REDIM'D ARRAY. Out-of-range subscript → ?BAD SUBSCRIPT. Max 255 dimensions, ≤32767 per dimension (memory-limited in practice). Element bytes: 2 (int), 5 (float), 3 + chars (string).

Operators and precedence

Highest to lowest (Table 1-3). Equal precedence evaluates left→right; relational operators have no precedence among themselves.

Prec	Operators	Notes
1	^	exponentiation (3^-2 = 1/9)
2	- (unary)	negation
3	* /
4	+ -	+ also concatenates strings
5	> = < <= >= <>	relational; true = -1, false = 0
6	NOT	bitwise/logical complement (one operand, on its right)
7	AND
8	OR

• Relational/logical results are integers: true is -1, false is 0. So A = (X>5) sets A to -1 or 0, and conditions can be done arithmetically.
• AND/OR/NOT operate bit-by-bit on integers (−32768…32767). A=96 AND 32 → 32; A=64 OR 32 → 96; NOT 96 → -97 (two's complement).
• XOR has no operator — it only exists inside the WAIT statement.
• Strings compare by PETSCII code, left to right; shorter string sorts lower when all else equal; leading/trailing blanks count.

Keyword categories

Abbreviation = type enough letters to be unambiguous, last one SHIFTed. PRINT abbreviates to ?.

Commands (mostly direct mode): CONT LIST LOAD NEW RUN SAVE VERIFY CLR. Statements: CLOSE CMD DATA DEF DIM END FOR…TO…STEP…NEXT GET GET# GOSUB GOTO IF…THEN INPUT INPUT# LET ON OPEN POKE PRINT PRINT# READ REM RESTORE RETURN STOP SYS WAIT. Numeric functions: ABS ATN COS EXP FRE INT LEN LOG PEEK POS RND SGN SIN SQR TAN USR VAL ASC STATUS(ST) TIME(TI). String functions: CHR$ LEFT$ MID$ RIGHT$ STR$ TIME$(TI$). Special: SPC( TAB( FN. Operators: .

Common abbreviations: ?=PRINT, G+SHIFTO=GOTO, GO+SHIFTS=GOSUB, P+SHIFTO=POKE, P+SHIFTE=PEEK, N+SHIFTE=NEXT, F+SHIFTO=FOR. Full list in references/abbreviations.md. Abbreviations save no memory (all keywords tokenize to one byte) and LIST back in full form.

INPUT vs GET

• INPUT A$ / INPUT A prompts and waits for RETURN; only works in PROGRAM mode (direct use → ?ILLEGAL DIRECT). Numeric INPUT fed text → ?REDO FROM START; extra commas → ?EXTRA IGNORED. Limited to ~80 chars (one logical line).
• GET A$ reads one key from the 10-char keyboard buffer and returns immediately — empty string if no key. It does not block. Idiomatic wait:

  10 GET A$: IF A$="" THEN 10
  

• Read the raw key matrix value with PEEK(197) (64 = no key). Stuff the buffer via locations 631–640 + count at 198.

Crunching (making programs smaller/faster)

• Renumber to low line numbers — GOTO 1 is cheaper than GOTO 100 (line refs store digits as bytes).
• Pack statements with : up to 80 chars; drop all spaces; remove REMs.
• Replace repeated literals with a 1–2 char variable (V=54296:POKEV,15).
• Use ? and other abbreviations to fit a line during entry (re-LIST inflates them; don't edit an over-80-char line afterward — excess is lost).

Gotchas

• Two-char variable significance silently aliases names — pick distinct first 2.
• String garbage collection can freeze the machine for seconds when free string space fills (BASIC V2's GC is O(n²)); FRE(0) triggers it. Reuse string vars and minimize churn in tight loops.
• Integer arrays save space (2 bytes vs 5) but integer scalars are slower (converted to float for every operation) — use % for big arrays, not loop vars.
• DEF FN must be executed before the function is used → ?UNDEF'D FUNCTION.
• An undimensioned array first touched in a loop auto-DIMs to 10 → later DIM errors with ?REDIM'D ARRAY.
• Largest result before ?OVERFLOW is 1.70141183E+38; underflow silently → 0.

Worked one-liners

• "Wait for any key": 10 GET K$:IF K$="" THEN 10
• "Random integer 1–6": R=INT(RND(1)*6)+1
• "Is it numeric?": ?VAL("12A") → 12 (stops at first non-numeric).
• "Embed a quote in a string": ?CHR$(34)"HI"CHR$(34)
• "Call machine code at $C000": SYS 49152

How to read the references

• references/basic-rules.md — PRG Ch1 verbatim: constants/variables/arrays, expressions, all operators with truth tables, the precedence table, string ops, data conversion, INPUT/GET tutorials, and the full crunching section. Read for the authoritative wording and worked examples.
• references/keyword-reference.md — PRG Ch2 verbatim: Table 2-1 (keyword / abbreviation / function type) plus the alphabetical "Description of BASIC Keywords" with TYPE, FORMAT, Action, and EXAMPLES for every keyword A–WAIT. This is the file to open for the exact syntax/behavior of any one keyword.
• references/abbreviations.md — PRG App A: the complete keyword→keystroke abbreviation table (and on-screen appearance). Read when typing fast or decoding an abbreviated listing.
• references/converting-basic.md — PRG App J: porting other-dialect BASIC (string DIM, A$(I,J) substrings, B=C=0, backslash separators, MAT).
• references/error-messages.md — PRG App K: every BASIC error message and its cause. Read this to diagnose a runtime/syntax error.

Cross-links

• Character codes used by CHR$/ASC/PRINT, and POKE-to-screen codes → c64-petscii.
• The physical keys and the screen editor's quote/insert modes → c64-keyboard.
• What addresses POKE/PEEK actually hit (VIC/SID/CIA, RAM banking) → c64-memory-map.
• Writing the machine code that SYS/USR calls → c64-assembly.