| 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: AND OR NOT.
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
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.
