BASIC Reference

A.C. Wright 6502 BIOS
Commands & Statements

Core Statements

Command	Syntax	Effect
PRINT	`PRINT [item [sep item …]]`	Output to console. `;`=no sep (trailing ; suppresses CRLF), `,`=14-col zone. Bare PRINT=CRLF only
INPUT	`INPUT ["prompt"{;\|,}] var [,var…]`	Read value(s) from user. `?REDO FROM START` on bad numeric input; `?EXTRA IGNORED` if too many
LET	`[LET] var = expr`	Assign expression to variable. `LET` keyword is optional
GOTO	`GOTO linenum`	Jump unconditionally to line
GOSUB	`GOSUB linenum`	Push position and jump. ≥20 levels always, ~27 for a simple one; beyond → `OUT OF MEMORY`
RETURN	`RETURN`	Pop GOSUB stack and resume after calling GOSUB
IF	`IF expr THEN stmt [ELSE stmt]`	Execute THEN if expr non-zero, else ELSE branch. `THEN`/`ELSE linenum` = `GOTO linenum`
FOR	`FOR var=init TO limit [STEP n]`	Counted loop. Default step 1. Up to 8 nested. Limit tested at `NEXT`, so the body always runs once
NEXT	`NEXT [var [,var…]]`	Increment loop var and branch back to matching FOR if condition holds
REM	`REM [text]`	Comment — rest of line ignored
END	`END`	Stop execution and return to `OK`. Variables preserved
STOP	`STOP`	Stop and print `BREAK IN nnnn`. Resume with CONT
CONT	`CONT`	Continue after STOP, END or Ctrl+C (immediate mode only); FOR and GOSUB frames survive. `CAN’T CONTINUE` after an error, NEW, CLR or RUN
ON	`ON expr GOTO l1,l2…`	Evaluate expr, branch to nth target. Also `ON … GOSUB`. Out-of-range silently continues
DATA	`DATA v1,v2,…`	Inline data (numeric or string literals) for READ. Skipped during normal execution
READ	`READ var [,var…]`	Read next value(s) from DATA. `OUT OF DATA` if exhausted
RESTORE	`RESTORE`	Reset DATA pointer to start of program
LIST	`LIST`	Print program in detokenized form. Ctrl+C interrupts
RUN	`RUN [linenum]`	Clear variables and run program, optionally from linenum
NEW	`NEW`	Erase program and clear variables
CLR	`CLR`	Clear variables and arrays; reset stacks. Program is kept
DIM	`DIM var(size) [,var(size)…]`	Dimension a 1-D array (numeric or string). Valid indices 0..size. `REDIM'D ARRAY` if already done
DEF FN	`DEF FN A(X) = expr`	Define single-argument numeric user function. Call with `FN A(value)`
POKE	`POKE addr, value`	Write byte value to memory address addr
BRK	`BRK`	Drop into machine-code monitor. Return to BASIC with `X`

Storage & System

Command	Effect
SYS <addr>	Call machine-code routine at addr; RTS returns to BASIC
LOAD "name"	Load named file from CompactFlash to $0800
SAVE "name"	Save current program to CompactFlash
LOAD	Receive program via XModem on serial port
SAVE	Transmit current program via XModem
DIR	List current disk's directory (prints DISK n header)
DEL "name"	Delete named file from current disk
DISK <n>	Select CF disk bank 0–255. Each disk = 1MB (2048 sectors); 256 disks = 256MB total. Resets to 0 on boot
BLOAD <addr>,"name"	Load file's raw bytes from current disk to memory address addr
BSAVE <addr>,<len>,"name"	Save len bytes from address addr to a named file on current disk
FORMAT	Erase current disk's file directory (prompts ERASE DISK n? Y/N)
BANK <n>	Select 1KB RAM bank n at $8000–$83FE
MEM	Print free bytes, HW_PRESENT (hex), and current DISK n

Video & Display

Command	Effect
CLS	Clear screen; reset cursor to (0,0)
LOCATE <row>,<col>	Move cursor to row 0–23, col 0–39
COLOR <fg>,<bg>	Set TMS9918 text foreground/background colours (0–15 each)

Sound

Command	Effect
SOUND <v>,<f>,<d>	Play tone on voice 1–3 at freq Hz for dur centiseconds, then silence (Kernal `SidPlayNote` numbers them 0–2)
VOL <n>	Set SID master volume (0–15)

Timing & I/O

Command	Effect
PAUSE <n>	Pause for n centiseconds (~10 ms each)
WAIT <addr>,<mask>	Spin until (addr) AND mask is non-zero; Ctrl+C aborts

Time & Date

Command	Effect
TIME	Print current RTC time as HH:MM:SS
DATE	Print current RTC date as CCYY-MM-DD
SETTIME <hh>,<mm>,<ss>	Set the RTC time
SETDATE <cc>,<yy>,<mm>,<dd>	Set the RTC date
NVRAM <addr>,<val>	Write byte to RTC NVRAM at address 0–255

BASIC Reference

A.C. Wright 6502 BIOS
Functions & Expressions

Functions & Expressions

Function	Returns
ABS(x)	Absolute value of x
SGN(x)	Sign of x: 1, 0, or −1
INT(x)	Floor — largest integer ≤ x
SQR(x)	Square root of x (error if negative)
EXP(x)	e raised to x
LOG(x)	Natural logarithm (error if x ≤ 0)
SIN(x) / COS(x) / TAN(x)	Trig functions, radians
ATN(x)	Arctangent, radians
RND(x)	Float [0,1) for x>0; repeats last for x=0; reseeds for x<0
PEEK(addr)	Byte value at memory address addr
FRE(x)	Free bytes between top of var space and string heap (arg ignored)
POS(x)	Current print column (arg ignored)
LEN(s$)	String length
VAL(s$)	Parse s$ as number; returns 0 if not numeric
ASC(s$)	ASCII code of first character of s$
CHR$(n)	One-character string with ASCII code n
STR$(n)	Numeric value n formatted as a string
LEFT$(s$,n)	First n characters of s$
RIGHT$(s$,n)	Last n characters of s$
MID$(s$,start[,len])	Substring from 1-based index start, optional length len
TAB(n)	In PRINT: advance cursor to column n (no-op if already past)
SPC(n)	In PRINT: emit n spaces
INKEY	Non-blocking key: ASCII code or 0. No parentheses
JOY(1) / JOY(2)	Joystick bitmask, bits R L D U Y X B A (7→0). Active low — a held button reads 0, an untouched stick reads $FF. Test with `IF (JOY(1) AND 16) = 0`
NVRAM(addr)	Read byte from RTC NVRAM; returns 0 if RTC absent
HEX(n)	In PRINT: output n as $xxxx hex; in expressions: returns n unchanged
MIN(a,b) / MAX(a,b)	Smaller / larger of a and b
FN A(value)	Call user function defined with DEF FN A(X)
var(index)	Array element access. Array must be DIM-med first

Operators

Operator	Description
+ − * /	Standard arithmetic
^	Exponentiation
+ (strings)	String concatenation
= <> < > <= >=	Comparisons — work on numbers and strings; return −1 (true) or 0 (false)
AND OR NOT	Bitwise logical on integer parts of operands

Operator Precedence (High → Low)

Level	Operators
Power	^
Unary	− (negate) +
Multiplicative	* /
Additive	+ −
Relational	= <> < > <= >=
Logical NOT	NOT
Logical AND	AND
Logical OR	OR

Graceful Degradation

Hardware Absent	BASIC Behavior
CompactFlash	`LOAD`, `SAVE`, `DIR`, `DEL`, `BLOAD`, `BSAVE`, `FORMAT` print `NO DEVICE`
Serial	XModem `LOAD`/`SAVE` return error
GPIO/VIA	`JOY()` returns `$FF` — every line released, as an untouched stick reads. `PAUSE` falls back to a calibrated software loop
SID	`SOUND`, `VOL` silently return
Video	`CLS`, `LOCATE`, `COLOR` silently skip; console auto-switches to serial
RTC	`TIME`, `DATE`, `SETTIME`, `SETDATE`, `NVRAM` (write) print `NO DEVICE`; `NVRAM()` read returns 0

A screen and a speaker have nothing to report back, so those statements skip silently; the ones that move data (CompactFlash, RTC) raise NO DEVICE, because there the program asked for something it did not get. Either way arguments are parsed and range-checked first — LOCATE 24,0 and VOL 16 are ILLEGAL QUANTITY even on a machine with neither card, so a program is wrong or right everywhere.

Notes

Variables: A–Z single-letter numeric (5-byte float, ~±1.7×10³⁸, 6 sig. digits). A$–Z$ strings. Arrays: DIM var(size) — indices 0..size, 1-D only.
Boolean: −1 = true, 0 = false. Multiple statements per line separated by :.
Stack Limits: GOSUB and FOR frames share the 6502 stack with the interpreter: at least 20 GOSUB levels always, ~27 for a simple subroutine · FOR/NEXT 8 nested loops. Exceeding either → OUT OF MEMORY.
Memory Layout: Programs grow up from $0800. Vars follow. String heap grows down from $8000. MEM reports free bytes.
XModem: LOAD/SAVE without filename uses XModem (128-byte blocks, checksum). System prints XMODEM RX/TX READY and waits ~60 s for terminal to begin transfer.
Sign Convention: Positive numbers print with a leading space; negative with −.
Program Files: .bas/.prg both just mean the bytes at $0800; the extension is cosmetic. A .prg is a 10 SYS 2060 stub with machine code attached. LOAD/SAVE round-trip it intact — don't edit its BASIC line, since that shifts the attached code.

HW_PRESENT Flags ($030D)

Bit	Mask	Card / Device
0	$01	RAM card low (IO 1)
1	$02	RAM card high (IO 2)
2	$04	RTC DS1511Y (IO 3)
3	$08	CompactFlash (IO 4)
4	$10	Serial R65C51 (IO 5)
5	$20	GPIO / VIA 65C22 (IO 6)
6	$40	SID / ARMSID (IO 7)
7	$80	Video TMS9918 (IO 8)