Locomotive BASIC

CPC Start Screen showing Locomotive copyright and BASIC version

Locomotive BASIC was a BASIC interpreter for the Amstrad CPC range of computers.

Description

Locomotive BASIC, was one of the best and fastest BASIC implementations of the era. The language benefited both from a clean, well-thought out implementation of the core language by Locomotive, and by the excellent firmware of the CPC, which lent most of its advanced features to the BASIC.

Unlike the competing Commodore 64, it featured a comprehensive graphic capabilities with its PLOT, DRAW, PAPER, INK, PEN, BORDER and (in BASIC 1.1) FILL commands. It had extensive sound commands, granting control of the AY-3-8912 via the firmware's volume and tone envelope system. With the SOUND command, you could select channels, set envelopes, pitch, noise and volume. That was something unmatched by other computers of that era.

Also there was simple interface for memory management, with MEMORY and LOAD commands. The latter allowed for loading of raw screen data, thus providing easy picture showing. Both through this (combined with CALL, PEEK and POKE) and the firmware's RSX system, it was easy to mix BASIC and assembly code, thereby speeding up programs by coding the slowest parts directly in machine code. Many successful programs, including games such as Radzone and applications such as PowerPage, made use of this technique.

With DEF FN, ON variable GOTO and ON variable GOSUB, Locomotive BASIC provided the rudiments of "structured programming", though nowhere near the extent of the PROCedures of BBC BASIC.

All in all, if you compare BASIC interpreters of that era, the Locomotive's seems to be the best 'all-rounder' regarding the combination of speed and complexity, and still some of its features were unmatched by others.

History

The CPC implementation of Locomotive BASIC was developed directly from Locomotive Software's existing Z80 BASIC. The existence of this is cited as one of the reasons Locomotive requested that Amstrad change the CPC's processor from a 6502 to a Z80.

The 464 shipped with BASIC 1.0 on ROM.

The language was revised and debugged for the 664, 6128 and Plus machines to become BASIC 1.1. Changes were minor but significant for the programmer, and included:

• DEC$ bug removed (in BASIC 1.0, it required two opening brackets and was undocumented)
• Better handling of string arguments to RSXs (|DIR,"*.BAS" rather than a$="*.BAS":|DIR,@a$)
• DATA statements can appear anywhere within a line; in BASIC 1.0, they had to be at the end of a line
• FILL command (fill area with solid colour)
• COPYCHR$ function (fetch character from screen)
• Better garbage collection
• Some number-handling bugs removed (e.g. in FOR loops with negative start/end values)
• FRAME (CALL &BD19)
• Extra, optional 'plotting mode' parameter for DRAW/PLOT commands (supported only through control codes on BASIC 1.0)
• GRAPHICS PAPER, GRAPHICS PEN commands
• ON BREAK CONT (disable ESCape)
• CLEAR INPUT (flush keyboard buffer)
• The AUTO command show the whole line if it exist, it only printed a * on the 464

Some parts of 'BASIC' were actually housed in the firmware ROM, but were not officially accessible to other programs. This included the line editor.

The 'pure BASIC' parts of Locomotive BASIC - i.e. those not concerned with CPC-specific firmware and hardware features - were upgraded to become Mallard BASIC, the CP/M language shipped with the PCW. This also featured exceptionally advanced random-access file handling, a feature missing from the CPC.

Command list

Commands and operators

AFTER I[,t] GOSUB Ln

Waits for i/50 seconds and then jumps to the subruotine at line Ln.

AUTO [Ln, i]

Automaticaly generates line numbers starting at line Ln with increment i between line numbers.

Use [ESC to leave AUTO mode. Default value for Ln and i is 10.

Example:

AUTO 100,5 - generates line numbers 100, 105, 110...

BORDER color

Changes the color of the border.

CALL add[,list of parameters]

Allows an externally developed subroutine to be called by BASIC

Example:

CALL 0 - resets the computer completely

CAT

Displays the names of all existing programes on the tape or disk.

Examples:

CAT [ENTER] - lists all disk files in alpha-numeric order

TAPE [ENTER]

CAT [ENTER] - lists names of all tape files in their storage order

CHAIN "filename"[,ln]

Enables the specified program to be loaded and RUN automatically. If the optional parameters ln is specified, the program execution will commence from line ln.

CHAIN MERGE "filename"[,ln][,DELETE1 nl - 1n2]

Loads the specified program from tape or disk, merges it into the program in memory, and starts execution of the merged program. The parameter DELETE1n1 - 1n2 is used to delete part of the original program before running it, if required.

CLEAR

Clears all variables from memory, leaving the program in memory unchanged. All open files are abandoned.

CLG [ink]

Clears the graphics screen to colour specified by ink. If parameter ink is not specified them the graphics screen is cleared to the colour specified by the GRAPHICS PAPER statement.

CLOSEIN

[...]

CLOSEOUT

[...]

CLS

Clears the screen. The text cursor is moved to the upper left corner.

CONT

[...]

CURSOR

[...]

DATA x1[,x2,x3...]

Defines a data section to be used by READ calls.

Data values can be of any type (integer, real or string) as long as the corresponding READ calls use a variable of the right type.

Example:

10 DATA "Hello, world!", 42
20 READ message$:PRINT message$
30 READ answer:PRINT "The answer is:";answer

See also: READ, RESTORE

DEF

[...]

DEFINT

[...]

DEFREAL

[...]

DEFSTR

[...]

DEG

Switch to degrees mode for trigonometric functions (SIN, COS...).

See also: RAD

DELETE [line]

Deletes the current program completely (without argument) or only the given line.

DI

Disables interrupts (but not[ESC]) until re-enabled by EI command or by RETURN at end of an interrupts service routine.

DIM

[...]

DRAW x,y [,[i1][,i2]]

Draws a line from the current cursor position to position x,y. i1 specifies colour, i2 is the logical colour.

i2 = 0   normal colour   i2 = 2 AND colour
i2 = 1   XOR colour      i2 = 3 OR colour

Example:

CLG 2

DRAW 500,400,0 - draws a line from 0,0 to 500,400

DRAWR xr, yr, [[i1][,i2]]

Draws a line from current graphics cursor position to current cursor x position + xr, current cursor y position + yr. i1 and i2 as DRAW.

Example:

Move 200,200

DRAWR 100,100,0 - draws a line from 200,200 to 300,300

EDIT line

Copies one program line to screen in edition mode.

EI

Enable interrupts which have been disabled by DI

END

Indicates end of program

ENT

[...]

ENV

[...]

ERASE v[$(i1[,i2])

Clears the contents of an array that is no longer required.

ERL

[...]

ERROR i

Returns the error message whose error code number is i.

EVERY

[...]

FILL i

Fills an area of a graphics screen i colour i (0-15). Default value of i is the current graphics pen colour. Only available in Basic 1.1.

FN

[...]

FOR TO STEP NEXT

[...]

FRAME

Smooths character and graphics movement and reduces flicker (waits for a VSYNC signal). Only available in Basic 1.1. On a CPC 464 you can use CALL &BD19 instead.

GOSUB i

Jumps to subroutine which is given as argument.

Example:

10 PRINT "Calling subroutine"
20 GOSUB 100
30 PRINT "Back from subroutine"
40 END
100 REM Begin of the subroutine
110 PRINT "Subroutine started"
120 RETURN

GOTO i

Jumps to the line number which is given as argument.

Example:

10 GOTO 100
20 REM not executed
30 REM not executed
100 PRINT "Hello World!"

GRAPHICS

[...]

IF THEN ELSE

[...]

INK

[...]

INPUT

[...]

KEY

[...]

LET

[...]

LINE

[...]

LIST

[...]

LOAD

[...]

LOCATE x,y

Moves the text cursor to the x,y location.

x starts at 1 on the left and goes up to 20 (in mode 0), 40 (in mode 1) or 80 (in mode 2).

y starts at 1 at the top and ends at 25 at the bottom.

MASK

[...]

MEMORY

[...]

MERGE

[...]

MID$

[...]

MODE

Changes the screen mode: MODE 0 is 160x200 in 16 colors, MODE 1 is 320x200 4 colors and MODE 2 is 640x200 2 colors.

MOVE

[...]

MOVER

[...]

NEW

[...]

ON BREAK

[...]

ON ERROR

[...]

OPENIN

[...]

OPENOUT

[...]

ORIGIN

[...]

OUT

[...]

PAPER

[...]

PEN

[...]

PLOT

[...]

PLOTR

[...]

POKE

[...]

PRINT

[...]

RAD

Switch to radians mode for trigonometric functions (SIN, COS...).

See also: DEG

RANDOMIZE [seed]

Resets the pseudo-random generator to the given seed. What is strange is that if no seed is given, one is interactively prompted for.

A common idiom to have a random random seed is to do:

RANDOMIZE TIME

READ variable

Gets the next data item (from DATA commands), stores it in the given variable and moves to the next item.

The variable must be of the correct type.

See also: DATA, RESTORE

RELEASE

[...]

REM [any text]

Introduces a comment.

RENUM [newLine],[oldLine],[step]

Renumbers the lines of the current program.

By default, the whole program is renumbered starting at line 10 with multiples of ten. It is important to note that jumps (GOTO, GOSUB and the like) are automatically converted to the new line numbers.

The whole set of parameters can be used to renumber only the last part of a program.

Example:

10 GOTO 20
20 GOTO 30
30 GOTO 10

becomes, after calling RENUM 100,20,5

10 GOTO 100
100 GOTO 105
105 GOTO 10

RESTORE [line]

Resets the data pointer used by READ. When used without parameters, resets the pointer to the first data in the program. Otherwise, resets the pointer to the given line number.

Example:

10 DATA 10,11,12,13,14
20 DATA 20
READ i:PRINT i
 10
Ready
READ i:PRINT i
 11
Ready
RESTORE
Ready
READ i:PRINT i
 10
Ready
RESTORE 20
Ready
READ i:PRINT i
 20
Ready

See also: DATA, READ

RESUME

[...]

RETURN

[...]

RUN [line]

Runs the current program, optionally starting at a given line. If no line number is given, starts at the first line.

SAVE

[...]

SOUND

[...]

SPC

[...]

SPEED

[...]

SQ

[...]

STOP

[...]

SUB

[...]

SWAP

[...]

SYMBOL n,i1[,i2,i3,i4,i5,i6,i7,i8]

Redefines the appearance of the character at index n.

Each of the following eight integers defines the contents of one pixel row, starting at the top of the character. Each character fits in an 8x8 pixel grid. Missing lines are considered as empty.

Example:

SYMBOL 255,255,129,129,129,129,129,129,255
PRINT CHR$(255)

defines character 255 to look like an empty square and prints it.

Initially, only characters with indices ranging from 240 to 255 can be redefined. See also SYMBOL AFTER to allow redefinition of arbitrary characters.

SYMBOL AFTER n

Allows the redefinition of character symbols from index n included.

Symbol redefinitions are made using the SYMBOL command above. Initially, only symbols from index 240 to 255 can be redefined. This initial situation can be restored with SYMBOL AFTER 240. Symbol previously redefined are restored to their original appearance.

TAB

[...]

TAG

[...]

TAGOFF

[...]

TROFF

[...]

TRON

[...]

USING

[...]

WAIT

[...]

WHILE WEND

[...]

WIDTH

[...]

WINDOW

[...]

WRITE

[...]

ZONE

[...]

Operators

AND

[...]

MOD

[...]

NOT

[...]

OR

[...]

XOR

[...]

Functions

ABS (n)

Returns the absolute value of n by ignoring the sign value.

Example

PRINT ABS(-3.5) - prints 3.5

ASC

[...]

ATN (n)

Returns the arctangent of n.

BIN$ (i1,[i2])

Returns binary representation of i1 between -32768 and 65535. The number of binary digits (0s and 1s) is specified by i2 (0-16)

Example:

PRINT BIN$(66,8) - prints 01000010

CHR$ (n)

Returns the character for a given index n. For instance CHR$(65) returns the character 'A'. Valid indices range from 0 (zero) to 255.

As an example, try the following basic program :

10 print chr$(208+rnd(2));:goto 10

It will draw a random maze with characters 208 and 209, which are an horizontal and a vertical bar.

CINT

[...]

COPYCHR$

[...]

COS

[...]

CREAL

[...]

DEC$

[...]

DERR

[...]

EOF

[...]

ERR

[...]

EXP

[...]

FIX

[...]

FRE

[...]

HEX$

[...]

HIMEM

[...]

INKEY

[...]

INKEY$

[...]

INP

[...]

INSTR

[...]

INT

[...]

JOY

[...]

LEFT$

[...]

LEN

[...]

LOG

[...]

LOG10

[...]

LOWER$

[...]

MAX

[...]

MIN

[...]

PEEK

[...]

PI

[...]

POS

[...]

REMAIN

[...]

RIGHT$

[...]

RND [(n)]

Generates the next random number in the current squence if n is positive or omitted. If n = 0, the random number generated will be the same as the last random number generated.

ROUND (n[,i1])

Rounds n to a number of decimal places or to the power of ten specified by i. If i is negative, the n is rounded to give an absolute integer with i zeros before the decimal point.

Example

PRINT ROUND(1562.357,2):PRINT ROUND(1562.375,-2) - prints 1562.36 1600

SGN (n)

Returns 1 if n is positive, 0 if n = 0, -1 if n is negative.

SIN (n)

Returns sine of n in degree or radian mode (see DEG and RAD)

SPACE$

[...]

SQ (channel)

Returns a bit significant integer showing state of the sound queue for specified channel where channel 1, 2, 3 = A, B, C.

Bits 0,1 and 2     number of free entries in the queue
Bits 3,4 and 5     redezvous state at head of the queue
Bit 6              head of the queue is held
Bit 7              channel is currently active

SQR (n)

Returns the square root of n.

STR$

[...]

STRING$

[...]

TAN

[...]

TEST

[...]

TESTR

[...]

TIME

Returns time elapsed since the computer was switched on or reset.

One second = TIME/300.

UNT

[...]

UPPER$

[...]

VAL

[...]

VPOS (#st)

Reports the current row (line) position of the text cursor relative to the top of the text windows of the specified stream.

XPOS

[...]

YPOS

[...]

(Please, fill in. Looks like a lot of work ;-) ...)

Other Basic Dialects avaliable for the CPC

BBC Basic

Web links

• Technical information at the Unofficial Amstrad WWW Resource
• Locomotive Basic Tutorial by Sean McManus