Expanded Basic



              Nine new commands for Spectrum

             Basic, courtesy of Robert Newman



I have written a machine-code routine which adds nine new

commands to Spectrum Basic which will run with or without

Interface 1.

  The machine-code routine must be loaded and initialised

before either writing a program, or loading a program which

uses the new commands. If you do this it is not necessary

to save the machine code again when you save your program.

  A short Basic program, listing 1 (16K) or listing 2

(48K), is used to load the machine code above RAMtop, and

perform the initialisation. This program should be typed

in, and then saved on a cassette or Microdrive cartridge to

auto-run from Line 3. If you use a Microdrive, the program

can be given the filename "run", ie:

SAVE *"m";1;"run" LINE 3 or

SAVE "BASIC+" LINE 3

  New the computer, then type in and Run Listing 3, the

machine code loader program. Copy the machine code bytes

from either listing 4 or 5 depending upon memory size.

Great care must be taken while copying from the machine

code dump, since a single mistake could cause the computer

to crash.

  You should now have a working copy of the Basic+ inter-

preter. To test it, rewind the take and clear the computer

by typing Randomise USR 0. Then load the initialisation

program and machine code, and type *ZAP which should pro-

duce a laser type sound effect.



 Graphic characters



  *USE (Nchars), (Memory): (Nchars) is the number of diffe-

rent graphic characters that you want to use, and can take

values from 1 to 255. (Memory) is the memory size in bytes

needed for storing those characters. To calculate the

number of bytes required to store each character, divide

its width by 8, and add 1 for any remainder, then multiply

by its height.

  This command causes part of the memory to be reserved for

the storage of your graphics characters. The total number

of bytes required is (Memory) + (Nchars)*4 bytes. For

example, the command *USE 8,256 reserves 288 bytes. If you

try to reserve more memory for your graphics than the spare

memory available, this will halt your program with a run-

time error.

  The graphics memory area is situated just before the

start of the machine code. If necessary, RAMtop will be

lowered - and the machine stack and Gosub stack moved - to

make room for the graphics memory. The routine will only

lower RAMtop, and never moves it to a higher address. This

is to allow your own machine-code routines to be used with

extended Basic provided that they are located between the

graphics memory and RAMtop.

  The *Use command clears the graphics memory and character

size definitions - see *Def - and can be used more than

once in a program for this purpose. If there is no *Use

command in a program, the command *USE 8,256 is assumed by

the routine.

  *DEF (Char), (Width), (Ht): After *Use, the *Def command

is used to set the size of a graphics character. (Char) is

the character number, which can take values from 1 to

(Nchars). (Width) and (Ht) are the sizes of this character

in pixels - from 1 to 256 pixels wide, and from 1 to 176

pixels high. You mist set the size of a character before

using it in one of the remaining commands, which are:-

*GET (char), (xpos), (ypos)

*PUT (char), (xpos), (ypos)

*CLR (char), (xpos), (ypos)

  Lines 2,3 and 4 of the initialisation program can be

deleted once the machine code has loaded, but you must not

delete line 1, otherwise none of the new commands will be

recognised when you Run your program.

  During runtime, if the command cannot be executed, the

error message produced is code Q - "Parameter error".

  *BRK 0 and *BRK 1: These commands allow the break key to

be disabled (*BRK 0), or re-enabled (*BRK 1). [Note that

the parameter has to be a literal 1 or 0. An expression

with the value 1 or 0 won't be accepted.]

  *ZAP: This command gives a laser-type zap sound.

  *NSE (Length): This command gives white noise for a time

depending upon (Length) which can take values from 1 to

255. Depending upon the length, this can sound like an

explosion, gunshot etc. A machine-gun type sound can be

made using a loop - eg.,

        FOR j=1 TO 10: *NSE 50: PAUSE 10: NEXT j

  *SCR (Paper), (Ink): This command changes the Paper and

Ink colours of the screen without erasing its contents.

Paper and Ink can take values from 0 to 9 as with the

normal colour commands.

  The remaining commands are concerned with printing of

user-defined graphics. These can be copied from the screen

into graphics memory with *Get and then printed anywhere on

the screen with *Put. The routine can handle graphics of

any size from 1 by 1 pixels to 256 by 176 pixels, and

caters for up to 255 different graphics characters.

  First, you need to decide how much memory you want to set

aside for the storage of graphics characters. This is done

with the *Use command.

  In all three commands, (Char) is the character number -

the size of which must have previously been defined with

*Def. (Xpos) and (Ypos) are the pixel co-ordinates of a

point on the screen at the top left hand corner of the area

from where the graphic character information is to be

copied from, printed to, or erased.

  The *Get command will copy a character from the speci-

fied position on the screen, and store it in the graphics

memory. It can then be printed anywhere on the screen using

*Put. If the values of (Xpos) and (Ypos) cause part of the

character to go over the edge, then it will wrap-around to

the opposite side of the screen. The *Clr command can be

used to erase a character, or alternatively it can be used

to clear a window on the screen.

  For example, the instructions

        *DEF 1,16,16: *GET 1,0,175: *PUT 1,240,15

would copy an area of the screen 16 by 16 pixels in size

from the top left of the screen to the bottom right.



[ And finally something about the TZX. This starts with the

  extended Basic itself, in both 48k and 16k versions. The

  16k version will not work on a 48k machine.

    Mr. Newman also provided two demonstration programs,

  and these follow on the TZX. They assume that the BASIC+

  machine code has been loaded, but work with either size

  memory. The first, "Demo", is a demonstration of all of

  the new commands. The second I have called "Game"; it is

  a 15-tiles puzzle.

    To run either demo program, load the desired version of

  BASIC+, then simply load the demo over the top. There is

  no need to retain line 1 in this case, since both demos

  have it included, so they will simply auto-run.

    By the way, you may have noted that the demo programs

  use the commands in lower case, while the article gives

  them in capitals or mixed case. In fact, the system will

  accept any case combination.

                                   Richard Bos, May 2012. ]