MESSAGE MOVER

by Martin Evans

from Your Spectrum 10 (Dec'84/Jan'85)



Have you ever wanted to put a message or instructions for a game

as scrolling text on the 23rd line of the screen display - just like

they do in the professional commercial packages? Well, here's

your chance. This machine code program can be used to display a

string of text which is constantly scrolled so that the words go off

one side of the screen as others enter from the opposite side.

	I've produced an assembly listing of the routine (listed from the

GENS3 HiSoft assembler), but there's also a Basic program which

will do the trick if you've not got an assembler at hand. The Basic

listing of the routine should be typed in directly as it stands; you'll

notice that a number is included at the end of each line of data as a

checksum and, if a mistake is made in your typing, the line number

containing the mistake will be displayed on-screen.

	Once you've typed the program in without error, it would be

wise to save it using SAVE "SCROLL" CODE 60000,95. It might

also be a good idea to save the Basic as well, so that you can make

the suggested changes that follow without corrupting your copy.

	The Basic can now be erased using the command NEW - the

machine code will, of course, still be present as it's positioned

above RAMTOP. To test the routine, type in the second Basic

program given and RUN it - you should now get the contents of z$

scrolling, pixel by pixel, across the 23rd line of the screen display.

To return to Basic, simply press any key.

	To incorporate this routine in one of your own programs, you

should use the second program given as a subroutine, define your

particular message as z$ and then GO SUB to the subroutine. The

machine code is completely re-locatable, but it must not be loaded

to the same address as the data for the message - otherwise you'll

crash the computer.

	As the routine stands the string will be scrolled round and round

until a key is pressed. However, to alter the routine so that the

message is only scrolled once, the value '40' in line 70 of the first

program must be changed to '200'; alternatively, you could load

the machine code, type POKE 60065,200 and re-SAVE the code

as shown above.

	16K Spectrum owners will be pleased to hear that they're not left

out - for once! Simply add these two lines to the second Basic

program:



15 RANDOMIZE (NEW ADDRESS)

16 POKE 60001,PEEK 23670: POKE 60002,PEEK 23671



You'll also have to alter the value '59399' in lines 30 and 50 to a

new value 'new address - 1'.

	As a last note, when POKEing z$ into memory, it's important

that the last address is POKEd with zero to tell the routine where

the end of the message is; look at line 50 of the second Basic

program.





PROGRAM 1

This program allows you to enter the machine code scrolling

routine in Basic. Once you've made sure it's error-free, SAVE

it to tape using the instructions given in the text.



Lines 10-90

Contain the data for the machine code routine. The last

number on each line of data is the value of the checksum -

so you'll know whether a mistake has been made as soon as

you RUN the program.



Lines 95-180

Lower RAMTOP, clear the checksum (b), and READ and

POKE the data making sure the checksum is correct for

each line.





PROGRAM 2

This program allows you to test the routine, providing a message

in z$ which will be scrolled on the 23rd line of the screen display

once the program is RUN. This program can also be used as a

subroutine in your own programming efforts. 16K owners should

read the text for the instructions on how to manipulate this program

for their machines.



Lines 5-10

Place the message (given in quotes) in the array z$.



Lines 20-40

POKE the message in z$ into locations 59399 onwards in memory.



Lines 50-60

Poke the value '0' at the end of the message in z$ and call

the routine.



Line 70

Prints the code of the key pressed.





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Another Fine Product transcribed by:

Jim Grimwood (jimg@globalnet.co.uk), Weardale, England

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