AUTOFADE

from Your Sinclair #4 (Apr.1986)



FADE AWAY



Have you got a burning desire to be on the telly? Well,

you're going the right way about it - unless you use this

program. Andy Pennell's AutoFade provides the perfect

protection policy for your TV.





Have you ever wondered what happens to your telly if you

decide to go walkabout while you're in the middle of a spot

of programming. Sod's law says that something always crops

up right in the middle of what you're doing - nature calls,

the cat dies, the telegram arrives to tell you that your

premium bond's just come up. Well, if you've just won a

hundred grand you're not going to worry too much about your

TV, but for the rest of us it's worth knowing what happens

if the Speccy's left on too long untouched. Well, if you

leave any image on the screen for a long time you may affect

the phosphor in the TV tube and that can result in a faint

image being burnt permanently into your screen.



So, how do you avoid it happening? Well, what you need is a

program that turns the screen off if you don't do anything

for a while. And that's exactly what this program, AutoFade,

does. If you don't touch a key for a certain period - you

can choose any time lapse up to twenty minutes - the screen

will go completely black. Well, all except for a single

flashing white square that lets you know what's happened.

The next time you press a key the screen will be restored to

its previous state. The idea for the program was suggested

to me by a YS reader who'd seen a similar routine for the

Beeb - the ancient historians amongst you may remember the

machine! As usual my filing system fell over and I lost the

original letter, but thanks anyway whoever you may be.





INTERRUPT US!



You've probably guessed already that the program uses

interrupts to do its stuff. And I use my interrupts so that

they work regardless of what add-ons are connected,

including Interface 1. That's what makes the program not

only genuinely useful but also a good illustration of how to

handle Interrupt Mode 2 neatly.



The usual problems with IM2 type routines is that they don't

work with non- Sinclair peripherals. That's because the byte

that's read when the Z80 tries to get the vector address is

not FF as normal but it can be any value. To take into

account all the values from 0 to FF you have to construct a

jump table that allows for all possibilities. And in spite

of what you'll read in many Z80 manuals, bit 0 isn't zeroed

when the vector is to be found. So, the interrupt routine

has to lie at an address that has two bytes the same - I've

used #FDFD. The I vector points to $FE00, which is a table

of 257 bytes of $FD, allowing for all possibilities. Take a

look at the Source Code and you'll see what I'm on about.





The Source



The easiest way of blanking the screen while saving its

previous contents is not to save all 6K of screen data but

just the attribute file of 768 bytes. You can then make the

display file black by zeroing the attributes, leaving the

display bit map well alone. The attributes are stored In the

area SCRBUF.



The INIT routine sets up the vector table and the Interrupt

mode. It also sets TIMER to 1 - this normally increases by 1

every 50th of a second until it reaches its maximum value,

when the screen is blanked, then it's set to 0 until a key

is pressed. The DISFAD routine turns off the fader by going

back to the usual Interrupt mode 1.



INTROU is my version of the Interrupt routine which normally

just scans the keyboard. It starts by calling the ROM

interrupt routine in either the Basic ROM or Interface 1,

then saves the registers before calling the extra routine

MYINT. It then restores the registers and returns in a

similar way to the ROM routine. The JP INTROU must always be

at #FDFD and it must immediately be followed by the 257 byte

vector table at #FE00. All these requirements do mean that

relocating the program to another memory address isn't just

a simple matter of changing the ORG directive. It's a job

that should only be attempted by the brave - or foolhardy!



MYINT is the special interrupt routine that controls the

screen. It starts by testing for a TIMER value of 0, which

means that the screen's been blanked. Now assuming that it

hasn't, it sees if the count has reached the magic number -

altering the LD DE instruction changes the period of the

delay in 50th seconds. If it's not timed out, the timer is

incremented unless a key's been pressed, when it's reset to

1.



BLANK handles the job of clearing the screen which it does

by copying the attributes to the buffer then setting them

all to 0 (black). The border's set to black as well and

you'll find the flashing cursor at the bottom right position

on the screen. It returns with a zero value on HL for TIMER.



TISBLA works when the screen's already blank and tests to

see if you've pressed a key. If you have, the old screen

attributes are restored, the correct border colour is set

and the TIMER is set to 1.





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Another Fine Product transcribed by:

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

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