Page 6 - Issue 6 - Memories ... are made of this

Memories ... are made of this

Mark Hutchinson, Belfast

Issue 6

Nov/Dec 83

When it comes to writing your own programs, a lot more can be done by using POKE statements than can be achieved using Basic. POKEs can save memory, enable you to write faster and let your programs run a bit faster. Following this introduction there is a list of locations that you can experiment with, many of which are not in the Basic Reference Manual. The experts amongst us can skip the introduction and look at the list to see if they can spot any new ones. I make no apologies if you do not, this article is mainly to help beginners.

What exactly is a memory location? It is the numeric position of a Register - a piece of hardware, that holds a bit (no pun intended) of information. Your Atari is what is termed an eight-bit machine which means that each register can hold eight bits (0 to 7) of information stored in BINARY (0 or 1). As all mathematical geniuses know, this is equal to 2 to the power of 8, or 256. This collection of bits is known as a BYTE and each Byte can hold a number up to 255 (0 is included).

Now the complications. A register can be used as a switch or a store. As a store it will hold information in the form 0 to 255, 0 being used for the empty or switch off state. As a switch, you will need to know the output lead that you want to switch. Binary is a representation of decimal numbers in the form 0 or 1 (termed BINARY CODED DECIMAL). This representation starts at 1. The next number is 1*2, or 2, and the next is 2*2, or 4, and so on up to 128. If you look at figure 1, you will see a register with eight leads (1 to 8). If you wish to turn on output 8, you need to make the register hold 128. This is done by POKEing [Register No],128. Similarly, if you wish to turn output 8 off then POKE with 0. You can turn any of the other outputs on by POKEing with the appropriate number but what do you do if you want to turn more than one lead on? Simple, you add together the numbers of the leads you want on. For instance to turn on leads 4 and 6 POKE with 80. Try the short program listing to see how numbers affect the byte.

Why is lead 1 in figure 1 equal to BIT 0? Well, mathematically, the number stored is equal to 2 to the power of the bit. Any number to the power of 0 is equal to 1 incidentally.

AtariLister - requires Java

Let's now take a quick look at appendix 1 of the Basic Manual. In some instances, a number greater than 255 needs to be stored and this is done by assigning two memory locations. The first location (LEAST SIGNIFICANT BYTE) will hold a number from 0 to 255 and when 256 is reached, the second location (MOST SIGNIFICANT BYTE) holds 1 and the first location returns to 0. How do you find out what is stored? Easy.

NUMBER STORED = 1st LOCATION + 2nd LOCATION * 256

Right, that's enough of the introductions, let's get down to some real locations to PEEK and POKE

16

POKE 16,64 to disable BREAK key. Complete keyboard disable is 0. Normal is 192. POKE these numbers into 53774 as well.

65

Input/Output noise flag. Tired of listening to the noise of all those bytes being transferred? Then POKE 65,0.

87

Graphics Mode register. Try 10 GR.18:POKE 87,0 followed by RUN and LIST. The number you poke here refers to the Graphics mode required. It does however depend on DISPLAY LIST pointers and POSITION statements to operate it properly.

106

This location gives the top of RAM in pages. By reducing this amount you can store anything above the new RAMTOP. Basic won't know it's there unless you tell it.

559

Use P=PEEK(559): POKE 559,0 to turn off the screen. POKE 559,P to switch it back on again. This is really Direct Memory Access Control (DMACTL) and is used in PM Graphics. The screen can draw up to 30% faster with the display off.

580

One of the nasties used to protect programs. When you power up, everything in memory is cleared. This is called COLDSTART. Hitting SYSTEM RESET is called WARMSTART and keeps everything in RAM. POKE 580,1 turns SYSTEM RESET into COLDSTART and when you press SYSTEM RESET, goodbye program. Normal is 0.

632-635

Instead of using S=STICK(O), try PEEKing 632-635 for STICK(0-3).

644-647

Try the same for STRIG(0-3).

656

Text window cursor. POKE with 0 to 3 for placing text on a row.

657

Text window cursor. POKE with 0 to 39 for placing text on a column. POSITION statements relate only to the upper area and not the text window.

842

Lets you write lines into a program without stopping, e.g. 10 POKE 842,13:POS.2,13:? "100 SE.2,2,2":? "CONT": POS.0,0: STOP: POKE 842,12. Now RUN it and LIST it.

53279

Console switch register. 7=no key pressed. 3=OPTION. 5=SELECT. 6=START. 4=SELECT and START. 2=OPTION and START. 1 =OPTION and SELECT. 0=all three pressed. Also controls keyboard speaker by POKEing with 0-7 e.g. 10 FOR P=1 TO 50:POKE 53279,0:FOR T=1 TO 40: NEXT T: NEXT P.

53770

Random number generator e.g. 10 ? INT(PEEK(53770)*100).

54273

Character control hardware register. Changes every 50th of a second. Use shadow location 755. Try this 10 POKE 54273,4: FOR T= 1 TO 5: NEXT T: GOTO 10.

You may have noticed a couple of words above which are worth explaining further.

PAGE. When talking about RAM, it means 256 bytes. 256*6=1536, the location of free RAM that you can write to for your own use (just like this magazine). Would you believe that's why they are both called PAGE 6? Page 6 RAM is for storing your own data. It is supposed to be a safe area but this is not 100% correct so be careful.

SHADOW LOCATION. Hardware registers are updated every screen cycle (every 50th of a second). If you were to POKE these registers directly the-information would change after just one cycle. The answer is to write to a location which will store data and pass it to the Hardware register on every pass. This is called the SHADOW. 54273 is a hardware register and if you run the program above you will see it being updated. The FOR . . NEXT loop slows it down for your benefit.

If you study these locations and the locations given in your Basic Manual, then begin to experiment, you should end up with some pretty good programs. Remember, the professionals use these tricks of the trade. One final tip, if you RUN a program and you can BREAK it and LIST it, then do so and study the PEEKs and POKEs that are used. If the writer has used a form such as POKE HM,UO then just go to a free area on your screen and ask your computer to work it out, e.g. ? HM,UO and your ever friendly Atari will tell you what these variables are. POKE a new number into the location, RUN it again and see what happens. You can't damage the computer, at worst you may 'lock-up' and will have to switch off and on again but for this small price you may learn a lot.

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