First Steps

Mark Hutchinson shows you how in his regular coulum for beginners


Issue 30

Nov/Dec 87


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Welcome back to First Steps. After last issue's break from the trials and tribulations of programming your Atari, it's time to get back to some programming. You'll never learn if we keep chatting about other things!

In previous issues I have covered several graphics topics and by now you should have had ample time to try your hand at the graphic modes and maybe have even tried a little bit of animation. Your program will draw a picture with its own routines (PLOT, DRAWTO), but quite often you may wish to save the picture for use elsewhere, or perhaps you wish to keep the same program routines and load in different backgrounds. So how is it done?


First we need to find out what is on the screen. If we were to set up a GRAPHICS 0 screen with some text on it, we could look at every point on the screen (GRAPHICS 0, 24 lines and 40 columns = 960 points) and obtain the ASCII value of the character using LOCATE. For example,

LOCATE X,Y,Z (The same as POSITION X,Y: GET #1,Z)

LOCATE sets the cursor at position X,Y (defined by the program) and reads the value of the character or point under the cursor. In text modes this will obtain a value for Z ranging from 0 to 255, depending on the character under the cursor. The same technique can be utilised for the graphics modes except that here the value will be 0 or 1 for the two colour modes, and 0 to 3 for the four colour modes. These colours correspond directly to the COLOR register used. Except when using the default colours, the SETCOLOR registers must be stored as well.

LISTING 1 shows how to read the screen, store the details and how to set up the screen after it has been cleared. The program itself is pretty simple and I hope that Steven Wayne agrees that the REM's will be explanation enough. The value (Z) under the cursor is changed to the ASCII value and stored in a string. This makes it easy to print the character directly from the string using a POSITION statement.

AtariLister - requires Java


The previous technique is slow, but even slower when it comes to the higher resolution modes. A better way would be to use screen memory. Each point on the screen has its origin in RAM. Stored in these locations are the details of the points i.e. colour or character. The position of the start of screen RAM is stored in locations 88 and 89 (termed pointers). To find out where exactly this is in your own computer enter the following and press RETURN,

PRINT PEEK(88)+256*PEEK(89)

The number given is the memory location that stores the value of the top left hand corner of the screen. Each point on the screen is stored sequentially in memory from that memory location.

So why use a pointer? Well, when different modes are used the RAM is increased or decreased according to the amount needed for that mode. The computer will shift the start of screen RAM to compensate for this and as it varies so much, you need to store the start of screen RAM somewhere so that you can keep track of where the screen starts. The technique of pointers is used quite a lot in the ATARI architecture.

Location 88 is equivalent to the units in decimal counting and location 89 can be compared to the tens. For instance, 21 in decimal is 1 +(10*2), so location 88 would store the 1 (units) and location 89 would store the 2 (tens). In the case of computers though groups of 256 are used instead of 10's, so a 1 in location 88 and a 2 in location 89 would equal 513. The reason for the magic figure of 256 is simple. The memory chips have eight read/write lines and each line has two states —either ON or OFF (just like a light switch). With two states and any combination of eight lines the mathematicians amongst the readers will deduce that you will have two to the power of eight possibilities — 256!

Directly reading the screen RAM means that we can do away with LOCATE and speed up the program. This can be demonstrated by listing 2. This program is slightly faster due to the reduced amount of program commands, but it is still very slow.

AtariLister - requires Java


So now that we have read our screen, where and how can we store it?

Storage in memory can be in two forms, either as a string or as a variable, both are temporary. The string can be held until filled then transmitted to disk or tape all at once but the variable must be transmitted at once otherwise the FOR/NEXT loop will overwrite it on the next pass.

Using PUT/GET commands with a variable, the data has to be passed to disk or tape one byte at a time. This may seem to be slow but it allows the use of a device called the Input Output Control Block (IOCB). This block is the socket you plug the tape or drive into. It is controlled by certain locations in memory and, if these locations are POKEd correctly then some great things can be achieved.

When you connect the tape or disk lead into the input/output socket of your computer you have given yourself a choice of eight communication channels, 0 – 7 (remember OPEN #1, etc?). Each channel has sixteen bytes of memory reserved for it in RAM, from locations 832 to 959. You will be told by various handbooks that you can use them all for your own use, all except channel 0 that is, because it is reserved for the screen display. This is not true, you can use the screen display, and to prove it I will introduce you to the ATARI 'Forced Read Mode'. I must confess though, this has been mentioned before in PAGE 6 by myself and others, but can you have enough of a good thing?

In this mode the program will stop running (the STOP command) and place information on the screen (using PRINT). Then it will reposition the cursor above the new program lines and enter the lines, directly from the screen, as if you had pressed RETURN. The CONT command will start the program where it left off, any new lines being run. LIST will show the modified program.

A self-modifying ATARI!  LISTING 3 will demonstrate this effect.

AtariLister - requires Java

That's it for this issue. Next time we'll take a closer look at those IOCB's. Meanwhile, don't forget that you can write with your problems or to suggest further topics for the column. Write to MARK HUTCHINSON, 1. HOLLYMOUNT, ERINVALE, FINAGHY, BELFAST, BT1O OGL



Chip – A piece of semiconductor material containing a microscopic integrated circuit.


Integrated circuit – A tiny circuit in which the electrical components are made of chemical elements diffused into a piece of semiconductive material.

Character – a single letter, number or symbol.

Daisy wheel – A plastic disk with spokes used as the print mechanism in letter quality printers.

Data – information to be processed by a computer.

Database – The entire collection of information available to a computer. A structured collection of information, or a collection of related files, considered as an entity.


Data set – A modem. A collection of data records with a logical relation to one another.

Device Handler – A collection of routines that connect the operating system and the user program with the input/output devices. There is one handler for each type of peripheral device in the hardware configuration.

Direct Memory Access – Transfer of data between a peripheral and main memory without intervention of the CPU.


More definitions next issue!