Instant Character Sets

by Les Howarth

 

Issue 29

Sep/Oct 87

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'Initialising ... please wait' Now you don't have to! Les Howarth has an instant solution

The great majority of programs use custom character sets but when a character set is created in BASIC there is usually an annoying long wait while the data is transferred and read into the new character base accompanied on screen by those famous words: "Initialising .... Please Wait"

Most programmers use the long winded loop FOR I =0 TO 1023 to construct the new set. It works but sometimes seems to take forever. There are two ways to speed up the process. Disk drive owners can create a character set as a file on disk and have it load into the program quite fast using an IOCB block but the second method is something quite different.

Suppose the entire character set was stored in a string? The set would then be instantly available because of the Atari's super fast string handing. Sounds good but there are problems. Unfortunately the Character Base register (address 756) requires the character set to begin on an even number of pages, that is the address of the string would have to be absolutely divisible by 256, otherwise the characters would be all wrong. The Atari stores strings in a 'dynamic' form so that the actual address of each string changes as your program is developed so this method cannot be used, or can it? Supposing the address of the character string was changed to point to a reserved part of memory, (with an even number of pages)?. It would then remain at that position because the program being written would not affect any protected memory. We could POKE the address of the reserved memory into CHBAS and it would work! The character set would truly be instantly set up.

HOW TO DO IT

Believe it or not you don't have to work all the details out for yourself. The accompanying program can be used without having to understand the workings of it. All you need to do is alter the data in CC$ for your own custom set.

Each line from LINE 31000 contains 10 characters so it should be fairly easy to work out the position of whichever character you want to alter. Remember that all the upper case characters are stored first then the Graphic and lower case letters.

The first character is CHR$(32)- a space, so line 31000 stores the 10 characters from CHR$(32) to CHR$(41). Line 31010 stores from CHR$(42) to CHR$(51) and so on.

One word of WARNING. It is vital that CC$ is the first variable that the program sees, therefore make sure you type line 10 before anything else. The ATARI stores variable names in the order that you type them in so if you add this routine to an existing program it will not work, as CC$ will be added to the end of the variable name table. To overcome this little problem, add the routine then list the entire program to tape or disk using LIST "C" or LIST "D:FILE". Then type NEW and re-enter the file using ENTER "C" or ENTER "D:FILE". The name table should then be correct. This procedure must be followed if you use TYPO to check the listing.

HOW DOES IT WORK?

The computer's memory is first lowered by 8 pages. This could be any number but you should never lower memory by less than five pages for a character set. The reason for this restriction is that when a 'clear screen' (CHR$(125)) instruction is encountered, ATARI BASIC tends to wipe out some of the memory above the screen and above RAMTOP which would of course wipe out some of your characters if they started immediately above the screen memory.

The Address of Strings and other variables is stored in a table. Address 134 and 135 (VVTP) points to the start of the variables, each one being allocated 8 bytes in this table. For strings the third and fourth bytes in VVTP gives a reference address for the string. This is not the actual address but an OFFSET number of bytes from the beginning of STARP (locations 140 and 141). These are the two bytes which need to be altered in order to redirect the string to your reserved memory.

In line 29030, 'D' is the address of the first variable in STARP, and 'R' is the address of VVTP. So R + 2 and R + 3 are the two bytes to change. In line 29040 'Q' finds the necessary OFFSET number of bytes which will direct the string to 4 pages above the screen. 'Q' is then divided into high and low bytes and these are POKEd into VVTP in line 29050.

The final instruction, which is placed at the end of all the new character data, is to POKE the string address into CHBAS POKE 756,MEMTOP

That's it. I hope that this technique will prove useful to you and speed up those agonising initialisation sequences in your programs. If you have any difficult understanding the program, a read through the COMPUTE! book REVISED MAPPING THE ATARI should help. It's a book that should be at every programmer's side.

AtariLister - requires Java

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