Tip of the Year: Save your work after MSX crash!

you forgot KILL "DEARPROG.BAS" as the last line

Can you explain it?

Sure...

To clear things out, let's take an example program:

10 REM Example program
20 PRINT"Hello world!"

In memory this program is tokenized this way:

#8000 =#00 This byte is always zero. If it is not, program can not run.
#8001-8002 =#8017 This is pointer to start of next line (20)
#8003-8004 =#000A = 10 this is line number
#8005 = #8F This is a token that represents REM command
#8006-8015 = " Example program"
#8016 =#00 This indicates end of line.
#8017-8018 =#802B This is pointer to start of next line
#8019-801A =#0014 = 20 this is line number
#801B = #91 This is a token that represents PRINT command
#801C-8029 = "Hello world!"
#802A =#00 This indicates end of line.
#802B-#802C =#0000 This is pointer to start of next line. Now it is 0 because there is no next line.

When your MSX is rebooted or NEW command is executed, pointer to start of next line in #8001 will be reseted to 0. If we want to restore the program, our problem is, that now we don't know what the next address for start of next line is!

When I was thinking about the tokenized program I was wondering, that how this can work when BASIC is not in default #8000 address? When loading of the data is done, these addresses must be calculated again and I was right. (This has been propably implemented to get BASIC files exchangeable with 16KB RAM machines as well.)

So here is actually what happens, when you execute those commands.

When you save empty program #FF will be put in start of the file to indicate start of tokenized program and BASIC program (this case #8001-8002) will be saved after it. Now we how ever change this address in #8001 to #8000. This number can be actually anything else but 0, but I used #8000 as in #8000 there is word #0000 so, if you use list command, you will not get crap to your screen or in worst case hang your MSX.

Now when this is saved, we will get file with following bytes: #FF,#00,#80
Now when this file is loaded, BASIC does not understand, that the file is so short and address in #8001 will be calculated again using data already in memory as information.

Now when you execute list, you can see the whole program. There is anyway still few problems... BASIC still thinks, that our program is 2 bytes long, so if you execute it, start of the program will be overwritten by variables of the program. Also if you try to save the program as tokenized BASIC program, only 3 bytes will be saved (This time they are anyway correct ). Solution is to save the program as ASCII file as then it is converted to text same way as when you use LIST command.

Rest of the steps are used to convert the ASCII file back to tokenized format in order to save disk space and loading time.

Ok, I was a bit too quick with the cassette version... It seems, that it has still some problems with GOTO, GOSUB and variable arrays... I think I still need to learn some more BASIC when I have more time

NYYRIKKI,

This tip works in Windows? It's much more necessarily in that enviroment...

I have needed this some times. In a brazillian book or magazine I've saw this tip 15 years ago, I guess.

Now when you execute list, you can see the whole program. There is anyway still few problems... BASIC still thinks, that our program is 2 bytes long, so if you execute it, start of the program will be overwritten by variables of the program.
The system variable (VARTAB? I don't have the docs at hand ) that tells BASIC where the start of the variable area is, will point to #8003 directly following the BASIC code. And therefor, the program gets overwritten.
Also if you try to save the program as tokenized BASIC program, only 3 bytes will be saved (This time they are anyway correct ).

That surprises me, actually. That means that the SAVE routine just writes out the block of memoty between TXTTAB (start of BASIC program) and (VARTAB). I woudl have thought that the save routine writes BASIC lines until it reaches the end of the program (3x #00)...
Solution is to save the program as ASCII file as then it is converted to text same way as when you use LIST command.
... whereas the SAVE,A routine seems to LIST the program to file.
Interesting...

POKE&H8002,128:DEFUSR=&HF434:IFUSR(0)THENMAXSB#""""PSETANDBASE

That MAXSB#""""PSETANDBASE: is that plain old obfuscation or functional code, somehow?