Sinclair ZX80 / ZX81 Forums

[Forthretro.new.fr]

Hello friends,

I am sure the answer to be somewhere, but let ask it again :
I have a program I call from the BASIC with the usual RAND USR XXXX

But in the program I will use all the REG and the Stack.
My question is : where is the information for to comeback to the BASIC ?
In the STACK ? In a REG ?
If in the stack I can do, at the start of the program something like
POP DE
LD (save_return),DE
....
Then
LD DE, (save_return)
PUSH DE
C9

But none of the solution seems to work

Merci

[sirmorris]

I seem to remember that BC has something to do with it

if the machine code is:
LD BC, returnvalue
RET

LET N = USR(..)

will put the return value in the variable N.

Don't use IX and IY, AF' registers in slow mode - you'll crash the computer. If you do use them in FAST, make sure that IY = $4000 on return.

Charlie

[siggi]

Hello friends,

I am sure the answer to be somewhere, but let ask it again :
I have a program I call from the BASIC with the usual RAND USR XXXX

But in the program I will use all the REG and the Stack.

If you run the program in SLOW mode, NEVER touch the IX, IY and AF' registers, because they are used by the display routine and /INT and /NMI. The only exception is, when you want to write your own display driver and you know, what you do ...

My question is : where is the information for to comeback to the BASIC ?
In the STACK ? In a REG ?
If in the stack I can do, at the start of the program something like
POP DE
LD (save_return),DE
....
Then
LD DE, (save_return)
PUSH DE
C9

But none of the solution seems to work

Merci

The solution is

RST 08H
DEFB 0xFF

That calls the Basic error routine with "error" 0, cleans up the stack and jumps back to BASIC.

HTH Siggi

[Forthretro.new.fr]

Many thanks for the answer. It helped me to understand the RAND USR !

With a debug I noted that the adress of the SP allway was 7FEA at the start.

While the SP was at 7FEA I could come back to the BASIC with C9.

However FORTH change the Stack adress later. It is why the computer crashed with any C9.

The solution was to put 7FEA in a 'return_adress' and for to exit, to use the instruction

ED 7B 'return_adress' ; LD SP, (return_adress) -> LD SP , 7FEA
C9


OK !!!