ZX81: zx_setstr() does not cut existing longer strings correctly

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siggi
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Posts: 343
Joined: Thu Jul 26, 2007 9:06 am

ZX81: zx_setstr() does not cut existing longer strings correctly

Post by siggi »

Test program:

Code: Select all

void main(void)
{
        char buffer[60];
        zx_setstr('a', "longlonglong");
        zx_getstr('a', buffer);
        printk("a=%s\n", buffer);

        zx_setstr('a', "short");
        zx_getstr('a', buffer);
        printk("a=%s\n", buffer);
}
Result on ZX81 is:

Code: Select all

A=LONGLONGLONG
A=SHORTO
The new length of the BASIC string is not set correctly. The same happens also, when the BASIC variable A$ is set (inside a BASIC line) during calls of zx_line()

Siggi
siggi
Well known member
Posts: 343
Joined: Thu Jul 26, 2007 9:06 am

Post by siggi »

Addendum:
BASIC shows the correct string "SHORT". LEN gives also the correct result 5.
?????
Siggi
siggi
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Posts: 343
Joined: Thu Jul 26, 2007 9:06 am

Post by siggi »

The function zx_getstr() seems to be buggy and returns one more char than BASIC, when printing the string variable. If crashes, when the string variable is empty ("").

Siggi
siggi
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Posts: 343
Joined: Thu Jul 26, 2007 9:06 am

Post by siggi »

The reason for the crash of "zx_getstr" caused by string length 0 is caused by check of the string length AFTER it has been copied to the destination:

Code: Select all

.outloop
        call        zx81toasc
        ld        (de),a
        inc        hl
        inc        de
        dec        bc
        ld        a,b
        or        c
        jr        nz,outloop
Thus string length 0 copies 64K ...

Now l am looking for the additional character at end of the string ....

Siggi
siggi
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Posts: 343
Joined: Thu Jul 26, 2007 9:06 am

Post by siggi »

The additional char at end of the string is caused by double incrementing DE at end of the string:

Code: Select all

.outloop
        call        zx81toasc
        ld        (de),a
        inc        hl
        inc        de
        dec        bc
        ld        a,b
        or        c
        jr        nz,outloop
;------------------------------
;        ldir
        inc        de
        xor        a
        ld        (de),a
        
        ld        hl,0
        ret
The last "inc de" is one increment too much...

Siggi
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dom
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Posts: 1195
Joined: Sun Jul 15, 2007 10:01 pm

Post by dom »

Thanks Siggi, I've applied that fix.
siggi
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Posts: 343
Joined: Thu Jul 26, 2007 9:06 am

Post by siggi »

But the problem of string length 0, resulting in a crahs (see my posting before) seems not to be solved ...

IMHO this should solve that problem:

Code: Select all

;-----------------------------
.outloop
        ld a,b
        or c
        jr z,strend 
        call        zx81toasc
        ld        (de),a
        inc        hl
        inc        de
        dec        bc
;        ld        a,b
;        or        c
        jr        outloop
;------------------------------
;        ldir
.strend
        xor        a
        ld        (de),a
        
        ld        hl,0
        ret
Siggi
siggi
Well known member
Posts: 343
Joined: Thu Jul 26, 2007 9:06 am

Post by siggi »

And then the "xor a" (at strend) could be replaced by the comment ";a is already 0 here", saving 1 byte ;-)

Siggi
siggi
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Posts: 343
Joined: Thu Jul 26, 2007 9:06 am

Post by siggi »

zx_setstr() is also buggy and does also handle strings of length 0 not correct.
Test program:

Code: Select all

// zcc +zx81 -create-app -startup=2 -vn -o test.bin test.c
#include <stdio.h>
#include <zx81.h>
#include <input.h>

void main(void)
{
    in_WaitForNoKey();
    printk("press any key to continue\n");
    in_WaitForKey();
    zx_setstr('a', "");
    printk("done\n");
}
After the program has been run, PRINT A$ (at BASIC level) gives "DONE?"

Siggi
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