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ti_kid
New Member (Posts: 78)

C-OS coding help
30 Jan 2026 06:58:52 pm

I am writing an os. I am doing parts in C to get started and then converting to ASM. I use
Code:

zcc +sos -crt0=my_crt0.s

to compile. Here is the main part:
Code:

__sfr __at 0x10 lcd_command;


__sfr __at 0x11 lcd_data;





#ifdef __linux__


#include <stdint.h>


uint8_t lcd_command, lcd_data;


#endif








#define LCD_6_BIT 0


#define LCD_8_BIT 1


#define LCD_ENABLE 3


#define LCD_DOWN 5


#define LCD_RIGHT 7


#define LCD_ROW 0x80


#define LCD_COL 0x20


#define LCD_CONTRAST 0xC0








typedef unsigned char u8;








u8 *cursor_x = (u8*)0x8000;


u8 *cursor_y = (u8*)0x8001;








#include "font.h"





void lcdDelay() __naked {


        __asm


                call $+3


                nop


                ret


        __endasm;


}





void lcdSetXY( int x, int y) {


        lcd_command = x + LCD_COL;


        lcdDelay();


        lcd_command = y + LCD_ROW;


        lcdDelay();


}





void lcdInit(void) {


        lcd_command = LCD_8_BIT;


        lcdDelay();


        lcd_command = LCD_ENABLE;


        lcdDelay();


        lcd_command = 0xf0;


        lcdDelay();


        lcd_command = 0x40;


        lcdDelay();


        lcd_command = LCD_DOWN;


        lcdDelay();


}





int putchar(char c) __z88dk_fastcall {


    __asm


        ld a,(_cursor_x)


        add a,0x20


        out (0x10),a


        call _lcdDelay





        ld a,(_cursor_y)


        add a,0x80


        out (0x10),a





        ld a,l


        sub a,0x20


        jr c,_putchar_err


        ld l,a


        ld h,0


        add hl,hl


        ld de,_CHAR_MAP


        add hl,de


        ld de,(hl)


        ld b,8


    _putchar_loop:


        ld a,(de)


        inc de


        out ($11),a


        call _lcdDelay


        djnz _putchar_loop


        ld hl,0


        jr _putchar_exit


    _putchar_err:


        ld hl,1


    _putchar_exit:


        push hl


        ld a,(_cursor_x)


        inc a


        cp 12


        jr c,_putchar_y_noinc


        ld a,(_cursor_y)


        inc a


        ld (_cursor_y),a


        xor a


    _putchar_y_noinc:


        ld (_cursor_x),a


        pop hl


    __endasm;


}

The ifdef Linux is so that my lsp doesn't complain as much.
Anyways here are some main() definitions and their output. https://imgur.com/a/main-c-outputs-4si8NwI

KermMartian
1
Site Admin (Posts: 64438)

31 Jan 2026 01:25:12 am

The evidence would suggest your `cursor_x` and `cursor_y` are always remaining zero, especially as supported by your "67" test, which seems to instead write "6" twice at (0,0). Your 256 bytes of scratch ram at 0x8000 (appData) should be fine.

ti_kid
New Member (Posts: 78)

31 Jan 2026 10:36:16 am

Yes but why wont B be at 0,5 and 6 at 0,6 , this also means that along with cursor_x and y not being updated the LCD coordinate set at the start of the ASM also dosen't work. I have no idea at all of what is causing this. When I had the entire routine in C It also didn't work.

ti_kid
New Member (Posts: 78)

31 Jan 2026 02:23:32 pm

Sorry for the double post
Okay now I am even more confused.
The new code (same except for start of putchar)
Code:




int putchar(char c) __z88dk_fastcall {


    lcdSetXY(*cursor_x, *cursor_y);


    __asm


        ;ld a,(_cursor_x)


        ;add a,0x20


        ;out (0x10),a


        ;call _lcdDelay





        ;ld a,(_cursor_y)


        ;add a,0x80


        ;out (0x10),a





        ld a,l


        sub a,0x20


        jr c,_putchar_err


        ld l,a


        ld h,0


        add hl,hl


        ld de,_CHAR_MAP


        add hl,de


        ld de,(hl)


        ld b,8


    _putchar_loop:


        ld a,(de)


        inc de


        out ($11),a


        call _lcdDelay


        djnz _putchar_loop


        ld hl,0


        jr _putchar_exit


    _putchar_err:


        ld hl,1


    _putchar_exit:


        push hl


        ld a,(_cursor_x)


        inc a


        cp 12


        jr c,_putchar_y_noinc


        ld a,(_cursor_y)


        inc a


        ld (_cursor_y),a


        xor a


    _putchar_y_noinc:


        ld (_cursor_x),a


        pop hl


    __endasm;


}

and
Code:

 int main(void) {


    lcdInit();


    *cursor_x = 6;


    *cursor_y = 0;


    putchar(*cursor_x+'0');


    putchar(*cursor_x+'0');


    return 0;


}

Now from this the output is.

I just realized that lcdSetXY corrupts hl and I use fastcall so the char was passed in hl. Here is the output with the code fix (still need to fix updating cursor_x and cursor_y and fix the ASM version of lcdSetXY).

ti_kid
New Member (Posts: 78)

31 Jan 2026 08:29:34 pm

Sorry for triple post
Out of curiosity in the ASM I replaced (_cursor_x) with (0x8000) and (_cursor_y) with (0x8001) and it worked.
At first I had no idea why but then I realized. In c variables are on the stack and I had declared cursor_x and y as vars instead of a define.
The reason this dosen't work is because _cursor_x and y would assemble to their offsets from the stack pointer (prolly 2 and 4) so I was reading (0x0002) and (0x0004). I had discovered one fix but another better one is to use define instead so that is what I am using know.

TLDR fixed code :

Code:

#define cursor_x (u8*)0x8000


#define cursor_y (u8*)0x8001


#define _cursor_x 0x8000


#define _cursor_y 0x8001

and
Code:

int putchar(char c) __z88dk_fastcall {


    __asm


        ld a,(_cursor_x)


        add a,0x20


        out (0x10),a


        call _lcdDelay





        ld a,(_cursor_y)


        add a,0x80


        out (0x10),a





        ld a,l


        sub a,0x20


        jr c,_putchar_err


        ld l,a


        ld h,0


        add hl,hl


        ld de,_CHAR_MAP


        add hl,de


        ld de,(hl)


        ld b,8


    _putchar_loop:


        ld a,(de)


        inc de


        out ($11),a


        call _lcdDelay


        djnz _putchar_loop


        ld hl,0


        jr _putchar_exit


    _putchar_err:


        ld hl,1


    _putchar_exit:


        ld a,(_cursor_x)


        inc a


        cp 12


        jr c,_putchar_y_noinc


        ld a,(_cursor_y)


        inc a


        ld (_cursor_y),a


        xor a


    _putchar_y_noinc:


        ld (_cursor_x),a


    __endasm;


}

And this all works. 🙂

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