topic Re: Getting access to a string in asm in 8-bit Microcontrollers

Getting access to a string in asm

mnemonic — Fri, 27 Nov 2009 20:32:49 GMT

Hello experts!

I want learn something about accessing a string. I found the application note AN1752. But I does not understand this exactly.

I done some coding on a LCD and displaysome chars by this way:

lcd_val: DS.B 1

write:

lda #'H'

sta lcd_val

jsr write_char

lda #'E'

sta lcd_val

jsr write_char

lda #'L'

sta ..........

...............

Now I want learn how to get access to 'HELLO' Array and store it to lcd_val:

MSG: DC.B 'HELLO'

Can anyone tell me how to do this?

best regards,

norticum

Re: Getting access to a string in asm

eckhard — Fri, 27 Nov 2009 23:19:13 GMT

Hello,

ist is done in a loop, using the X Register

                clrx
L1             lda       MSG,X          ;load AccA w/char from msg
                beq      OUTMSG       ;end of msg?
                jsr       write_char     ;write data to LCD
                incx
                bra     L1                ;loop to finish msg
OUTMSG    rts

Eckhard

Re: Getting access to a string in asm

bigmac — Sat, 28 Nov 2009 00:26:37 GMT

Hello norticum,

A couple of comments about Eckhard's sub-routine -

The sub-routine assumes that the string data is terminated with a zero byte value.
The sub-routine will work only if the value H:X remains unaltered by the write_char sub-routine. Perhaps check that this is so. If not, you might consider modifying write_char by saving the H:X entry value(s) to the stack, and restoring them on exit.
The H-register must have a value of zero for the sub-routine to operate correctly. This might be explicitly initialised with ldhx #0 as the first instruction (in lieu of clrx).

For a more general sub-routine that is capable of sending many different strings to the LCD, consider entering the sub-routine with the H:X value already set to the start address of a particular string.

MSG1:

dc.b "Hello",0

...

ldhx #MSG1

jsr send_string

...

send_string:

lda ,x

beq end_str ; Exit if end of string

jsr write_char

aix #1 ; Next string character

bra send_string ; Loop always

end_str:

rts

Again, the write_char sub-routine must not alter the H:X value.

Regards,

Mac

Message Edited by bigmac on 2009-11-28 03:36 AM

Re: Getting access to a string in asm

mnemonic — Sat, 28 Nov 2009 19:56:07 GMT

Hello Mac Hello Ego!

Thanks a lot! Both Listings are very interesting. I tried the code from Mac, it works fine!

Can you please explain me "lda ,x" ? I do not understand this line.

Regards,

norticum

Re: Getting access to a string in asm

bigmac — Sun, 29 Nov 2009 01:42:35 GMT

Hello norticum,

mnemonic wrote:

Can you please explain me "lda ,x" ? I do not understand this line.

This instruction uses indexed addressing mode with no offset. That is the accumulator is loaded with a byte value fetched from an address that is specified by the 16-bit value contained within the H:X register. Another way of writing the instruction is lda 0,x.

Regards,

Mac