documentation about in and out instructions

documentation about in and out instructions

Jannes Braet 18 Nov 2013 19:48

I have just learned about the in and out instructions but I do not really understand it. If I simply follow the explanation on this site (http://z80-heaven.wikidot.com/direct-input-output#toc5) I would make this simple program, but it does not work. Is the documentation wrong or did I understand it wrongly?
This is the code:

    ld hl,0400h  ;210004
    ;random value

    ld a,%11111110  ;3EFE
    ;group 0

    out ($01),a  ;D301
    ;tell hardware to look for group zero keypress

    in a,($01)  ;DB01
    ;receive keypress

    bit 0,a  ;CB47
    ;test if down arrow was pressed

    jr nz,$-8 ;20F6
    ;if it was not pressed then return to ld a, for another key press

    Bcall(_DispHL)  ;EF0745
    ;If it was pressed then display the value of hl.

    ret ;C9

EDIt: If I read the example on this site (http://wikiti.brandonw.net/?title=83Plus:Ports:01) it seems like no loops is needed but If I use this code it simply quits the program right after it started so I don't even had the time to press anything.

    ld hl,0400h  ;210s004
    ld a,%11111110  ;3EFE
    out ($01),a  ;D301 
    in a,($01)  ;DB01
    bit 0,a  ;CB47
    call z,disp ;CCA49D
    ret ;C9

disp:
    Bcall(_DispHL)  ;EF0745
    ret ;C9

Last edited on 18 Nov 2013 20:41 by Jannes Braet Show more

Re: documentation about in and out instructions

Xeda Elnara 18 Nov 2013 20:11

Your code worked as I expected it to (Wait until the Down key is pressed, then display HL). What were you trying to do?

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Z80 Assembly>English>TI-BASIC>Python>French>C>0

Re: documentation about in and out instructions

Jannes Braet 18 Nov 2013 20:40

Huh wow… I'm sure it did not work last time, but now it does work when I try it again. Well I assume I made a mistake somewhere.
And can you tell me about my edit?

Another question:
All the links on http://z80-heaven.wikidot.com/ports are broken. Do these pages exist somewhere? or is it just an uncompleted section of this website? You might want to link them to http://wikiti.brandonw.net/?title=83Plus:Ports:01 and the other ports too, if the pages do not exist here.

Last edited on 18 Nov 2013 20:40 by Jannes Braet Show more

Re: documentation about in and out instructions

Xeda Elnara 19 Nov 2013 12:54

To answer your edit, if you don't use a loop, the program will finish in 67 t-states which is approximately .00001 seconds. You will probably want a loop in there. For example, the following exits on Clear:

Start:
    ld a,FDh    ;3EFC    ;reads both the arrows and Enter~Clear. key codes will overlap
    out (1),a    ;D301
    in a,(1)    ;DB01
    inc a        ;3C    ;If A=FF (no keys pressed), then inc a sets to zero and sets the zero flag
    jr z,$-7    ;28F7
    dec a        ;3D
    bit 6,a        ;CB77    ;Test if Clear is pressed
    ret z        ;C8
    ld l,a        ;6F
    ld h,0        ;2600    ;key value is now HL
    push hl        ;E5    ;push HL onto the stack since the next bcalls destroy HL
    bcall(_ClrScrnFull) ;EF4645
    bcall(_HomeUp)    ;EF5845
    pop hl        ;E1    ;pop the last item on the stack, to HL
    bcall(_DispHL)    ;EF0745
    jp Start    ;C3959D

And to answer your other question, it is an incomplete page (there are a few others, too). I will post a link to WikiTI until the pages can be fleshed out.

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Z80 Assembly>English>TI-BASIC>Python>French>C>0