Re: A85: questions

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Re: A85: questions


Michael Pearce wrote:

> >2)  what does the adding hl and de with the (programaddr) (or
> whatever)
> >stuff do?  why is it necesary in Zshell, but nowhere else?  I fi'm
> porting
> >a prog to the 86 say, what would some of the equivalent stuff be?
> >thanx much
> >
>
> On the ti85, the location of the variables (strings specifically) in
> the calculator's RAM are always changing.  Because of this, the
> location of the addresses are always changing within the program that
> you are making.  the first line of code that you write is the ".org
> 0".  when using instructions such as call, jp or ld, and you want to
> use a 16-bit address within the program, the actual machine code is
> calculated by adding the origin of the program (0 in this case) to the
>
> 16-bit address.  for example:
>         ld hl,MyData    ;MyData is a label that is the number of
>                         ;bytes from the .org of the program.  this is
>                         ;a 16-bit number that tells the *offset* from
>                         ;the beginning of your program.  You
>                         ;then add the origin number (0) to find the
>                         ;actual machine code.
>         ld de,(PROGRAM_ADDR)
>                         ;hl now has the address of my data relative
>                         ;to the start of the program, but this does
>                         ;not take into account the fact that the
>                         ;program could be any where in memory.
>                         ;so we look in PROG_ADDR to find the
>                         ;location of the first byte of the code
>         add hl,de
>                         ;now, once these 2 addresses are added,
>                         ;the exact location of MyData in memory
>                         ;is known
>         ......
>         more code
>         ......
> MyData:
>         .db 7,7
>
> To overcome this repetitive action, shells such as Usgard and Rigel
> take away the need for PROGRAM_ADDR addition using a table of offsets
> or having the program always moved to a fixed address (the ".org"
> value is the address of where it is moved), respectively.  And i
> believe that the 86 uses fixed address relocation (like Rigel), but i
> am not certain.  Maybe someone else can answer that for sure.
> BTW, does anyone else the that this list is completely dead?  then
> again, maybe its not the list, its the ti85 :(.
> -mike
>  mgp4007@omega.uta.edu

   You are correct on all counts.
 Jonathan Kaus
References: