[Coco] 6x09/hello_os9-Commented

[Joel Rees]

On Mon, Oct 2, 2023 at 10:04 AM coco--- via Coco <coco at maltedmedia.com> wrote:
>
> All
>
> I received a "Hello World" sample program From Alex Evans, It works
> well.
> I have looked at it line by line trying to understand everything.
> I still have some questions, so I have posted the code followed by a
> table
> with line numbers, code and comments/questions. This shows exactly what
> I
> understand or am guessing and what I am clueless about. Please take a
> look at
> it and using line numbers you can comment on the accuracy of my
> understanding
> of the code and the meaning of the lines of code I am clueless about.
>
> The location of the post is:
>      http://jechar.ca/coco/Download/source/6x09/hello_os9-Commented.html

I suppose I shouldn't kibbutz, but it would be a lot easier to comment
if you had put that in you email instead of in your blog.

There are reasons to ORG things to non-zero in OS-9 assembler, but I
don't think you want to know those just yet.

As has been indicated, EQUating size to 128 and passing it as the 6th
parameter to the mod directive tells the assembler to tell the linker
the program will need 128 bytes of data space, as I recall the
explanation.

Asterisk in an expression could be called the "here" operand. It
evaluates as the current location to assemble at. Thus, when endpgm is
EQUated to *, because we started with ORG 0, endpgm is the size in
bytes of the executable object.

FCS is "form constant string", meaning a NUL terminated string. It
implicitly tags a 0 byte on the end of the quoted string that gets
assembled into the object code there.

Compare that to FCC, "form constant character", which does not
automatically append a o byte.

LEA -- load effective address. The 6809 has a number of useful
addressing modes. You tend to think of B,Y in

LDA B,Y

as simply the address to load A from. But LEAX B,Y computes the
address and puts the address into X, instead of grabbing the contents
of memory at that address.

In the case of LEAX hello, PCR, it's calculating the address of the
FCC string at label hello relative to the PC, and loading the address
into X. This is one of the keys to relocatable 6809 code. The
assembler figures out the offset from the instruction to the string,
and assembles that offset as the offset argument to the LEA in PC
relative mode. That way, when the OS loads the code at, say, &7800
instead of 0, the LEA still gets the right address for the string
hello.

Hopefully, together with the other responses, this answers your questions.

-- 
Joel Rees

http://reiisi.blogspot.jp/p/novels-i-am-writing.html