From: Maxime Beauvais <maximebeauvais@xxxxxxxxx>
Reply-To: stella@xxxxxxxxxxx
To: stella@xxxxxxxxxxx
Subject: Re: [stella] Kirk tutorial question
Date: Mon, 15 Jul 2002 06:27:21 -0700 (PDT)
I had the same problem with this red line program.
Check my previous email concerning the vcs.h file.
I was missing one line and that was the reason why I
was not able to compile!!!!!
Good luck...
--- Gary Szwiec <gszwiec@xxxxxxxxxxx> wrote:
> Thanks for your help.
>
> Here is the code. Clock does compile and run fine.
>
> I am using DASM V2.12.04 and running Stella 1.1.a
>
> ++++++++++++++++++++
> ; thin red line by Kirk Israel
> ;
> ; (anything after a ; is treated as a comment and
> ; ignored by DASM)
> ;
> ; First we have to tell DASM that we're
> ; coding to the 6502:
> ;
>
> processor 6502
>
> ;
> ; then we have to include the "vcs.h" file
> ; that includes all the "convenience names"
> ; for all the special atari memory locations...
> ;
> include vcs.h
>
> ;
> ; now tell DASM where in the memory to place
> ; all the code that follows...$F000 is the preferred
> ; spot where it goes to make an atari program
> ; (so "org" isn't a 6502 or atari specific
> command...
> ; it's an "assembler directive" that's
> ; giving directions to the program that's going to
> ; turn our code into binary bits)
> ;
>
> org $F000
>
> ;
> ; Notice everything we've done so far is "indented"
> ; Anything that's not indented, DASM treats as a
> "label"
> ; Labels make our lives easier...they say "wherever
> the
> ; next bit of code ends up sitting in physical
> memory,
> ; remember that location as 'labelname'. That way
> we
> ; can give commands lke "JMP labelname" rather than
> ; "JMP $F012" or what not.
> ; So we'll call the start of our program "Start".
> ; Inspired genius, that. Clever students will have
> ; figured out that since we just told DASM "put the
> ; next command at $F000", and then "Call the next
> memory
> ; location 'Start':, we've implicitly said that
> ; "Start is $F000"
> ;
>
> Start
>
> ;
> ; The next bit of code is pretty standard. When the
> Atari
> ; starts up, all its memory is random scrambled. So
> the first
> ; thing we run is "SEI" "CLD" and "TXS".
> ; Look these up if you want,
> ; for now know that they're just good things to
> cleanse
> ; the palette...
>
> SEI ;Disable Any Interrupts (hey look! we can put
> comments here)
> CLD ; Clear BCD math bit.
> LDX #$FF ; put X to the top...
> TXS ; ...and use it reset the stack pointer
>
> ;
> ; Now this is another pretty standard bit of code to
> start
> ; your program with..it makes a noticeable delay
> when your
> ; atari program starts, and if you're a hot shot you
> could consider
> ; zeroing out only the memory locations you care
> about, but
> ; for now we're gonna start at the top of memory,
> walk our way
> ; down, and put zeros in all of that.
> ;
> ; One thing you may notice is that a lot of atari
> programming
> ; involves starting at a number, and counting your
> way down
> ; to zero, rather than starting at zero and counting
> your
> ; way up. That's because when you're using a
> Register to
> ; hold your counter, it's faster/easier to compare
> that
> ; value to zero than to compare it to the target
> value
> ; you want to stop at.
> ;
> ; So X is going to hold the starting memory location
> ; (top of memory, $#FF), the "A"ccumulator is going
> to
> ; hold what we put into each memory location (i.e.
> zero)
>
> LDX #$FF ;X is top of memory
> LDA #0 ;Put Zero into A
> ClearMem
> STA 0,X ;Now, this doesn't mean what you think...
> DEX ;decrement X (decrease X by one)
> BNE ClearMem ;if the last command resulted in
> something
> ;that's "N"ot "Equal" to Zero, branch back
> ;to "ClearMem"
> ;
> ; Ok...a word of explanation about "STA 0,X"
> ; You might assume that that said "store zero into
> the memory
> ; location pointed to by X..." but rather, it's
> saying
> ; "store whatever's in the accumulator at the
> location pointed
> ; to by (X plus zero)"
> ;
> ; So why does the command do that? Why isn't there
> just a
> ; "STA X" command? (Go ahead and make the change if
> you want,
> ; DASM will give you an unhelpful error message when
> you go
> ; to assemble.) Here's one explanation, and it has
> to do with
> ; some handwaving I've been doing...memory goes from
> $0000-$FFFF
> ; but those first two hex digits represent the
> "page" you're dealing
> ; with. $0000-$00FF is the "zero page", $0100-$01FF
> is the first
> ; page, etc. A lot of the 6502 commands take up
> less memory
> ; when you use the special mode that deals with the
> zero page,
> ; where a lot of the action in atari land takes
> place.
> ; ...sooooo, STA $#nnnn would tell it to grab the
> next two bytes
> ; for a full 4 byte address, but this mode only
> grabs the one
> ; value from the zero page
> ;
>
> ;
> ; Now we can finally get into some more interesting
> ; stuff. First lets make the background black
> ; (Technically we don't have to do this, since
> $00=black,
> ; and we've already set all that memory to zero.
> ; But one easy experiment might be to try different
> two
> ; digit hex values here, and see some different
> colors
> ;
> LDA #$00 ;load value into A ("it's a black
> thing")
> STA COLUBK ;put the value of A into the
> background color register
>
> ;
> ; Do the same basic thing for missile zero...
> ; (except missiles are the same color as their
> associated
> ; player, so we're setting the player's color
> instead
> ;
>
> LDA #33
> STA COLUP0
>
> ;
> ; Now we start our main loop
> ; like most Atari programs, we'll have distinct
> ; times of Vertical Sync, Vertical Blank,
> ; Horizontal blank/screen draw, and then Overscan
> ;
> ; So every time we return control to Mainloop.
> ; we're doing another television frame of our humble
> demo
> ; And inside mainloop, we'll keep looping through
> the
> ; section labeled Scanloop...once for each scanline
> ;
>
> MainLoop
> ;*********************** VERTICAL SYNC HANDLER
> ;
>
=== message truncated ===
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