Assembly is hard.

[+atari2600land]

Somehow I managed to cobble up two games, but I'm having some troubles on this one, mainly due to the fact that I can't figure out how to display just one part of a sprite. Suppose I have some code that looks like this:

enemy
   db  01111100b
   db  11010110b
   db  11111110b
   db  10000010b
   db  11111110b
   db  01000100b
   db  01000100b
   db  11000110b    

   db    01111000b
   db    11111110b
   db    11111110b
   db    01111100b
   db    00000000b
   db    00011110b
   db    00111111b
   db    00011110b            
   

   
setshape
; r0=spritepointer, r1=pointer to image in rom
   mov    r7,#8        ; 8 bytes

   
   
copyspriteloop:
   mov    a,r1
   movp    a,@a        ; get byte
   movx    @r0,a
   inc    r0
   inc    r1
   djnz    r7,copyspriteloop
   ret

At the beginning, I called the certain shape to appear using this:

   mov    r0,#vdc_spr2_shape
   mov    r1,#enemy & 0ffh
   call    setshape

My question is, how would I get the next 8 lines of the sprite to appear instead of the first 8?

[Nukey Shay]

I don't know the Z80 equalivant to (indirect),y addressing...or much else pertaining to Z80 for that matter...but couldn't you just alter the value of r1 before entering the loop?

[+Gemintronic]

No. YOU'RE hard atari2600land. You've got the stones to persevere until it makes sense. Keep on going!

Edited August 2, 2012 by theloon

[GroovyBee]

I don't know the Z80 equalivant to (indirect),y addressing...or much else pertaining to Z80 for that matter...but couldn't you just alter the value of r1 before entering the loop?

 

Its not Z80. Its 8048 which is the CPU in the Philips G7000.

 

My question is, how would I get the next 8 lines of the sprite to appear instead of the first 8?

 

Try :-

 

   mov    r0,#vdc_spr2_shape
   mov    r1,#enemy+8 & 0ffh
   call    setshape

[+atari2600land]

Thanks, groovybee, but I've given up on that game. It was just too hard for me to contemplate how to do other stuff.

 

OK, well, next question. I'm working on a Freeway type game, and I was just wondering how to make a non-appearing timer like in Freeway? I struggled with this for days on end and what I've tried doesn't make any sense whatsoever.

traff4.txt

Edited August 5, 2012 by atari2600land

[GroovyBee]

I can't find your software timer code in that source. You aren't keeping track of either the seconds or the number of vblanks passing by.

[+atari2600land]

There is no software timer in the code. How would I keep track of the number of vblanks?

[GroovyBee]

There is a vsyncirq in the vector table but no source code to handle it in your example. Put an increment of a RAM variable in there. When it gets to 50 or 60 (depending on TV region) then reset it back to 0 and subtract 1 from your seconds counter. When the seconds counter reaches 0 then your time has run out. With a single byte containing the seconds you can have a maximum time out of 255 seconds.

[+atari2600land]

Why does this not work?

   ;timer part
   mov r0,#thissucks
   movx a,@r0
   add a,#1
   movx @r0,a    

   mov r0,#thissucks
   movx a,@r0    
   xrl a,#60
   jz loop_000

   jmp loop_00

loop_000

   call add_score

   mov r0,#thissucks
   mov @r0,#000h

   mov r0,#seconds
   movx a,@r0    
   add a,#0ffh
   movx @r0,a    

   mov r0,#seconds
   movx a,@r0    
   xrl a,#000h
   jz gameover_4
   
   jmp loop_00

gameover_4
   jmp gameover

loop_00
  -rest of program-

[GroovyBee]

In G7000.h there are ram locations called iram_clock, eram_minutes and eram_seconds. What happens in those locations?

[+atari2600land]

In my code? Nothing. I didn't know they existed. How would I go about using those variables?

[GroovyBee]

You could convert them to digits and put them on screen to see if the BIOS does anything with them. Or look at them in the emulator.

[+atari2600land]

I got it working! By the way, what's the difference between PAL and NTSC in terms of seconds? Would PAL be 50 and NTSC be 60, or PAL is 60 and NTSC is 50?

traff4.txt

[GroovyBee]

PAL has 50 vblanks per second and NTSC 60.

[+atari2600land]

Since the clock is different on different machines, can someone do a quick look at my code to see whether this would help detect whether the game is running on a PAL Videopac or an NTSC Odyssey²?

traff5.txt

[+atari2600land]

Never mind. As someone pointed out in the other thread, and I didn't know this, the whole game would run slower, so there's no need to make the clock run slower.

[GroovyBee]

Never mind. As someone pointed out in the other thread, and I didn't know this, the whole game would run slower, so there's no need to make the clock run slower.

 

Not true! You need to account for the difference in the number of vblanks per second. If you are counting to 60 on NTSC the clock will roll over every second as expected. If you are counting to 60 on PAL the clock will roll over every 1.2 seconds. For a 60 second time limit you get (60*60)/60 seconds running on NTSC. If you run on PAL without changing the code you'd get (60*60)/50 seconds. So PAL people would get 72 seconds to complete the same course. That wouldn't be good if the game was ever used in an international high score contest.

[+atari2600land]

Well, then, look through this amended code and see if this would work.

traff7.txt

[+atari2600land]

OK, I'm stumped. How do you get a number to equal another number? Say for instance, that I want sprite0x to equal char1x. How would I do this?

[GroovyBee]

You need to do the following steps:

 

- Load a data register with the source address (the variable you want to read).

- Move the contents of the address pointed to by the register into the accumulator.

- Load a data register with the destination address (the variable you want to write).

- Move the accumulator into the address pointed at by that register.

[+atari2600land]

Cool, thanks, I ended up with this:

   mov r0,#011h
   movx a,@r0
   mov r1,#001h
   movx @r1,a

(In Odyssey² assembly, #11h is char0x and #001h is sprite0x. I am still wondering what exactly @ does, though. Keep in mind I learned/ am learning how to do different things by tweaking different examples)

[GroovyBee]

I am still wondering what exactly @ does, though.

 

It roughly means "the contents of the address". Its a bit like saying "let fred=1" in BASIC except that in the Videopac's case "1" would have moved into the accumulator first and the address of "fred" would be moved into a data register.

[+atari2600land]

last question for a while.

How would I do

"if a>80 && a<88"

in assembly?

[GroovyBee]

We discussed "if statements" back in February. I'm not an 8048 programmer but the code would be something like :-

 

; Check if val>=88
   add a, #168
   jc OutOfRange

; Val is in the range 0 to 87 at this point.

; You need to restore A here it was just trashed with the add.
...

; Check if val>=81
   add a, #175
   jnc OutOfRange

; Get here if val>80 and val<88
InRange:

; Do something because its in range...

; Jump to somewhere.

; Get here if val<=80 or val >=88
OutOfRange:

; Do something because its out of range...

[+atari2600land]

So then would "if a<3" would be:

       add a, #253
       jc OutOfRange