Pointer to alphadata

[BydoEmpire]

Hi all.  After 2 years away, I finally got some time to pick up a game I had started.  Some of the code is based on tutorial projects so it might look familiar.   I want to be able to change the screen map room-to-room.  I've been trying to (if this were C) create a pointer to the screen data, which is stored as alphadata.  Snippets:

 

  rem ** screen and map variables, ideally it's a pointer to current screen alphadata
  dim currentscreen=l
  dim currentscreenhi=m

 

...

  rem ** const pointers to the alphadata - this probably isn't correct

  rem ** I have two screens - screenmap00 and screenmap01
  const screen00lo=#<screenmap00
  const screen00hi=#>screenmap00
  const screen01lo=#<screenmap01
  const screen01hi=#>screenmap01

 

  rem ** I was hoping I could update the map I want based on this (for now, hard-coded to screenmap01 to test)

  currentscreen=screen01lo
  currentscreenhi=screen001hi

 

...

 

  rem ** seems to be plotting garbage data, passing in screenmap00 or screenmap01 works fine, of course

  plotmap currentscreen 1 0 0 20 12

 

 

...

 

 alphadata screenmap00 tileset_colortest
 'abcdef        fgfghi'
 'hi                hi'
 'hi                hi'
 'fg                fg'
 'h                   '
 'h                   '
 'h      dddddd       '
 'h                   '
 'hi                hi'
 'hihihihihh        hi'
 'hi              dehi'
 'fgfgfg        fgfgfg'
end

 alphadata screenmap01 tileset_colortest
 'abcdef        fgfghi'
 'hi                hi'
 'hiiii             hi'
 'fg                fg'
 '                   i'
 '                   i'
 '       dd  dd      i'
 '                   i'
 'hi                hi'
 'hihi         i    hi'
 'hi           i  dehi'
 'fgfgfg       ffgfgfg'
end

 

------------------------------------------

Looking at the assembly from the .list.txt file, the addresses seem  to check out
2b
   1108  802b                   .L0165        ;;  currentscreen = screen01lo
   1109  802b
   1110  802b               a9 ef              LDA    #screen01lo
   1111  802d               85 f1              STA    currentscreen
   1112  802f                   .L0166        ;;  currentscreenhi = screen01hi
   1113  802f
   1114  802f               a9 8b              LDA    #screen01hi
   1115  8031               85 f2              STA    currentscreenhi

 

0x8bef is the address screenmap01 is at:

   4117  8bef                   screenmap01
   4118  8bef               02              .byte.b    (<tileset_colortest + $02)
   4119  8bf0               04              .byte.b    (<tileset_colortest + $04)
   4120  8bf1               06              .byte.b    (<tileset_colortest + $06)

Edited December 29, 2020 by BydoEmpire

[BydoEmpire]

This is throwing me for a loop:

 

currentscreenhi and currentscreen are 01a1 and 01a0 respectively:

    154  10000 ????        01 a1        currentscreenhi =    var97
    156  10000 ????        01 a0        currentscreen =    var96

 

they get assigned the correct location of screenmap01 (0x8bf1):

   1115  802b
   1116  802b               a9 f1              LDA    #screen01_ptr_lo
   1117  802d               8d a0 01           STA    currentscreen
   1118  8030                   .L0166        ;;  currentscreenhi = screen01_ptr_hi
   1119  8030
   1120  8030               a9 8b              LDA    #screen01_ptr_hi
   1121  8032               8d a1 01           STA    currentscreenhi

 

plotmap gets the map file pointer from what seems to be currentscreen (01a0)

   1370  8191               a9 a0              lda    #<currentscreen
   1371  8193               85 42              sta    temp1
   1372  8195
   1373  8195               a9 01              lda    #>currentscreen
   1374  8197               85 43              sta    temp2

 

AHA, it's passing in 01a0 instead of the contents of currentscreen (0x8bf1).  Not sure exactly how to fix yet, but it's been helpful to at least write this down and logically walk through it.  The list.txt file has been SUPER helpful.

 

Q: Can you pass in a (non-const) pointer/variable to plotmap?

Q: Do I have to (or should I) just copy the "current" screenmap data to some other location (like $2200) and always pass that in?

Edited December 29, 2020 by BydoEmpire

[BydoEmpire]

Yeah, that worked fine.  I just copy the current alphadata to $2200 and use that block for everything.  Don't need to change screens too often, so it should be fine.  I *would* like to know if there's a way to use my initial approach, just to satisfy my curiosity.

[+Karl G]

The problem is that plotmap has no idea that it's dealing with a pointer, so it would just treat currentscreen like a normal memory address (the address of the L variable, in your case).

[RevEng]

35 minutes ago, BydoEmpire said:

 I *would* like to know if there's a way to use my initial approach, just to satisfy my curiosity.

The "plotmap" command in your example doesn't just use "currentscreen". It creates a ton of character display objects, each of them pointing to some offset of "currentscreen". So each of these display objects would need to be recalculated and modified under-the-hood, which is a lot more work than just changing one pointer. It could be done if it was really required, but most game designs would be better served by one of the more common screen change models...

 

1. use plotmap with a ram buffer of characters, and change the ram buffer contents whenever required. (it sounds like this is what you settled on)

2. use clearscreen and use plotmap to point to some other rom. (with the understanding this may take a few frames to happen, and you likely want to black the screen while it happens)

 

[BydoEmpire]

Thanks so much, I really appreciate the pointers (pun intended). The 7800basic community is so helpful, and all or the examples made it pretty easy to get up and running for the most part.