Protecting Memory For P/M And Character Sets
Fred Pinho
Redefined character sets and player/missile graphics both require protected memory. This article shows how to avoid memory conflicts.
Other articles in this book explain how to properly locate either a redefined character set or player/missile data. But what if you want to use both at the same time? You can't use the formulas given in these articles directly, because the two data sets will interfere with each other. To simplify matters, I've prepared the table below. It will allow you to position both your P/M and character data so that they won't clash.
Note that the two sets of data are stored in memory below the display list. Because of this, a couple of cautions are in order:
1. Be careful when changing graphics modes. If you go to a graphics mode requiring increased display memory, you could overwrite your P/M data. It's probably best to locate your data to accommodate the graphics mode with the largest memory requirement. To help, I've included the memory requirements for each graphics mode.
2. You have to watch your BASIC program to insure that it doesn't expand into your data-storage area.
In your reading, you'll come across other methods of storing these data sets (above a lowered RAMTOP, in a string, etc.). However, these methods have some serious limitations of their own. The method given here is straightforward and easy to trouble-shoot.
The table gives the offset from RAMTOP (in pages) needed to properly locate each data set. Note that a page is a fancy way of describing a block of 256 data bytes. Also, the offset is subtracted from RAMTOP to get the proper memory location. I've given an example of use of the table below:
Desired: GRAPHICS 7
Single-Line Player/Missile Graphics
Redefined Full Character Set
Code: 10 PM=PEEK(106-32:REM Calculate page setback for player/missile data 20 POKE 54279,PM:REM Set page number of PMBASE 30 PMBASE=256*PM:REM Calculate memory location of PMBASE 40 CHRSTART=256*(PM-4):REM Calculate page offset for new character set 50 POKE 756,CHRSTART:REM Set CHBASE to point to new character set location
From here, you can go on to implement the player/missile system and your redefined character set as described elsewhere in this book. May all your missiles be on target.
Simultaneous Positioning Of Player/Missile And Redefined Character Set Data in Memory.
|
Memery required for Display Data + Display List |
Player/Missile Data Locate PMBASE at the indicated offset (in pages) from RAMTOP |
Relocation of full character set beneath player/missile data Relocate at the indicated offset (in pages) from RAMTOP |
||||
|
Graphics mode |
Total Bytes |
As whole pages |
Double-line resolution |
Single-line resolution |
Character set with double-line P/M |
Character set with single-line P/M |
|
0 1 2 3 4 5 6 7 8 |
992 674 424 434 694 1174 2174 4190 8112 |
4 3 2 2 3 5 9 17 32 |
8 8 8 8 8 12 16 24 36 |
16 16 16 16 16 16 24 32 40 |
12 12 12 12 12 16 20 28 40 |
20 20 20 20 20 20 28 36 44 |
Notes:
A. General
- 1. All page calculations are to the nearest, highest whole page.
2. RAMTOP (location 106) defines the top of available memory. The display data lies just beneath RAMTOP. The display list resides just beneath the display data.
B. Player/Missile
- Player/missile offsets are calculated by observing the following restrictions for the location of PMBASE.
| Double-Line resolution |
Single-Line resolution |
|
| Offset from any other data | 1K | 2K |
| Boundary location for PMBASE | 1K | 2K |
C. Character-Set
- The full redefined character set must start on a 1K boundary (i.e., the first memory location must be a multiple of 1024).
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