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+P4

This website contains an archive of files for the Acorn Electron, BBC Micro, Acorn Archimedes, Commodore 16 and Commodore 64 computers, which Dominic Ford has rescued from his private collection of floppy disks and cassettes.

Some of these files were originally commercial releases in the 1980s and 1990s, but they are now widely available online. I assume that copyright over them is no longer being asserted. If you own the copyright and would like files to be removed, please contact me.

Tape/disk: Home » Personal collection » Acorn ADFS disks » Electron_User_Group » EUG_38.ADF
Filename: +P4
Read OK: ✔
File size: 123D bytes
Load address: 2B204556
Exec address: D3450
Duplicates

There is 1 duplicate copy of this file in the archive:

File contents
Firstly, it is important to know how screen memory is laid out in memory.
The exact position of each character within the memory of your computer
varies from one graphics mode to another, but the basic principals are the
same:

- The screen area is divided up into characters. These are blocks of eight 
  pixels by eight pixels, and are the same size as the rectangular block
  of space taken up by one letter when you print text to the screen. Screen
  memory is divided up into blocks, with each the characters being defined
  by one of these blocks.
- The screen is stored row by row, with the top row first and the bottom
  row last.
- Within each of these rows, characters are stored one by one, from left to
  right.
- In all modes, screen memory ends at &7FFF (ie. at the end of RAM)

Footnote: By convention, memory addresses are specified in hexidecimal. I
          use the standard Acorn '&' prefix to indicate hexidecimal values.
          Those used to dealing with PCs are any other computers may be used
          to the American convention of using a dollar prefix in its place.
          It should be noted that the Elk expects & and will not accept $.

Example:

Suppose I want to draw a blob in character square (6,7), in mode 1. What
is the address of this square? (given that each character takes 16 bytes
of screen memory to define in mode 1 - we'll come to how to calculate this
in a minute).

Well - looking at the table earlier, screen memory starts at &3000 in mode
1. We want to find the start of row 7 in screen memory. This row has 7
rows of characters above it, and which will be stored before it. Each of
these rows consists of 40 characters, which take up 16 bytes each.
Therefore the total memory taken up by these rows is 7 * 40 * 16 = 4480
bytes. [Each character is 16 bytes, each row consists of 40 of these
characters, therefore we multiply by 40, and as there are 7 of these
complete rows, we multiply by 7 to get the value for seven of these rows]

Now we have that the starting address of row 7 is at:

&3000 + 4480 = &4180

Now that we have found where abouts in memory this row starts, we need to
locate character 6. This should be fairly easy - we know that characters are
stored from left to right, so just add on the memory required to store six
characters to the left of the one we want, i.e. 16 * 6 = 96 bytes [Six
characters, each taking 16 bytes to store].

Therefore the address of character (6,7) will be:

&4180 + 96 = &41E0

Answer: Character (6,7) is stored from &41E0 to &41EF

Just to check the answer is correct, type this simple loop:

10 MODE 1
20 FOR ad=&41E0 TO &41EF
30 ?ad=255
40 NEXT

And yes, a white splodge does appear in square (6,7).

This whole procedure can actually be refined down, and expressed as a simple
formula:

Address = Base + ( y * No. characters per line + x ) * Memory per character

Where: Address= The memory location of the start of the character you want.
       Base   = Start of screen memory in current graphics mode.
       (x,y)  = Co-ordinate of square you want to access (same as PRINT TAB)

Since the number of characters per line, the number of bytes per character,
and the base address of screen memory are all constant for any particular
graphics mode, the values for the graphics mode you are using can be
substituted into the equation in your program. For mode 1, for example, you
would use:

Address = &3000 + (40 * y + x) * 16

Before continuing, one note should be borne in mind. You may remember that
a while back I mentioned that when you type text which overflows off the
bottom of the screen, the computer makes a note that each line is stored
in memory one line lower than it should be, but does not actually move
anything around in screen memory. The same effect can cause problems with
direct access to screen memory as well. The simple rule for beginners is,
NEVER attempt to use direct screen memory access when there is even the
slightest possibility that the screen might have been allowed to scroll up
at all (e.g. after using INPUT to get input from the user - how do you
know that they didn't type so much that it went of the bottom of the
screen?) If you know what you are doing, this rule can be ignored, and
high-speed scrolling can be achieved as a result, but that is not for the
beginner! I can tell you from first hand experience, that creating
scrolling in Jupiter 3 by this technique was a nightmare - especially when
the operating system calls you need to make to achieve this are completely
different on the BBC to the Elk, so you need to find out which platform
you are operating on, and then make the relavant calls for that platform!


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00000a50  35 0d 34 30 20 4e 45 58  54 0d 0d 41 6e 64 20 79  |5.40 NEXT..And y|
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00000b40  72 65 73 73 3d 20 54 68  65 20 6d 65 6d 6f 72 79  |ress= The memory|
00000b50  20 6c 6f 63 61 74 69 6f  6e 20 6f 66 20 74 68 65  | location of the|
00000b60  20 73 74 61 72 74 20 6f  66 20 74 68 65 20 63 68  | start of the ch|
00000b70  61 72 61 63 74 65 72 20  79 6f 75 20 77 61 6e 74  |aracter you want|
00000b80  2e 0d 20 20 20 20 20 20  20 42 61 73 65 20 20 20  |..       Base   |
00000b90  3d 20 53 74 61 72 74 20  6f 66 20 73 63 72 65 65  |= Start of scree|
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00000bb0  65 6e 74 20 67 72 61 70  68 69 63 73 20 6d 6f 64  |ent graphics mod|
00000bc0  65 2e 0d 20 20 20 20 20  20 20 28 78 2c 79 29 20  |e..       (x,y) |
00000bd0  20 3d 20 43 6f 2d 6f 72  64 69 6e 61 74 65 20 6f  | = Co-ordinate o|
00000be0  66 20 73 71 75 61 72 65  20 79 6f 75 20 77 61 6e  |f square you wan|
00000bf0  74 20 74 6f 20 61 63 63  65 73 73 20 28 73 61 6d  |t to access (sam|
00000c00  65 20 61 73 20 50 52 49  4e 54 20 54 41 42 29 0d  |e as PRINT TAB).|
00000c10  0d 53 69 6e 63 65 20 74  68 65 20 6e 75 6d 62 65  |.Since the numbe|
00000c20  72 20 6f 66 20 63 68 61  72 61 63 74 65 72 73 20  |r of characters |
00000c30  70 65 72 20 6c 69 6e 65  2c 20 74 68 65 20 6e 75  |per line, the nu|
00000c40  6d 62 65 72 20 6f 66 20  62 79 74 65 73 20 70 65  |mber of bytes pe|
00000c50  72 20 63 68 61 72 61 63  74 65 72 2c 0d 61 6e 64  |r character,.and|
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