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eug_titles/PT3/+P3

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 hard disk » misc » misc3
Filename: eug_titles/PT3/+P3
Read OK: ✔
File size: 1F11 bytes
Load address: 0000
Exec address: 0000
Duplicates

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

File contents
If you are not familiar with machine code, then you can probably skip the next
section on how to directly access screen memory, as it is impossible
to achieve any speed by accessing screen memory directly in BASIC - your
program will crawl along incredibly slowly. You could still use such a
technique in preparing a screen which you intend to save to floppy using
the technique described before with *SAVE and *LOAD.

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!

Now that you can locate a the memory occupied by a character in screen
memory, you need to know how the bytes within this block are laid out, so that
you can have pixel by pixel control over your monitor screen. This is also
required if you are to be able to calculate the amount of memory taken up
by each character's definition in a particular mode.

The graphics modes can be divided into three categories: monochrome modes
(ie. two colours), four colour modes, and sixteen colour modes. All of the
Elk's modes fall into these three groupings. Which of these colours each
pixel of the screen is depends upon the status of a number of binary bits.
These have two states, 0 or 1. Now, in a monochrome mode, one bit is
sufficient to define the colour of one pixel (bits have two states, 0 or 1,
and the pixels have two states, colour 0 or colour 1). In a four colour mode,
on the other hand, one bit will not be sufficient, as four different states
are required. Two bits will be enough to store the colour of one pixel,
however:

Bit 1 state     Bit 2 state     Colour
  0               0               0
  0               1               1
  1               0               2
  1               1               3

Those familiar with binary will spot the pattern straight away - simply
convert the binary value of the two bits into decimal, and you have the
logical colour number. A very similar pattern applies to MODE 2 - the Elk's
16 colour mode. Each pixel requires 4 bits to store its colour (2^4=16).
Once again, the colour of the pixel is the logical colour whose number is
given when four bits are put together to form a binary value:

Bit 1 state  Bit 2 state  Bit 3 state  Bit 4 state  Colour  Default
  0            0            0            0            0       BLK
  1            0            0            0            1       RED
  0            1            0            0            2       GRN
  1            1            0            0            3       YEL
  0            0            1            0            4       BLE
  1            0            1            0            5       MGN
  0            1            1            0            6       CYN
  1            1            1            0            7       WHT
  0            0            0            1            8     BLK/WHT
  1            0            0            1            9     RED/CYN
  0            1            0            1           10     GRN/MGN
  1            1            0            1           11     YEL/BLE
  0            0            1            1           12     BLE/YEL
  1            0            1            1           13     MGN/GRN
  0            1            1            1           14     CYN/RED
  1            1            1            1           15     WHT/BLK

[Default: Default setting of this logical colour when MODE 2 is first
entered]

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00000ae0  68 61 72 61 63 74 65 72  73 2c 20 65 61 63 68 20  |haracters, each |
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00001a80  20 43 6f 6c 6f 75 72 20  20 44 65 66 61 75 6c 74  | Colour  Default|
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00001aa0  30 20 20 20 20 20 20 20  20 20 20 20 20 30 20 20  |0            0  |
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00001b30  20 30 20 20 20 20 20 20  20 20 20 20 20 20 30 20  | 0            0 |
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00001c10  20 35 20 20 20 20 20 20  20 4d 47 4e 0d 20 20 30  | 5       MGN.  0|
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00001c50  20 20 20 36 20 20 20 20  20 20 20 43 59 4e 0d 20  |   6       CYN. |
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00001cf0  20 20 20 20 30 20 20 20  20 20 20 20 20 20 20 20  |    0           |
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00001ec0  0d 0d 5b 44 65 66 61 75  6c 74 3a 20 44 65 66 61  |..[Default: Defa|
00001ed0  75 6c 74 20 73 65 74 74  69 6e 67 20 6f 66 20 74  |ult setting of t|
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00001ef0  75 72 20 77 68 65 6e 20  4d 4f 44 45 20 32 20 69  |ur when MODE 2 i|
00001f00  73 20 66 69 72 73 74 0d  65 6e 74 65 72 65 64 5d  |s first.entered]|
00001f10  0d                                                |.|
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