Re: Pointer to video memory, standalone

2 Aug 2006

      Waldek Hebisch wrote:
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[ Charset ISO-8859-1 unsupported, converting... ]
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Hi there,
It looks like it's not possible to declare a pointer to char and 
initialise it to the video memory, e.g.
type
    VideoMemory: ^char;
var
    VideoMemory := 0x80000;
Excuse the syntax if its wrong, I'm new to Pascal. :-)
The following shows how to store a character at address 0x80000:
program VideoMemory;
type
    VideoMemory = ^char;
var
    VideoMemoryVar : VideoMemory value VideoMemory($80000);
begin
  VideoMemoryVar^ := 'a';
end
.
Note that it depends on your OS what will be at the address 0x80000.
If you try to access video memory after entering protected mode,
but before you initialize memory management your OS will crash.
Also variables and pointer types used to access hardware should be
marked `volatile', but I just noticed that currently GPC does not
allow this...
It does, but as an "attribute" (in order to reduce syntax pollution
for a very rarely used feature).

program VideoMemory;
type
    VolatileChar = char attribute (volatile);
    VideoMemory = ^VolatileChar;

procedure p;
var
  VideoMemoryVar : VideoMemory value VideoMemory($80000);
  c: Char;
begin
  c := VideoMemoryVar^;
  c := VideoMemoryVar^;
  c := VideoMemoryVar^;
  c := VideoMemoryVar^;
  c := VideoMemoryVar^;
  c := VideoMemoryVar^;
  c := VideoMemoryVar^;
  c := VideoMemoryVar^;
  c := VideoMemoryVar^;
end;

begin
  p
end.

(The procedure body is there to look at the generated assmebler code
and see the multiple accesses, which vanish without the attribute
when compiling with optimization.)

Frank

-- 
Frank Heckenbach, f.heckenbach@fh-soft.de, http://fjf.gnu.de/, 7977168E
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