William Chops" Westfield" wrote:
>     /*
>      * Our console uart is memory mapped
>      */
>     struct uart_type *console = (struct uart_type *)0x801000;

I guess this defines CONSOLE as a pointer to a record of type UART_TYPE and
sets the pointer pointing to address 801000h?  A comment or two would help.

Anyway to show the flavor I created a whole program in Pascal that pretends
to use memory mapped hardware registers, which I think was your point.  I
also wrote it more in the style and naming convetions I would have written
such a thing:

program x;

const
  adr_uart = 16#801000;         {address of memory mapped UART registers}

type
  uart_p_t = ^uart_t;
  uart_t = record               {memory mapped UART hardware layout}
    stuff: integer32;
    morstuff: integer32;
    end;

var
  console_p: uart_p_t;          {points to console output HW registers}

begin
  console_p := uart_p_t(adr_uart); {point to the console output regs}
  console_p^.stuff := 27;       {send value to console}
  end.

I did a whole program so that I could actually build it to make sure I
didn't mess up something.  This program builds and runs on Windows.  Not
surprisingly is results in a popup about the program performing a illegal
operation when run.  Since my Pascal is implemented as a source to source
translator and the output is for MSVC on Windows, this also created a
temporary C file:

extern void string_cmline_set (
  int,
  char *,
  char *);

/*****************************
**
**   Start of program X.
*/
#define adr_uart_k 8392704
typedef struct uart_t {
  int stuff;
  int morstuff;
  } uart_t;

typedef uart_t *uart_p_t;

__declspec(dllexport) int main (
    int argc,
    char * argv) {

  static uart_p_t console_p;

  static int int_1 = 8392704;
  /*
  **   Executable code for program X.
  */
  string_cmline_set (argc, argv, "x");
  console_p = *(void * *)&int_1;       /* 8392704 */
  console_p->stuff = 27;
  return 0;

  #undef adr_uart_k
  }

Ignore the STRING_CMLINE_SET stuff.  That is implicit initialization added
to the start of all top level programs in my environment on Windows.  The
__DECLSPEC is unique to the Windows target platform.  Also keep in mind that
the above C was written by a machine for a machine, so its appearance and
style may look a bit strange and awkward, and it should really be judged by
the machine code a reasonable C compiler would produce from it.

> Please provide a pascal example that works with at least two different
> vendors' pascal compilers.

No, you're missing the point again.  We're talking about computer science
concepts, not particular implementations.  The point is that what you want
to do is possible in a more tightly typed language like Pascal.  A single
instance is proof enough.  Anyone can install my Windows host build
environment and verify this for themselves if they think I'm cooking the
data somehow.

> but I think it's a fine example of exactly the sort of thing that
> caused less dangerous languages to be dismissed from consideration for
> "systems" programming.

I don't see why.  A variant of Pascal (call it Pascal-like if you prefer)
was used to write the Apollo Aegis operating system.  Someone else pointed
out that a similar language was used to write the original Apple OS.

People seem to forget that tight type checking only means that typing rules
are enforced unless you explicitly say otherwise, as I did in using the data
type UART_P_T as a "type transfer function" in the first executable
statement.  I could have also used UNIV_PTR instead, but that would have
turned off more type checking than necessary.  UNIV_PTR is roughly like a C
void*.


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