Subversion Repositories openrisc

@node syscalls,machine,reentrancy,Top

@chapter NEC V70 system calls

The library needs a certain amount of system-specific support in order

to operate.  These routines have to be written for each unique

target environment.  For testing purposes,

a set of system calls specific to an NEC V70 Unix PC running AT&T Unix

System V R2 are included.  These files are in the @file{sys/sysvnecv70}

directory.

All the calls have to be implemented in order to avoid link time

errors, but the implementation need not include all possible

functionality; in general, for any functionality that isn't

available, returning an error code is sufficient.

@section Input/Output

The V70 target may not have any character I/O devices.

In this case, the @code{fstat}, @code{ioctl}, @code{isatty},

@code{lseek}, @code{read} and @code{write} routines may all return @code{-1},

to signal failure (inspect @file{cerror.s} to see how to do this).

Sometimes it is correct to implement the functions in a very

simple and machine specific way.  For instance, the target board may

have one serial line.

In this case, the @code{write} system call can be ``hard-wired'' to

always print to the serial device, no matter what the supplied file

handle says. Similarly, the other I/O system calls can be written to

take advantage of a known configuration.

Note that the library starts up assuming that three files are already

open.  File handles used are:

@table @code

@item 0

Is used for all input from @code{stdin}.

@item 1

Is used for all output to @code{stdout}.  This includes functions like

@code{putc} and @code{printf}.

@item 2

Is used for all output to @code{stderr}.  The library will use this

file to print error messages from the math functions.  Output can

also be sent to @code{stderr} by @code{fprintf(stderr,@dots{})}

@end table

@section Example @code{write} routine

On a board with a very simple I/O structure, this would be adequate:

@example

@group

char *duart_status = DUART_ADDR;

void poll()

@{

   /* Dummy function to fool optimizer */

@}

int write(fd, string, len)

int fd;

char *string;

int len;

@{

  int i;

  for (i = 0; i < len; i++)

  @{

    while (*duart_status & DUART_BUSY)

     poll();

    *duart_port = string[i];

  @}

  return len;

@}

@end group

@end example

@section Memory allocation

The library allocates memory from the heap either for its own use, or when

you explicitly call @code{malloc}.  It asks the system for

memory by calling the @code{sbrk} function.

On a Unix system, @code{sbrk} keeps track of the heap's extent by keeping a

pointer to the end of the @code{bss} section.  Unix linkers

traditionaly mark the end of @code{bss} by creating a symbol

@code{_end}.  When the library wants more memory, it calls

@code{sbrk} with the size of the request.  @code{sbrk} must then

perform an operation specific to the target environment, and return a pointer

to the new area.  For a simple application, the following fragment may

be sufficient:

@example

@group

char *moving_end = &end;

char *sbrk(request)

int request;

@{

  char *return_address;

  return_address = moving_end;

  moving_end += request;

  return return_address;

@}

@end group

@end example

@section Initialization and termination

The system dependent support routines are responsible for

initializing the library for use by an application, and cleaning up

when the application is complete.

This functionality is traditionally provided in @code{crt0} and

@code{_exit}.

The @code{crt0} function usually contains the instructions first run

by the operating system when an application starts.  The

@code{crt0} function can take advantage of this and prepare the way

for the libary.

Another task for @code{crt0} is to call the @code{main} function

provided by the application writer, and also to call @code{exit} if

the main function ever returns.

@code{exit} tells the operating system that the application has

finished.