@node syscalls,machine,reentrancy,Top
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@node syscalls,machine,reentrancy,Top
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@chapter NEC V70 system calls
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@chapter NEC V70 system calls
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The library needs a certain amount of system-specific support in order
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The library needs a certain amount of system-specific support in order
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to operate. These routines have to be written for each unique
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to operate. These routines have to be written for each unique
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target environment. For testing purposes,
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target environment. For testing purposes,
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a set of system calls specific to an NEC V70 Unix PC running AT&T Unix
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a set of system calls specific to an NEC V70 Unix PC running AT&T Unix
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System V R2 are included. These files are in the @file{sys/sysvnecv70}
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System V R2 are included. These files are in the @file{sys/sysvnecv70}
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directory.
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directory.
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All the calls have to be implemented in order to avoid link time
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All the calls have to be implemented in order to avoid link time
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errors, but the implementation need not include all possible
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errors, but the implementation need not include all possible
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functionality; in general, for any functionality that isn't
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functionality; in general, for any functionality that isn't
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available, returning an error code is sufficient.
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available, returning an error code is sufficient.
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@section Input/Output
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@section Input/Output
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The V70 target may not have any character I/O devices.
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The V70 target may not have any character I/O devices.
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In this case, the @code{fstat}, @code{ioctl}, @code{isatty},
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In this case, the @code{fstat}, @code{ioctl}, @code{isatty},
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@code{lseek}, @code{read} and @code{write} routines may all return @code{-1},
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@code{lseek}, @code{read} and @code{write} routines may all return @code{-1},
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to signal failure (inspect @file{cerror.s} to see how to do this).
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to signal failure (inspect @file{cerror.s} to see how to do this).
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Sometimes it is correct to implement the functions in a very
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Sometimes it is correct to implement the functions in a very
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simple and machine specific way. For instance, the target board may
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simple and machine specific way. For instance, the target board may
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have one serial line.
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have one serial line.
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In this case, the @code{write} system call can be ``hard-wired'' to
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In this case, the @code{write} system call can be ``hard-wired'' to
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always print to the serial device, no matter what the supplied file
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always print to the serial device, no matter what the supplied file
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handle says. Similarly, the other I/O system calls can be written to
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handle says. Similarly, the other I/O system calls can be written to
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take advantage of a known configuration.
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take advantage of a known configuration.
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Note that the library starts up assuming that three files are already
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Note that the library starts up assuming that three files are already
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open. File handles used are:
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open. File handles used are:
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@table @code
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@table @code
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@item 0
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@item 0
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Is used for all input from @code{stdin}.
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Is used for all input from @code{stdin}.
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@item 1
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@item 1
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Is used for all output to @code{stdout}. This includes functions like
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Is used for all output to @code{stdout}. This includes functions like
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@code{putc} and @code{printf}.
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@code{putc} and @code{printf}.
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@item 2
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@item 2
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Is used for all output to @code{stderr}. The library will use this
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Is used for all output to @code{stderr}. The library will use this
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file to print error messages from the math functions. Output can
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file to print error messages from the math functions. Output can
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also be sent to @code{stderr} by @code{fprintf(stderr,@dots{})}
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also be sent to @code{stderr} by @code{fprintf(stderr,@dots{})}
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@end table
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@end table
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@section Example @code{write} routine
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@section Example @code{write} routine
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On a board with a very simple I/O structure, this would be adequate:
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On a board with a very simple I/O structure, this would be adequate:
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@example
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@example
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@group
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@group
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char *duart_status = DUART_ADDR;
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char *duart_status = DUART_ADDR;
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void poll()
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void poll()
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@{
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@{
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/* Dummy function to fool optimizer */
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/* Dummy function to fool optimizer */
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@}
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@}
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int write(fd, string, len)
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int write(fd, string, len)
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int fd;
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int fd;
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char *string;
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char *string;
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int len;
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int len;
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@{
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@{
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int i;
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int i;
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for (i = 0; i < len; i++)
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for (i = 0; i < len; i++)
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@{
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@{
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while (*duart_status & DUART_BUSY)
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while (*duart_status & DUART_BUSY)
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poll();
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poll();
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*duart_port = string[i];
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*duart_port = string[i];
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@}
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@}
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return len;
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return len;
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@}
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@}
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@end group
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@end group
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@end example
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@end example
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@section Memory allocation
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@section Memory allocation
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The library allocates memory from the heap either for its own use, or when
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The library allocates memory from the heap either for its own use, or when
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you explicitly call @code{malloc}. It asks the system for
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you explicitly call @code{malloc}. It asks the system for
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memory by calling the @code{sbrk} function.
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memory by calling the @code{sbrk} function.
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On a Unix system, @code{sbrk} keeps track of the heap's extent by keeping a
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On a Unix system, @code{sbrk} keeps track of the heap's extent by keeping a
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pointer to the end of the @code{bss} section. Unix linkers
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pointer to the end of the @code{bss} section. Unix linkers
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traditionaly mark the end of @code{bss} by creating a symbol
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traditionaly mark the end of @code{bss} by creating a symbol
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@code{_end}. When the library wants more memory, it calls
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@code{_end}. When the library wants more memory, it calls
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@code{sbrk} with the size of the request. @code{sbrk} must then
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@code{sbrk} with the size of the request. @code{sbrk} must then
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perform an operation specific to the target environment, and return a pointer
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perform an operation specific to the target environment, and return a pointer
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to the new area. For a simple application, the following fragment may
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to the new area. For a simple application, the following fragment may
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be sufficient:
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be sufficient:
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@example
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@example
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@group
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@group
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char *moving_end = &end;
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char *moving_end = &end;
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char *sbrk(request)
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char *sbrk(request)
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int request;
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int request;
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@{
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@{
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char *return_address;
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char *return_address;
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return_address = moving_end;
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return_address = moving_end;
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moving_end += request;
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moving_end += request;
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return return_address;
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return return_address;
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@}
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@}
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@end group
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@end group
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@end example
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@end example
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@section Initialization and termination
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@section Initialization and termination
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The system dependent support routines are responsible for
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The system dependent support routines are responsible for
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initializing the library for use by an application, and cleaning up
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initializing the library for use by an application, and cleaning up
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when the application is complete.
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when the application is complete.
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This functionality is traditionally provided in @code{crt0} and
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This functionality is traditionally provided in @code{crt0} and
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@code{_exit}.
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@code{_exit}.
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The @code{crt0} function usually contains the instructions first run
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The @code{crt0} function usually contains the instructions first run
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by the operating system when an application starts. The
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by the operating system when an application starts. The
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@code{crt0} function can take advantage of this and prepare the way
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@code{crt0} function can take advantage of this and prepare the way
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for the libary.
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for the libary.
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Another task for @code{crt0} is to call the @code{main} function
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Another task for @code{crt0} is to call the @code{main} function
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provided by the application writer, and also to call @code{exit} if
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provided by the application writer, and also to call @code{exit} if
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the main function ever returns.
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the main function ever returns.
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@code{exit} tells the operating system that the application has
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@code{exit} tells the operating system that the application has
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finished.
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finished.
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