Nios II Semihosting Protocol
----------------------------

The runtime (libgloss) indicates a semihosting request to the debug
agent by issuing a "break 1" instruction.  r4 and r5 are used to pass
parameters per the normal C ABI on nios2.

r4 contains a request code.  r5 is typically a pointer to a 4-word
parameter block, except for the exit operation where it is an
immediate integer value.

The result of the operation is returned in the first word of the
parameter block.  The second word is used to return an errno value,
encoded per the "Errno Values" section of the RSP documentation in the
GDB User Manual.

The supported r4 request codes are:

#define HOSTED_EXIT  0

  Terminate program execution; send a 'W' stop reply to GDB.

  r5 contains the exit code, as an immediate integer rather than indirectly
  in a parameter block.  This semihosting request isn't expected to return.

#define HOSTED_INIT_SIM 1

  Reserved/unimplemented.

#define HOSTED_OPEN 2

  Open file; 'Fopen' GDB fileio request.

  r5 points to a parameter block containing:
  [0] pointer to filename
  [1] filename length
  [2] open flags, encoded per the GDB RSP documentation
  [3] mode, encoded per the GDB RSP documentation

  Return values in parameter block:
  [0] file descriptor or -1 on error
  [1] errno, encoded per the GDB RSP documentation

#define HOSTED_CLOSE 3

  Close file; 'Fclose' GDB fileio request.

  r5 points to a parameter block containing:
  [0] file descriptor

  Return values in parameter block:
  [0] return status
  [1] errno, encoded per the GDB RSP documentation

#define HOSTED_READ 4

  Read from file; 'Fread' GDB fileio request.

  r5 points to a parameter block containing:
  [0] file descriptor
  [1] pointer to buffer
  [2] buffer size
  
  Return values in parameter block:
  [0] number of bytes read
  [1] errno, encoded per the GDB RSP documentation

#define HOSTED_WRITE 5

  Write to file; 'Fwrite' GDB fileio request.

  r5 points to a parameter block containing:
  [0] file descriptor
  [1] pointer to buffer
  [2] byte count
  
  Return values in parameter block:
  [0] number of bytes written
  [1] errno, encoded per the GDB RSP documentation

#define HOSTED_LSEEK 6

  File seek; 'Flseek' GDB fileio request.

  r5 points to a parameter block containing:
  [0] file descriptor
  [1] high word of 64-bit offset
  [2] low word of 64-bit offset
  [3] seek flag, encoded per the GDB RSP documentation

  Return values in parameter block:
  [0] high word of 64-bit result
  [1] low word of 64-bit result
  [2] errno, encoded per the GDB RSP documentation

#define HOSTED_RENAME 7

  File rename; 'Frename' GDB fileio request.

  r5 points to a parameter block containing:
  [0] oldname pointer
  [1] oldname length
  [2] newname pointer
  [3] newname length

  Return values in parameter block:
  [0] return status
  [1] errno, encoded per the GDB RSP documentation

#define HOSTED_UNLINK 8

  File unlink/delete; 'Funlink' GDB fileio request.

  r5 points to a parameter block containing:
  [0] filename pointer
  [1] filename length

  Return values in parameter block:
  [0] return status
  [1] errno, encoded per the GDB RSP documentation

#define HOSTED_STAT 9

  File information; 'Fstat' GDB fileio request.

  r5 points to a parameter block containing:
  [0] filename pointer
  [1] filename length
  [2] pointer to stat buf, using the structure definition in the GDB RSP
      documentation 

  Return values in parameter block:
  [0] return status
  [1] errno, encoded per the GDB RSP documentation

#define HOSTED_FSTAT 10

  File information; 'Ffstat' GDB fileio request.
  
  r5 points to a parameter block containing:
  [0] file descriptor
  [1] pointer to stat buf, using the structure definition in the GDB RSP
      documentation 

  Return values in parameter block:
  [0] return status
  [1] errno, encoded per the GDB RSP documentation

#define HOSTED_GETTIMEOFDAY 11

  Get current time; 'Fgettimeofday' GDB fileio request.

  r5 points to a parameter block containing:
  [0] timeval pointer, using the structure definition in the GDB RSP
      documentation

  Return values in parameter block:
  [0] return status
  [1] errno, encoded per the GDB RSP documentation

#define HOSTED_ISATTY 12

 Return true if the file descriptor is the GDB console; 'Fisatty' GDB fileio
 request.

  r5 points to a parameter block containing:
  [0] file descriptor

  Return values in parameter block:
  [0] return status
  [1] errno, encoded per the GDB RSP documentation

#define HOSTED_SYSTEM 13

  System call; 'Fsystem' GDB fileio request.

  r5 points to a parameter block containing:
  [0] command pointer
  [1] command length

  Return values in parameter block:
  [0] return status
  [1] errno, encoded per the GDB RSP documentation