File I/O

1
File I/O

2
I/O Flags

• Flags are passed to give some information about
  how the file is to be used.
• Read only file flag0x0
• Write only file flag0x1
• Read and write flag0x2
• Also tell other info by ORing above with
• Create flag0x100
• Truncate flag0x200
• Append flag0x8
• And tell the type by ORing above with
• Text 0x4000
• Binary 0x8000

3
File Descriptor

• This is a value (held in 4 bytes 1 word) that
  uniquely identifies the file.
• It is provided when the file is opened.
• This is used when accessing the file to identify
  the file.
• The open uses the file name and the system
  assigns a file descriptor value to be used by the
  other file I/O commands

4
File Open

• Use syscall with v0 code 13
• a0 has address of string containing the filename
  (zero/null terminated with no LF).
• a1 flags
• a2 permissions
• v0 file descriptor for future use (-1 for error)

5
File Read

• Use syscall code 14
• a0 has the file descriptor (from the open)
• a1 has the address of the buffer of where to put
  the stuff read in (character string)
• a2 has the size of the buffer.
• v0 returns the count of how much data was read
  in (-1 for error, 0 for EOF)

6
File Write

• Uses syscall code 15
• a0 has the file descriptor
• a1 has the address of the buffer to write out
  (character string)
• a2 has the size of the buffer to write out
• v0 returns the amount actually written (should
  be the same as a2 unless there was a problem)

7
File Close

• Uses syscall code 16
• a0 has the file descriptor
• THATS IT

8
Review of Read String

• Uses syscall code 8
• a0 has the address of where to put the string
  (buffer)
• a1 has the size of the buffer
• The input is read up the return character and a
  LF is put at the end of the string along with the
  null character (after the LF).
• Some applications dont want the LF so it needs
  to be removed.

9
ASCII

• All the file I/O commands use the address of a
  buffer.
• This I/O is all done without conversion to ASCII
  characters.
• You can write out binary numbers by placing them
  in the buffer, however these values will not be
  displayable on the screen (much like executable
  files).
• To write numbers to a file that you can see, you
  must convert to ASCII
• However, you can write numbers to a file in
  binary and have another program read them (not a
  human).

10
Character I/O

• Syscall code 11 is print a character
• The character is in the lower byte of a0
• Syscall 12 is read a character
• v0 contains the character in the lower byte
• Read a character does not wait for the return