File I/O

I/O Flags Flags are passed to give some information about how the file is to be used. – Read only file – flag=0x0 – Write only file – flag=0x1 – Read and write – flag=0x2 Also tell other info by ORing above with – Create – flag=0x100 – Truncate – flag=0x200 – Append – flag=0x8 And tell the type by ORing above with – Text – 0x4000 – Binary – 0x8000

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

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)

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)

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)

File Close Uses syscall code 16 $a0 has the file descriptor THAT’S IT

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 don’t want the LF so it needs to be removed.

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).

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