Steven Herbst, 4.23.05

FILE I/O with SPIM:
The 'open' system call has the code '13', and requires
the following arguments:
$a0: filename (string)
$a1: flags - bitwise-or O_RDONLY, O_WRONLY, or O_RDWR 
	     with any of the other 'O_' constants
	     Type 'man 2 open' for details on the
	     meaning of each constant
$a2: mode  - when O_CREAT is specified, $a2 must be set
	     to one of the 'S_' constants
	     Type 'man 2 open' for details on the 
	     meaning of each constant
Returns file descriptor in $v0

The 'read' system call has the code '14', and requires
the following arguments:
$a0: file descriptor (integer from 'open' system call)
$a1: buffer - storage location of characters being read
$a2: length - number of characters to read
Returns the number of characters read in $v0

The 'write' system call has the code '15', and requires
the following arguments:
$a0: file descriptor (integer from 'open' system call)
$a1: buffer - location of characters being written
$a2: length - number of characters to write
Returns the number of characters written in $v0

The 'close' system call has the code '16', and requires
the following argument:
$v0: file descriptor (integer from 'close' system call)

CONSTANTS:

O_RDONLY: 0
O_WRONLY: 1
O_RDWR: 2

O_CREAT: 64
O_EXCL: 128
O_NOCTTY: 256
O_TRUNC: 512
O_APPEND: 1024
O_NONBLOCK: 2048
O_NDELAY: 2048
O_SYNC: 4096

S_IRWXU: 448
S_IRUSR: 256
S_IREAD: 256
S_IWUSR: 128
S_IWRITE: 128
S_IXUSR: 64
S_IEXEC: 64
S_IRWXG: 56
S_IRGRP: 32
S_IWGRP: 16
S_IXGRP: 8
S_IRWXO: 7
S_IROTH: 4
S_IWOTH: 2
S_IXOTH: 1