MARS syscalls

To make simulation interactive, we use system calls in the way MARS and SPIM simulators do:

rvice	Code in $v0	Arguments	Result
print integer	1	$a0 = integer to print
print float	2	$f12 = float to print
print double	3	$f12 = double to print
print string	4	$a0 = address of null-terminated string to print
read integer	5		$v0 contains integer read
read float	6		$f0 contains float read
read double	7		$f0 contains double read
read string	8	$a0 = address of input buffer	See note below table
		$a1 = maximum number of characters to read
sbrk (allocate heap memory)	9	$a0 = number of bytes to allocate	$v0 contains address of allocated memory
exit (terminate execution)	10
print character	11	$a0 = character to print	See note below table
read character	12		$v0 contains character read
open file	13	$a0 = address of null-terminated string containing filename	$v0 contains file descriptor (negative if error). See note below table
		$a1 = flags
		$a2 = mode
read from file	14	$a0 = file descriptor	$v0 contains number of characters read (0 if end-of-file, negative if error). See note below table
		$a1 = address of input buffer
		$a2 = maximum number of characters to read
write to file	15	$a0 = file descriptor	$v0 contains number of characters written (negative if error). See note below table
		$a1 = address of output buffer
		$a2 = number of characters to write
close file	16	$a0 = file descriptor
exit2 (terminate with value)	17	$a0 = termination result	See note below table

Service 8 - Follows semantics of UNIX 'fgets'. For specified length n, string can be no longer than n-1. If less than that, adds newline to end. In either case, then pads with null byte If n = 1, input is ignored and null byte placed at buffer address. If n < 1, input is ignored and nothing is written to the buffer.

Service 11 - Prints ASCII character corresponding to contents of low-order byte.

Service 13 - MARS implements three flag values: 0 for read-only, 1 for write-only with create, and 9 for write-only with create and append. It ignores mode. The returned file descriptor will be negative if the operation failed. The underlying file I/O implementation uses java.io.FileInputStream.read() to read and java.io.FileOutputStream.write() to write. MARS maintains file descriptors internally and allocates them starting with 3. File descriptors 0, 1 and 2 are always open for: reading from standard input, writing to standard output, and writing to standard error, respectively (new in release 4.3).

Services 13,14,15 - In MARS 3.7, the result register was changed to $v0 for SPIM compatability. It was previously $a0 as erroneously printed in Appendix B of Computer Organization and Design,.

Service 17 - If the MIPS program is run under control of the MARS graphical interface (GUI), the exit code in $a0 is ignored.