MARS syscalls

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

MARS kernel is enabled by --mars option. Currently only little-endian MIPS and RISC-V are supported.

To make syscall in MIPS, use following instruction sequence:

   <load arguments>
   li $v0, <code>
   syscall

To make syscall in RISC-V, use following instruction sequence:

   <load arguments>
   li x2, <code>
   ecall

Table of system calls

Type	Code in $v0 / x2	Arguments	Result
print integer	1	$a0 / x4 = integer to print
print float	2	$f12 = float to print
print double	3	$f12 = double to print
print string	4	$a0 / x4 = address of null-terminated string to print
read integer	5		$v0 / x2 contains integer read
read float	6		$f0 contains float read
read double	7		$f0 contains double read
read string	8	$a0 / x4 = address of input buffer $a1 / x5 = maximum number of characters to read	See note below table
sbrk (allocate heap memory)	9	$a0 / x4 = number of bytes to allocate	$v0 / x2 contains address of allocated memory
exit (terminate execution)	10
print characte	11	$a0 / x4 = character to print
read character	12		$v0 / x2 contains character read
open file	13	$a0 / x4 = address of null-terminated string containing filename $a1 / x5 = flags $a2 / x6 = mode	$v0 / x2 contains file descriptor (negative if error).
read from file	14	$a0 / x4 = file descriptor	$v0 / x2 contains number of characters read (0 if end-of-file, negative if error).
write to file	15	$a0 / x4 = file descriptor $a1 / x5 = address of output buffer $a2 / x6 = number of characters to write	$v0 / x2 contains number of characters written (negative if error).
close file	16	$a0 / x4 = file descriptor
exit2 (terminate with value)	17	$a0 / x4 = termination result