MIPS Syscalls

MIPS syscalls are used to make requests to the operating system. They are not instructions that are executed by the CPU, but rather instructions that are used by the simulator to make requests to the operating system.

Each syscall has a unique code that is used to identify it. You must put the syscall code inside the the $v0 register before calling the syscall instruction.

As an example:

# Load the integer 42 into register $a0, which is
# the register that will be printed by the syscall 
li $a0, 42 

# Set the syscall code for printing an integer
li $v0, 1
syscall

1 - Print integer

Arguments

$a0

integer to print

2 - Print float

Arguments

$f12

float to print

3 - Print double

Arguments

$f12

double to print

4 - Print string

Arguments

$a0

address of null-terminated string to print

5 - Read integer

Result

$v0

contains integer read

6 - Read float

Result

$f0

contains float read

7 - Read double

Result

$f0

contains double read

8 - Read string

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.

Arguments

$a0

address of input buffer

$a1

maximum number of characters to read

9 - Sbrk (allocate heap memory)

Result

$v0

contains address of allocated memory

Arguments

$a0

number of bytes to allocate

10 - Exit (terminate execution)

11 - Print character

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

Arguments

$a0

character to print

12 - Read character

Result

$v0

contains character read

13 - Open file

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. 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).

Result

$v0

contains file descriptor (negative if error)

Arguments

$a0

address of null-terminated string containing filename

$a1

flags

$a2

mode

14 - Read from file

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,.

Result

$v0

contains number of characters read (0 if end-of-file, negative if error)

Arguments

$a0

file descriptor

$a1

address of input buffer

$a2

maximum number of characters to read

15 - Write to file

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,.

Result

$v0

contains number of characters written (negative if error)

Arguments

$a0

file descriptor

$a1

address of output buffer

$a2

number of characters to write

16 - Close file

Arguments

$a0

file descriptor

17 - Exit2 (terminate with value)

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

Arguments

$a0

termination result

30 - Time (system time)

Service 30 - System time as milliseconds since 1 January 1970.

Result

$a0

low order 32 bits of system time

$a1

high order 32 bits of system time

34 - Print integer in hexadecimal

Displayed value is 8 hexadecimal digits, left-padding with zeroes if necessary.

Arguments

$a0

integer to print

35 - Print integer in binary

Displayed value is 32 bits, left-padding with zeroes if necessary.

Arguments

$a0

integer to print

36 - Print integer as unsigned

Displayed as unsigned decimal value.

Arguments

$a0

integer to print

41 - Random int

Each stream (identified by $a0 contents) is modeled by a different Random object. There are no default seed values, so use the Set Seed service (40) if replicated random sequences are desired.

Result

$a0

contains the next pseudorandom, uniformly distributed int value from this random number generator's sequence

Arguments

$a0

i.d. of pseudorandom number generator (any int)

42 - Random int range

Each stream (identified by $a0 contents) is modeled by a different Random object. There are no default seed values, so use the Set Seed service (40) if replicated random sequences are desired.

Result

$a0

contains pseudorandom, uniformly distributed int value in the range 0 <= [int] < [upper bound], drawn from this random number generator's sequence

Arguments

$a0

i.d. of pseudorandom number generator (any int)

$a1

upper bound of range of returned values

43 - Random float

Each stream (identified by $a0 contents) is modeled by a different Random object. There are no default seed values, so use the Set Seed service (40) if replicated random sequences are desired.

Result

$f0

contains the next pseudorandom, uniformly distributed float value in the range 0.0 <= f < 1.0 from this random number generator's sequence

Arguments

$a0

i.d. of pseudorandom number generator (any int)

44 - Random double

Each stream (identified by $a0 contents) is modeled by a different Random object. There are no default seed values, so use the Set Seed service (40) if replicated random sequences are desired.

Result

$f0

contains the next pseudorandom, uniformly distributed double value in the range 0.0 <= f < 1.0 from this random number generator's sequence

Arguments

$a0

i.d. of pseudorandom number generator (any int)

50 - ConfirmDialog

Result

$a0

contains value of user-chosen option 0: Yes 1: No 2: Cancel

Arguments

$a0

address of null-terminated string that is the message to user

51 - InputDialogInt

Result

$a0

contains int read

$a1

contains status value 0: OK status -1: input data cannot be correctly parsed -2: Cancel was chosen -3: OK was chosen but no data had been input into field

Arguments

$a0

address of null-terminated string that is the message to user

52 - InputDialogFloat

Result

$f0

contains float read

$a1

contains status value 0: OK status -1: input data cannot be correctly parsed -2: Cancel was chosen -3: OK was chosen but no data had been input into field

Arguments

$a0

address of null-terminated string that is the message to user

53 - InputDialogDouble

Result

$f0

contains double read

$a1

contains status value 0: OK status -1: input data cannot be correctly parsed -2: Cancel was chosen -3: OK was chosen but no data had been input into field

Arguments

$a0

address of null-terminated string that is the message to user

54 - InputDialogString

See Service 8 note below table

Result

$a1

contains status value 0: OK status. Buffer contains the input string. -2: Cancel was chosen. No change to buffer. -3: OK was chosen but no data had been input into field. No change to buffer. -4: length of the input string exceeded the specified maximum. Buffer contains the maximum allowable input string plus a terminating null.

Arguments

$a0

address of null-terminated string that is the message to user

$a1

address of input buffer

$a2

maximum number of characters to read

55 - MessageDialog

N/A

Arguments

$a0

address of null-terminated string that is the message to user

$a1

the type of message to be displayed: 0: error message, indicated by Error icon 1: information message, indicated by Information icon 2: warning message, indicated by Warning icon 3: question message, indicated by Question icon other: plain message (no icon displayed)

56 - MessageDialogInt

N/A

Arguments

$a0

address of null-terminated string that is an information-type message to user

$a1

int value to display in string form after the first string

57 - MessageDialogFloat

N/A

Arguments

$a0

address of null-terminated string that is an information-type message to user

$f12

float value to display in string form after the first string

58 - MessageDialogDouble

N/A

Arguments

$a0

address of null-terminated string that is an information-type message to user

$f12

double value to display in string form after the first string

59 - MessageDialogString

N/A

Arguments

$a0

address of null-terminated string that is an information-type message to user

$a1

address of null-terminated string to display after the first string