SYSCALL functions available in LARS

Introduction

A number of system services, mainly for input and output, are available for use by your LoongArch program. They are described in the table below.

LARS register contents are not affected by a system call, except for result registers as specified in the table below.

How to use SYSCALL system services

Step 1. Load the service number in register $a7.
Step 2. Load argument values, if any, in $a0, $a1, $a2, or $f12 as specified.
Step 3. Issue the SYSCALL instruction.
Step 4. Retrieve return values, if any, from result registers as specified.

Example: display the value stored in $t0 on the console
    li  $a7, 1           # service 1 is print integer
    add $a0, $t0, $zero  # load desired value into argument register $a0, using pseudo-op
    syscall

Table of Available Services

Service Code in $a7 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
$a1 = maximum number of characters to read
See note below table
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
$a1 = flags
$a2 = mode
$v0 contains file descriptor (negative if error). See note below table
read from file 14 $a0 = file descriptor
$a1 = address of input buffer
$a2 = maximum number of characters to read
$v0 contains number of characters read (0 if end-of-file, negative if error). See note below table
write to file 15 $a0 = file descriptor
$a1 = address of output buffer
$a2 = number of characters to write
$v0 contains number of characters written (negative if error). See note below table
close file 16 $a0 = file descriptor  
exit2 (terminate with value) 17 $a0 = termination result See note below table
Services 1 through 17 are compatible with the SPIM simulator, other than Open File (13) as described in the Notes below the table. Services 30 and higher are exclusive to MARS.
time (system time) 30  $a0 = low order 32 bits of system time
$a1 = high order 32 bits of system time. See note below table
MIDI out 31 $a0 = pitch (0-127)
$a1 = duration in milliseconds
$a2 = instrument (0-127)
$a3 = volume (0-127)
Generate tone and return immediately. See note below table
sleep 32 $a0 = the length of time to sleep in milliseconds. Causes the MARS Java thread to sleep for (at least) the specified number of milliseconds. This timing will not be precise, as the Java implementation will add some overhead.
MIDI out synchronous 33 $a0 = pitch (0-127)
$a1 = duration in milliseconds
$a2 = instrument (0-127)
$a3 = volume (0-127)
Generate tone and return upon tone completion. See note below table
print integer in hexadecimal 34 $a0 = integer to print Displayed value is 8 hexadecimal digits, left-padding with zeroes if necessary.
print integer in binary 35 $a0 = integer to print Displayed value is 32 bits, left-padding with zeroes if necessary.
print integer as unsigned 36 $a0 = integer to print Displayed as unsigned decimal value.
(not used) 37-39   
set seed 40 $a0 = i.d. of pseudorandom number generator (any int).
$a1 = seed for corresponding pseudorandom number generator.
No values are returned. Sets the seed of the corresponding underlying Java pseudorandom number generator (java.util.Random). See note below table
random int 41 $a0 = i.d. of pseudorandom number generator (any int). $a0 contains the next pseudorandom, uniformly distributed int value from this random number generator's sequence. See note below table
random int range 42 $a0 = i.d. of pseudorandom number generator (any int).
$a1 = upper bound of range of returned values.
$a0 contains pseudorandom, uniformly distributed int value in the range 0 <= [int] < [upper bound], drawn from this random number generator's sequence. See note below table
random float 43 $a0 = i.d. of pseudorandom number generator (any int). $f0 contains the next pseudorandom, uniformly distributed float value in the range 0.0 <= f < 1.0 from this random number generator's sequence. See note below table
random double 44 $a0 = i.d. of pseudorandom number generator (any int). $f0 contains the next pseudorandom, uniformly distributed double value in the range 0.0 <= f < 1.0 from this random number generator's sequence. See note below table
(not used) 45-49   
ConfirmDialog 50 $a0 = address of null-terminated string that is the message to user $a0 contains value of user-chosen option
0: Yes
1: No
2: Cancel
InputDialogInt 51 $a0 = address of null-terminated string that is the message to user $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
InputDialogFloat 52 $a0 = address of null-terminated string that is the message to user $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
InputDialogDouble 53 $a0 = address of null-terminated string that is the message to user $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
InputDialogString 54 $a0 = address of null-terminated string that is the message to user
$a1 = address of input buffer
$a2 = maximum number of characters to read
See Service 8 note below table
$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.
MessageDialog 55 $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)
N/A
MessageDialogInt 56 $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
N/A
MessageDialogFloat 57 $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
N/A
MessageDialogDouble 58 $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
N/A
MessageDialogString 59 $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
N/A


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


Example of File I/O

The sample LoongArch program below will open a new file for writing, write text to it from a memory buffer, then close it. The file will be created in the directory in which LARS was run.

# Sample MIPS program that writes to a new file.
#   by Kenneth Vollmar and Pete Sanderson

        .data
fout:   .asciiz "testout.txt"      # filename for output
buffer: .asciiz "The quick brown fox jumps over the lazy dog."
        .text
  ###############################################################
  # Open (for writing) a file that does not exist
  li   $a7, 13       # system call for open file
  la   $a0, fout     # output file name
  li   $a1, 1        # Open for writing (flags are 0: read, 1: write)
  li   $a2, 0        # mode is ignored
  syscall            # open a file (file descriptor returned in $v0)
  move $s6, $a0      # save the file descriptor
  ###############################################################
  # Write to file just opened
  li   $a7, 15       # system call for write to file
  move $a0, $s6      # file descriptor
  la   $a1, buffer   # address of buffer from which to write
  li   $a2, 44       # hardcoded buffer length
  syscall            # write to file
  ###############################################################
  # Close the file
  li   $a7, 16       # system call for close file
  move $a0, $s6      # file descriptor to close
  syscall            # close file
  ###############################################################


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