A number of system services, mainly for input and output, are available for use by your MIPS program. They are described in the table below.
MIPS register contents are not affected by a system call, except for result registers as specified in the table below.
li $v0, 1 # service 1 is print integer add $a0, $t0, $zero # load desired value into argument register $a0, using pseudo-op syscall
Service | 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 $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 |
# 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 $v0, 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, $v0 # save the file descriptor ############################################################### # Write to file just opened li $v0, 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 $v0, 16 # system call for close file move $a0, $s6 # file descriptor to close syscall # close file ###############################################################
Service 31 will generate the tone then immediately return. Service 33 will generate the tone then sleep for the tone's duration before returning. Thus it essentially combines services 31 and 32.
This service requires four parameters as follows:
pitch ($a0)
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duration in milliseconds ($a1)
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instrument ($a2)
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volume ($a3)
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System service 31 was developed and documented by Otterbein student Tony Brock in July 2007.
Items in [ ] are optional. Valid options (not case sensitive,
separated by spaces) are:
Option | Description | Since |
---|---|---|
a | assemble only, do not simulate | 1.0 |
aen | terminate MARS with integer exit code n if assembly error occurs | 4.1 |
ascii | display memory or register contents interpreted as ASCII codes. (alternatives are dec and hex) | 4.1 |
b | brief - do not display register/memory address along with contents | 2.2 |
d | display MARS debugging statements (of interest mainly to MARS developer) | 1.0 |
db | MIPS delayed branching is enabled. | 3.3 |
dec | display memory or register contents in decimal. (alternatives are ascii and hex) | 2.2 |
dump | dump memory contents to file. Option has 3 arguments, e.g. dump <segment> <format> <file>. Current supported segments are .text and .data. Also supports an address range (see m-n below). Current supported dump formats are Binary, HexText, BinaryText, AsciiText. See examples below. | 3.4 |
hex | display memory or register contents in hexadecimal - this is the default. (alternatives are ascii and dec) | 2.2 |
h | display this help. Use this option by itself and with no filename. | 1.0 |
ic | display instruction count; the number of MIPS basic instructions 'executed' | 4.3 |
mc | set memory configuration. Option has 1 argument, e.g. mc <config>. Argument <config> is case-sensitive and its possible values are Default for the default 32-bit address space, CompactDataAtZero for a 32KB address space with data segment at address 0, or CompactTextAtZero for a 32KB address space with text segment at address 0. | 3.7 |
me | display MARS messages to standard err instead of standard out. Allows you to separate MARS messages from MIPS program output using redirection. | 4.3 |
nc | copyright notice will not be displayed. Useful if redirecting or piping program output. | 3.5 |
np | pseudo-instructions or extended instruction formats are not permitted. | 3.0 |
p | project option - will assemble the specified file and all other assembly files (*.asm; *.s) in its directory. | 3.1 |
sen | terminate MARS with exit code n if simulate (run) error occurs | 4.1 |
sm | start execution at statement having global label 'main' if defined | 3.8 |
smc | Self Modifying Code - Program can write and execute in either text or data segment | 4.4 |
we | assembler warnings will be considered errors. | 3.5 |
n | where n is an integer maximum count of execution steps to simulate. If 0, negative or not specified, there is no maximum. | 1.0 |
$reg | where reg is number or name (e.g. 5, t3, f10) of register whose content to display at end of run. Even-numbered float register displays both float and double. Option may be repeated. NOTE: Depending on your command shell, you may need to escape the $, e.g. \$t3 | 2.2 |
reg_name | where reg_name is the name (e.g. t3, f10) of register whose content to display at end of run. Even-numbered float register displays both float and double. Option may be repeated. $ not required. | 2.2 |
m-n | memory address range from m to n whose contents to display at end of run. m and n may be decimal or hexadecimal (starts with 0x), m <= n, both must be on word boundary. Option may be repeated. | 2.2 |
pa | program arguments - all remaining space-separated items are argument values provided to the MIPS program via $a0 (argc - argument count) and $a1 (argv - address of array containing pointers to null-terminated argument strings). The count is also at the top of the runtime stack ($sp), followed by the array.This option and its arguments must be the last items in the command! | 3.5 |
Example: java -jar mars.jar h
Displays command options and explanations.
Example: java -jar mars.jar $s0 $s1 0x10010000-0x10010010
fibonacci.asm
Assemble and run fibonacci.asm. At the end of the run, display
the contents of registers $s0 and $s1, and the
contents of memory locations 0x10010000 through 0x10010010. The contents
are displayed in hexadecimal format.
Example: java -jar mars.jar a fibonacci.asm
Assemble fibonacci.asm. Does not attempt to run the program, and
the assembled code is not saved.
Example: java -jar mars.jar 100000 infinite.asm
Assemble and run infinite.asm for a maximum of 100,000 execution
steps.
Example: java -jar mars.jar p major.asm
Assemble major.asm and all other files in the same directory,
link the assembled code, and run starting with the first instruction in major.asm.
Example: java -jar mars.jar major.asm minor.asm sub.asm
Assemble and link major.asm, minor.asm and sub.asm.
If successful, execution will begin with the first instruction in major.asm.
Example: java -jar mars.jar a dump .text HexText
hexcode.txt fibonacci.asm
Assemble fibonacci.asm without simulating (note use of 'a'
option). At end of assembly, dump the text segment (machine code) to file
hexcode.txt in hexadecimal text format with one instruction per
line.
Example: java -jar mars.jar dump 0x10010000-0x10010020
HexText hexcode.txt fibonacci.asm
Assemble and simulate fibonacci.asm. At end of simulation, dump
the contents of addresses 0x1001000 to 0x10010020 to file hexdata.txt
in hexadecimal text format with one word per line.
Example: java -jar mars.jar t0 process.asm pa counter 10
Assemble and run process.asm with two program argument values,
"counter" and "10". It may retrieve the argument count (2) from $a0,
and the address of an array containing pointers to the strings "count" and
"10", from $a1. At the end of the run, display the contents of
register $t0.
The ability to run MARS from the command line is useful if you want to develop scripts (macros) to exercise a given MIPS program under multiple scenarios or if you want to run a number of different MIPS programs such as for grading purposes.
This document is available for printing on the MARS home page http://www.cs.missouristate.edu/MARS/.