This is how the program returns control to the operating system without expecting to get control back.
On SPIM, the exit service stops execution of the simulated MIPS processor.
li $v0,4 # code 4 == print string
la $a0,string # $a0 == address of the string
syscall # Ask the operating system to
# perform the service.
. . . .
.data
string: .asciiz "Hello SPIM!\n"
The exit service is normally used when the program is finished. A real computer is always running, and control has to go somewhere, so this is how control is sent back to the OS. The OS then goes about its own tasks, possibly starting another user program.
The print string SPIM exception handler service prints
a null terminated string to the simulated monitor.
The address of the string is loaded into register $a0.
Typically the string is in the data section of memory.
The print string service sends bytes to the simulated monitor
one by one starting
with the byte pointed to by $a0.
It continues until it hits a null byte.
It does not check that the bytes are ASCII.
You will print garbage if you point $a0
at a location that is not an ASCII string.
If you want to advance to a new line,
use the newline character '\n'
inside or at the end of the string.
Bug Alert: the la instruction
is used to load the address of the first byte of
the string to be printed.
Bug Alert: values in t registers
may be altered by the operating system service,
for this and other services.
Fill in the blanks.
$v0,4 # code 4 == print string
$a0,message # $a0 == address of the string
# Ask the operating system to perform the service.
. . . .
.data
message: .asciiz "Intruder Alert!\n"