#This program demonstrates the use of syscalls. See the MARS online help for further information.
#Simulators like MARS usually provide a small set of "service routines", which emulate the services usually
#provided by an operating system, and accessible via the syscall instruction.
#author: Hornick
#CS2710 - 1/5/13

.data
welcome_msg:
.asciiz "Hello World!"	#defines an ascii string terminated with a null (0) character
error_msg:
.asciiz "Oh no! An error occurred!"	
goodbye_msg:
.asciiz "Bye!"

.text	#.text directive (default) just tells the assembler that the following instructions are for the .text segment of memory

main:	#this label is assigned the value of the address of the next instruction
	
# The following instructions demonstrate how to prepare to invoke a syscall. Every syscall
# requires $v0 to be set to a value corresponding to the service that will be invoked by the 
# syscall. Depending on the particular syscall, other registers may also have to be set to 
# specific values. 
	li	$v0, 	4		#function code for "print string" service
	la	$a0,	welcome_msg	#load the address of the welcome message string into $a0
	syscall				#invoke the service routine
	
# The following instructions demonstrate how to use procedures to invoke various syscalls.	
	la	$a0,	error_msg	#load the address of the string to be printed
	jal 	ErrorMessageDialog	#call the ErrorMessageDialog procedure
	
	la	$a0,	goodbye_msg	#load the address of the string to be printed
	jal	print_msg		#call the print_msg procedure
	jal	InfoMessageDialog	#call the InfoMessageDialog procedure
	
	jal	exit			#call the procedure that causes the program to exit

##################################################################################################
# This is procedure print_msg. It encapsulates the syscall to the "print string" service routine.
# Note that this procedure assumes that the calling routine has loaded the address of the string
# to be printed into $a0
print_msg:
	sw	$ra, ($sp)		# push the return address onto the stack
	addi	$sp, $sp, -4		# decrement $sp (by 4 bytes) to point at the next available stack location
	
	li	$v0,	4		# function code for "print string" system service
	syscall				# invoke the service routine
	
	addi	$sp, $sp, 4		# increment $sp (by 4 bytes) to point to the return address
	lw	$ra, ($sp)		# pop the return address from the stack
	jr	$ra			# return to the instruction referenced by $ra (return address register)
# end procedure print_msg
##################################################################################################

##################################################################################################
# This is procedure InfoMessageDialog. It encapsulates the syscall to the "MessageDialog" service routine,
# specifying that an informational dialog should be displayed.
# Note that this procedure assumes that the calling routine has loaded the address of the string
# to be printed into $a0. 
InfoMessageDialog:
	sw	$ra, ($sp)		# push the return address onto the stack
	addi	$sp, $sp, -4		# decrement $sp (by 4 bytes) to point at the next available stack location
	
	li	$v0,	55		# function code for "print string" system service
	li	$a1,	1		# function code for informational message
	syscall				# invoke the service routine
	
	addi	$sp, $sp, 4		# increment $sp (by 4 bytes) to point to the return address
	lw	$ra, ($sp)		# pop the return address from the stack
	jr	$ra			# return to the instruction referenced by $ra (return address register)
# end procedure InfoMessageDialog
##################################################################################################

##################################################################################################
# This is procedure ErrorMessageDialog. It encapsulates the syscall to the "MessageDialog" service routine,
# specifying that an error dialog should be displayed.
# Note that this procedure assumes that the calling routine has loaded the address of the string
# to be printed into $a0. 
ErrorMessageDialog:
	sw	$ra, ($sp)		# push the return address onto the stack
	addi	$sp, $sp, -4		# decrement $sp (by 4 bytes) to point at the next available stack location
	
	li	$v0,	55		# function code for "print string" system service
	li	$a1,	0		# function code for error message
	syscall				# invoke the service routine
	
	addi	$sp, $sp, 4		# increment $sp (by 4 bytes) to point to the return address
	lw	$ra, ($sp)		# pop the return address from the stack
	jr	$ra			# return to the instruction referenced by $ra (return address register)
# end procedure ErrorMessageDialog
##################################################################################################

##################################################################################################
# This is procedure exit. It encapsulates the syscall to the "terminate execution" service routine.
# Note that this procedure never returns, so it is not necessary to implement stack management.
exit:
	li	$v0, 	10		# function code for "terminate execution" 
	syscall				# invoke the service routine
# end procedure exit
##################################################################################################