1. System Calls & Arithmetic
Updated 10/12/2010

2. System Calls
Print string
Print integer
Read integer

3. Print String Example # HELLO WORLD .data
# Symbolic label Hello: data type is ASCII string
Hello: .asciiz "Hello\n"
.text # Text segment is unused for now.
main: # Start of code section
# Load register $v0 with code 4 for print string.
li $v0, 4
# Load $a0 with address of string.
# Label names are case sensitive.
la $a0, Hello
# call operating system to perform print operation
syscall
# END OF PROGRAM - MUST LEAVE ONE BLANK LINE AT END.

4. Printing an Integer Value
Use a MIPS system call to print the value of a 32-bit integer
# system call code for print_int = 1
li $v0, 1
# put integer value in register $a0
li $a0, 75
syscall

5. Register-to-Register Move
Use the move instruction to copy a value from one register into another
Assign register $a0 the same value as $t0
# $a0 = $t0
move $a0, $t0

6. Printing an Integer Value
Print the value in a register
# system call code for print_int = 1
li $v0, 1
# suppose the value to print is in $t3
# Assign $a0 the value contained in $t3
move $a0, $t3
syscall

7. Print Integer Example # PRINT AN INTEGER .data
.text # Text segment is unused for now.
main: # Start of code section
# Load register $v0 with code 1 for print integer
li $v0, 1
# Load $a0 with the integer value.
li $a0, 75
# call operating system to perform print operation
syscall
# END OF PROGRAM - MUST LEAVE ONE BLANK LINE AT END.

8. Programming Exercise
Enhance your Find Exponent program so that it prints the message:
The exponent is X
Where X is the value of the actual unbiased power of 2 exponent left in a register

9. Read Integer Example # READ AN INTEGER .data
.text # Text segment is unused for now.
main: # Start of code section
# Load register $v0 with code 5 for read integer
li $v0, 5
# call operating system to do read integer
syscall
# Integer value is returned in register $v0
# END OF PROGRAM - MUST LEAVE ONE BLANK LINE AT END.

10. Read & Echo Print Integer
# READ AN INTEGER AND ECHO PRINT ITS VALUE.
.data
Prompt:.asciiz “Enter an integer: “
Echo: .asciiz “Your number is “
.text # Text segment is unused for now.
main:
# Print prompt message to enter an integer.
li $v0, 4
la $a0, Prompt
syscall
# Load register $v0 with code 5 for read integer
li $v0, 5
# Integer value is returned in register $v0
move $t0, $v0
# Continued...

11. Read & Echo Print Integer
# ...CONTINUED:
# Print echo message.
li $v0, 4
la $a0, Echo
syscall
# Load register $v0 with code 1 for print integer
li $v0, 1
# Put saved input integer value in register $a0
move $a0, $t0
# End of program

12. Arithmetic Instructions
Integer multiply
Integer divide

13. Integer Multiply
Example # Load operands for multiplication li $v0, 5 li $v1, 3 # $a0 = $v0 x $v1 mul $a0, $v0, $v1

14. Integer Multiply (big #s)
Multiply two 32-bit integers
Product is a 64-bit integer
MIPS processor provides two special registers called HI and LO to store the product

15. Integer Multiply
# Set operands. li $t0, 7 li $t1, 11 # multiply $t0 x $t1 mult $t0, $t1 # Save lower 32-bits of product to $t2 mflo $t2 # Save upper 32-bits of product in $t3 mfhi $t3

16. Integer Multiply Example
# Set operands li $t0, 7 $t0 = 0x li $t1, 11 $t1 = 0x B # Compute 7 x 11 mult $t0, $t1 # Product in special registers LO = 0x D (77 base 10) HI = 0x

17. BIG Integer Multiply Example
# Set both operands 1,000,000 li $t0, $t0 = 0x000F4240 li $t1, $t1 = 0x000F4240 # Compute 1,000,000 x 1,000,000 mult $t0, $t1 # Product is more than 32-bits! LO = 0xD4A5100 HI = 0x000000E8

18. Integer Division
Example # Load operands for division li $v0, 15 li $v1, 3 # $a0 = $v0 / $v1 div $a0, $v0, $v1

19. Integer Divide (dual action)
Divide two 32-bit integers
Quotient is stored in the LO register
Remainder is left in the HI register

20. Integer Division
# Set operands. li $t0, 25 li $t1, 3 # Compute $t0 / $t1 div $t0, $t1 # Save quotient to register $t2 mflo $t2 # Save remainder to register $t3 mfhi $t3

21. Integer Division Example
# Set operands li $t0, 25 $t0 = 0x li $t1, 3 $t1 = 0x # Compute 25 / 3 div $t0, $t1 LO = 0x (Quotient) HI = 0x (Remainder)

22. Cycles Per Instruction
MIPS integer multiply takes 5-12 clock cycles
MIPS integer divide takes clock cycles
Bitwise shift instructions execute in only one clock cycle!

23. Programming Exercise
Create string prompt for user and a string output as a label quarters: . asciiz "Quarters: "
Prompt user to enter number of pennies 1-99
Compute & print number of quarters needed to make change (fewest coins) output the label with the value– test it and get it working before proceeding..
Compute & print number of dimes – test it
Compute & print number of nickels – test is
Example: 67 cents
Quarters: 2
Dimes:
Nickels: 1
Pennies: 2