[SOLVED] mips Java assembly Microsoft PowerPoint CSE220 Unit04 MIPS Assembly Branches and Loops.pptx

Microsoft PowerPoint CSE220 Unit04 MIPS Assembly Branches and Loops.pptx

1

1Kevin McDonnell Stony Brook University CSE 220

CSE 220:

Systems Fundamentals I

Unit 4:

MIPS Assembly:

Branches and Loops

2Kevin McDonnell Stony Brook University CSE 220

Branches
There are no if-statements or loops in MIPS
Instead there are different kind of branch statements that

direct the CPU to execute instructions out of sequential
order

In addition to the 32 registers we can use as programmers,
there is also the program counter (PC), which holds the
address of the next instruction to execute

After an instruction is fetched (the address of which is in
the PC), the value in the PC is incremented by 4 (i.e., 4
bytes)

The assumption is that the next instruction to execute is in
the neighboring memory cell

Branch instructions provide a different value to the PC

3Kevin McDonnell Stony Brook University CSE 220

Types of Branches
Conditional branches

The PC is updated if a condition is true
branch on equal (beq)
branch on not equal (bne)
branch on less than zero (bltz)

many others
Unconditional branches

The PC is changed directly
jump (j)
jump register (jr)
jump and link (jal)

4Kevin McDonnell Stony Brook University CSE 220

Conditional Branching
Used for implementing if-statements, switch-statements

and loops
beq: if two registers have the same data, jump to the

instruction at a provided memory address
bne: if two registers have different data, jump to the

instruction at a provided memory address
Example usage:

beq $a0, $s1, Equal_Case

2

5Kevin McDonnell Stony Brook University CSE 220

beq Example
addi $s0, $0, 4# $s0 = 0 + 4 = 4

addi $s1, $0, 1# $s1 = 0 + 1 = 1

sll $s1, $s1, 2 # $s1 = 1 << 2 = 4beq $s0, $s1, target # branch is takenaddi $s1, $s1, 1# not executedsub$s1, $s1, $s0 # not executedtarget:add$s1, $s1, $s0# $s1 = 4 + 4 = 86Kevin McDonnell Stony Brook University CSE 220bne Exampleaddi $s0, $0, 4# $s0 = 0 + 4 = 4addi $s1, $0, 1# $s1 = 0 + 1 = 1sll $s1, $s1, 2 # $s1 = 1 << 2 = 4bne $s0, $s1, target # branch not takenaddi $s1, $s1, 1# $s1 = 4 + 1 = 5sub$s1, $s1, $s0# $s1 = 5 4 = 1target:add$s1, $s1, $s0# $s1 = 1 + 4 = 57Kevin McDonnell Stony Brook University CSE 220Conditional Branching Other conditional branching instructions: bgez: branch to label if register contains a value greater than or equal to zero Example: bgez $a0, target bgtz: branch on greater than zero blez: branch on less than or equal to zero bltz: branch on less than zero bge: branch on greater than or equal to Example: bge rs, rt, label Branch to label if rs rt8Kevin McDonnell Stony Brook University CSE 220Conditional Branching The four relational operators <,>,, are actually

pseudoinstructions
They can be implemented with the help of the R-type slt

instruction: set on less than
slt rd, rs, rt

Set rd to 1 if rs < rt; otherwise, set rd to 039Kevin McDonnell Stony Brook University CSE 220If-statement Example Java code: MIPS code:if (i == j)f = g + h;f = f i; Note that the MIPS assembly code tests the opposite case ( ). You will see this convention used a lot.# $s0 = f, $s1 = g,# $s2 = h, $s3 = i,# $s4 = jbne $s3, $s4, L1add $s0, $s1, $s2L1: sub $s0, $s0, $s310Kevin McDonnell Stony Brook University CSE 220Unconditional Branching An unconditional branch is akin to a go to statement Ill show you how to use one in a loop in a few minutesaddi $s0, $0, 4 # $s0 = 4addi $s1, $0, 1# $s1 = 1j target# jump to targetsra $s1, $s1, 2 # not executedaddi $s1, $s1, 1 # not executedsub $s1, $s1, $s0# not executedtarget:add $s1, $s1, $s0# $s1 = 1 + 4 = 511Kevin McDonnell Stony Brook University CSE 220Unconditional Branching The j instruction simply takes an immediate value that gives part of the address (26 bits) to jump to The 32-bit target address is formed by concatenating the first 4 bits of the PC to the 26-bit immediate after shifting them 2 bits to the left The jr instruction is an R-type instruction that jumps to the address given in a register Example: jr $s0 Used when returning from function calls The jal instruction is used when making a function call More on jr and jal in a later Unit12Kevin McDonnell Stony Brook University CSE 220MIPS Program: Find Given a in $s0, b in $s1 and c in $s2, find the maximum of the three and store the maximum in $s3 Java code: if (a > b)
if (a > c)

max = a;
else

max = c;
else

if (b > c)
max = b;

else
max = c;

4

13Kevin McDonnell Stony Brook University CSE 220

MIPS Program: Find
# s0 = a, $s1 = b, $s2 = c, $s3 = max
li $s0, 255# a
li $s1, 11 # b
li $s2, 9# c

ble $s0, $s1, a_LTE_b # a <= b, so either b or c is maxmble $s0, $s2, maxC # a > b but a <= c, so max = cmove $s3, $s0 # a > b and a > c, so max = a
j done

a_LTE_b:
ble $s1, $s2,maxC # a <= b and b <= c, so max = cmove $s3, $s1# a <= b and b > c, so max = b
j done

maxC:
move $s3, $s2# max = c

done:

14Kevin McDonnell Stony Brook University CSE 220

while-loop Example
Let see how to write a while-loop in MIPS
Java code: MIPS code:
// determines the power
// of n such that 2n = 128
int pow = 1;
int n = 0;

while (pow != 128) {
pow = pow * 2;
n = n + 1;

}

# $s0 = pow, $s1 = n

addi $s0, $0, 1
add$s1, $0, $0
addi $t0, $0, 128

while:
beq $s0, $t0, done
sll $s0, $s0, 1
addi $s1, $s1, 1
jwhile

done:

15Kevin McDonnell Stony Brook University CSE 220

for-loop Example #1
Recall that we typically use for-loops when we know the

exact number of iterations
Java code: MIPS code:
// add the numbers
// from 0 to 9
int sum = 0;
int i;
for (i=0; i!=10; i++) {
sum = sum + i;

}

# $s0 = i, $s1 = sum
add$s1, $0, $0
add$s0, $0, $0
addi $t0, $0, 10

for: beq $s0, $t0, done
add$s1, $s1, $s0
addi $s0, $s0, 1
jfor

done:

16Kevin McDonnell Stony Brook University CSE 220

for-loop Example #2
Java code: MIPS code:
// sums the powers of
// 2 from 1 to 256

int sum = 0;

int i;

for (i=1; i < 257; i=i*2) {sum = sum + i;}# $s0 = i, $s1 = sumadd$s1, $0, $0addi $s0, $0, 1addi $t0, $0, 257loop: slt $t1, $s0, $t0beq $t1, $0, doneadd$s1, $s1, $s0sll $s0, $s0, 1jloopdone:517Kevin McDonnell Stony Brook University CSE 220switch-statement Example Java code: MIPS code:switch (amount) {case 20: fee = 2; break;case 50:fee = 3; break;case 100: fee = 5; break;default:fee = 7;}case20:li $t0, 20bne $s0, $t0, case50li $s1, 2j donecase50:li $t0, 50bne $s0, $t0, case100li $s1, 3j donecase100:li $t0, 100bne $s0, $t0, defaultli $s1, 5j donedefault:li $s1, 7done:18Kevin McDonnell Stony Brook University CSE 220Example: Count # of Ones Lets write a MIPS program that counts the number of binary 1s in a 32-bit word num Java code:counter = 0;position = 1;for (i = 0; i < 32; i++) {bit = num & position;if (bit != 0)counter++;position = position << 1;}19Kevin McDonnell Stony Brook University CSE 220Example: Print First N Primes Lets write a program to print the first N prime numbers, where N is hard-coded Its helpful first to look a Java implementation and then turn it into MIPS20Kevin McDonnell Stony Brook University CSE 220Example: Leap Year A year after 1582 is a leap year if it is divisible by 4 with the exception of centenary years (years ending in 00) that are not divisible by 400 2015 was not a leap year because 2015 is not divisible by 4 1900 was not a leap year because although 1900 is divisible by 100, it is not divisible by 400 2000 was a leap year because 2000 is divisible by 400if (year % 4 != 0) then ordinary_yearelse if (year%100 == 0) and (year%400 != 0) thenordinary_yearelse leap_year