Code to MIPS Assembly
I was trying to convert C code to MIPS assembly. There are these two following C code snippets. The problem is that my solution differs from the standard solution. Also, I don't understand the standard solution. I hoped, someone could explain me the two following mips assembly code snippets.
First of all some additional information for the task. Only the following MIPS instructions are allowed: lw, add, beq, bne and j .
The register:
$s3 contains i
$s4 contains j
$s5 contains k
A is an array of 32-Bit-Integer and the initial address of A is in $s6
$t0 and $t1 can be used for storing temporary variables
The first one is a simple do-while loop:
do {
i = i + j;
} while(A[i] == k);
MIPS Assembly
loop: add $s3, $s3, $s4 // this is i = i+j
add $t1, $s3, $s3 // from now on
add $t1, $t1, $t1 // I cant follow anymore
add $t1, $t1, $s6 // What happens in these three lines?
lw $t0, 0($t1) // 0($t1) is new for me. What does this zero do?
beq $t0, $s5, loop
Now the second C code:
if ( i == j )
i = i + A[k];
else if( i == k )
i = i + A[j];
else
i = i + k;
Here is the MIPS assembly code:
bne $s3, $s4, Else1 // this line is if(i==j)
add $t1, $s5, $s5 // from here on
add $t1, $t1, $t1 // till
add $t1, $t1, $s6 //
lw $t0, 0($t1) //
add $s3, $s3, $t0 // here I don't understand
j done
ELSE1: bne $s3, $s5, Else2 // this line is if(i==k)
add $t1, $s4, $s4 // The same game as above
add $t1, $t1, $t1
add $t1, $t1, $s6
lw $t0, 0($t1)
add $s3, $s3, $t0 // till here
j done
ELSE2: add $s3, $s4, $s5
Could anyone explain me what really goes on? That would be very helpful
Ok the most interesting and not-understandable piece of code is the following:add $t1,
add $t1, $s5, $s5
add $t1, $t1, $t1
add $t1, $t1, $s6
this code multiplies $s5 by four, and stores it in $t1, then adds to $t1 $s6. That is equivalent to:
$t1 = A[k]
after you understand this, the code looks much clearer.
about the lw $t0, 0($t1) :
you can use an offset point in the address. Here the offset is 0.
lw is load. The mode here is indirect addressing, the most common is:
lw $t2, ($t0)
But you can also include a byte offset,
lw $t2, 4($t0) # load word at RAM address ($t0 + 4) into register $t2
The compiler is just putting the 0 placeholder in the non-offset version.
So, to load A[i] you need to do two things. Take the base address of A[] and then add i times the sizeof(A[0]) . I am guessing that the values in A are 32 bit.
add $t1, $s3, $s3
$t1 = j + j or 2 * j
add $t1, $t1, $t1
$t1 = $t1 + $t1 or j +j +j +j or 4 * j
So why did it do it this way? Well, doing a straight multiply is slow in terms of clock cycles. The next choice would be a shift operation, but in this case, the compiler designers decided that two adds beat a shift.
add $t1, $t1, $s6
I would guess that $s6 is the base address of A[] . So ultimately '$t1 = A[] + 4 * j`
I have a answer.The loop, a is array of elements have basic address :0x0FE3B128. Thanks all so much.. and this is my homework, I don't sure that it is correct.
for(i=1; i!=20;i+=3){
a[i]= a[5]+1;
}
lui $s0, 0x0FE3
ori $s0, $0, B128
lw $t1, 20($s0)
addi $s1, $0, 1
addi $s2, $0, 20
LOOP beq $s1, $s2, DONE
add $t1, $t1, $s1
sll $t1, $t1, 2
sw $t2, 0($t1)
addi $s1, $0, 3
j LOOP
DONE上一篇: 优化C代码中的汇编代码冗余
下一篇: 代码到MIPS汇编