Question

I am studying the MIPS assembly language and came across this example in the book and to me it seems incorrect. If it is it wouldn’t be the first mistake I found in this book.

The variables f and g are assigned registers $s0 and $s1 respectively, the base addresses for the arrays A and B are $s6 and $s7 respectively.

The c code example is:

f = g - A[B[4]];

And the corresponding MIPS assembly provided is:

lw  $t0, 16($s7)
lw  $s0, 0($t0)
sub $s0, $s1, $s0

From my understanding the above MIPS code would load some random data from memory at the address provided by $t0 and then subtract it from $s1 and not access the index $t0 of the array denoted in $s6.

The correct MIPS assembly from my understanding would be along the lines of:

lw  $t0, 4($s7)
add $t0, $t0, $s6
sll $t0, $t0, 2
lw  $s0, 0($t0)
sub $s0, $s1, $s0

I am correct that this is an error in the book or am I misunderstanding something.

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Edit: Fixed an error in the corrected mips code as pointed out by Chris Dodd

Answer 1

As pointed out my many there was an error in the book. Since discovering this error I found several such errors.