Assembly language problem [repost using template :D]

[Khaled Kord]

Homework Statement

given that f,g,h,i and j are assigned to registers s0,s1,s2,s3 and s4
and the base addresses for the arrays A,B are in s6,s7

convert this C statement to assemply language: B[8] = A[i-j];

Homework Equations

none

The Attempt at a Solution

image attachedi attached both manual solutions and my answers, i can't understand the second line in his, he is adding the address of array A with variable f, while he should multiply f by 4 to get the offset

 

[Khaled Kord]

Homework Statement

given that f,g,h,i and j are assigned to registers s0,s1,s2,s3 and s4
and the base addresses for the arrays A,B are in s6,s7

convert this C statement to assemply language: B[8] = A[i-j];

Homework Equations

none

The Attempt at a Solution

image attachedi attached both manual solutions and my answers, i can't understand the second line in his, he is adding the address of array A with variable f, while he should multiply f by 4 to get the offset

the manual solution answer

 

[cpscdave]

What is the array type?

 

[Khaled Kord]

What is the array type?

32-bit integers

 

[jedishrfu]

In C code when you add one to pointer of a given type it will handle the size of the element.

http://www.tutorialspoint.com/cprogramming/c_pointer_arithmetic.htm

 

[cpscdave]

32-bit integers

Between that and what jedishrfu said you should be able to figure out why its multiplied by 4 :)

 

[Khaled Kord]

In C code when you add one to pointer of a given type it will handle the size of the element.

http://www.tutorialspoint.com/cprogramming/c_pointer_arithmetic.htm

you mean mean if i added a number to a reference it automatically multiply it by the size
i know about pointers arithmetic from previous programming course, didn't think about it .. thanks !

 

[Khaled Kord]

Between that and what jedishrfu said you should be able to figure out why its multiplied by 4 :)

i know why do we multiply by 4, i was asking why the manual solution didn't :)

 

[Mark44]

you mean mean if i added a number to a reference it automatically multiply it by the size
i know about pointers arithmetic from previous programming course, didn't think about it .. thanks !

In C and C++, if you add a number to a pointer, the result of the addition depends on the type of the pointer. So, adding 1 to an int pointer actually results in an address 4 bytes higher in memory than the address in the pointer. Adding 1 to a double pointer results in an address 8 bytes higher in memory. Adding 1 to a char pointer results in an address 1 byte higher in memory.

Subtracting a number from a pointer works in a similar fashion, but results in addresses that are lower in memory.

In contrast, when you write assembly code you have to do those extra multiplications yourself.

 

[Khaled Kord]

In C and C++, if you add a number to a pointer, the result of the addition depends on the type of the pointer. So, adding 1 to an int pointer actually results in an address 4 bytes higher in memory than the address in the pointer. Adding 1 to a double pointer results in an address 8 bytes higher in memory. Adding 1 to a char pointer results in an address 1 byte higher in memory.

Subtracting a number from a pointer works in a similar fashion, but results in addresses that are lower in memory.

In contrast, when you write assembly code you have to do those extra multiplications yourself.

if in assembly i should do this extra, so why the manual solution didn't do that?! "it's answer attached"

 

[Mark44]

if in assembly i should do this extra, so why the manual solution didn't do that?! "it's answer attached"

Posting an image of the code in your book is not very helpful. You should also state what sort of assembly code you're working with. Yours appears to be MIPS.

sub $t0, $s3, $s4
add $t0, $s6, $t0
lw $t1, 16($t0)
sw $t1, 32($s7)

given that f,g,h,i and j are assigned to registers s0,s1,s2,s3 and s4
and the base addresses for the arrays A,B are in s6,s7

convert this C statement to assemply language: B[8] = A[i-j];
i can't understand the second line in his, he is adding the address of array A with variable f, while he should multiply f by 4 to get the offset

What are the values of f, g, h, and i? To understand what a piece of code is doing, you have to know what values are being added, subtracted, loaded, or stored.

In the first line of code, the value in s4 is being subtracted from the value in s3, with the result being placed in t0.
Start at the first line, and see if you can figure out what gets stored in t0. Then do the same for the second, third, and fourth lines.

Do you know what the LW and SW instructions do?