Question

How do I write a get_min code in assembly? The code has to get the minimum value in an array.

Answer 1

I'm doing this in raspberry pi's ARM.

Answer 2

Can you tell me what instructions you have avaiable?

Answer 3

Assembly languages differ

Answer 4

What do you mean by which instructions?

Answer 5

sorry, i am very new to assembly. :(

Answer 6

what operations can you do? Do you have like 'mov' or 'add' or stuff like that?

Answer 7

yes, all those are availabe.

Answer 8

THose are actually the ones we learned.

Answer 9

Do you have sub? What sort of jumps do you have?

Answer 10

Show me what you have so far.

Answer 11

I literally have nothing so far. I have no clue how to start.

Answer 12

Do you have something like 'jz'?

Answer 13

I just know how each of the arithmetic assembly operations work like add r0, r1, r2 mean r0 = r1 + r2. We learned every arithmetic operation. but that's about it. not really how to use it.

Answer 14

Let me think. First let us start with the C code.

Answer 15

Then convert it and optimize it.

Answer 16

You return the first and last, not the max and min.

Answer 17

But since it is sorted, the min is the first?

Answer 18

yes.

Answer 19

it's ARMv6, but i'm not sure how we can predict the memory layout of structs without some sort of compiler directive for packing, and without that i'm not sure how to access the elements in a safe way

Answer 20

How is `list` defined?

Answer 21

@wio: in the C code it's given as:

typedef struct List {
int *sortedList;
int size;
int maxSize;
} list;

Answer 22

um, list is in a struct. one sec.

Answer 23

how big are addresses? 64-bits? do you know how the struct is aligned by the compiler? when you compile your C code add -S and paste the backend assembly code

Answer 24

the C code containing the struct and stuff

Answer 25

32 bit.

Answer 26

i compiled with gcc -o -S array_test.c arraysort.c

Answer 27

remove -o and make sure you're compiling this code targeting the ARMv6 for raspberry pi

Answer 28

how do I know if I'm targetting the ARMv6?

Answer 29

I have to leave now, but i'll ocme back and post the results of the compilation.

Answer 30

BUt thanks for all your help.

Answer 31

well, in pseudocode for get_max you want something like:

move [r0 + 4] to A /* now A = ls->size */
add [r0] to A /* now A = ls->sortedList + ls->size = &ls->sortedList[ls->size] */
sub 1 from A /* A = &ls->sortedList[ls->size - 1] */
mov [r0] to r0 /* set return value to ls->sortedList[ls->size - 1] */

Answer 32

then for get_min it's somewhat simpler since you just want ls->sortedList[0]

Answer 33

how would i change it for min?

Answer 34

why would moving r0 + 4 to a mean ls-> size? wouldn't that just be the first 4 bytes? which is ls->sortedList[0]?

Answer 35

@oldrin.bataku

Answer 36

No, ls->sortedList is at r0 + 0, and ls->size is at r0 + (sizeof ls->sortedList). We expect that ls->sortedList, or any pointer for that matter, is 4 bytes in a 32 bit processor.

Answer 37

wait isn't sizeof(ls->sortedList) the amount i malloced?

Answer 38

or is it just the size of the int pointer?

Answer 39

actually my code is broken anyways, it should [4*r0+16] into A and then subtract 4 since each element of sortedList is 4 bytes

Answer 40

@Curry well ls->sortedList is just int * so it's just the size of a pointer (you said 4 bytes)

Answer 41

move [r0 + 0] to A /* now A = ls->sortedList[0] */

Answer 42

is that right ^^

Answer 43

er oops, it should move [r0+4] as before and then multiply that by 4 to get a byte offset from ls->sortedList

Answer 44

well, sortedList[0] is *(sortedList + 0). The actual address is sortedList + 0

Answer 45

@Curry no, that is A = ls->sortedList or &sortedList[0]

Answer 46

hmm, so for max val, the sudo code should be

Answer 47

ls is a pointer; dereferencing it gives us the value at *ls which is sortedList, a pointer to the array of the values

Answer 48

mov r0+4 to A
add [r0] to 4*A /* now A = ls->sortedList + ls->size = &ls->sortedList[ls->size] */
sub 1 from A /* A = &ls->sortedList[ls->size - 1] */
mov [r0] to r0 /* set return value to ls->sortedList[ls->size - 1] */

Answer 49

so do i deference it twice?

Answer 50

well, move [r0+4] to A, sub 1 from A, then multiply A by 4, then add [r0] to A, and then lastly move [A] to r0

Answer 51

and for min, it'd be mov r0 to A, multiply A by 4, add r0 to A and move to r0?

Answer 52

r0+4 is a pointer to ls->size, so [r0+4] is ls->size, and then we subtract 1 to get ls->size - 1, and then we multiply by 4 to get a byte offset from the start of ls->sortedList, and then we add that to ls->sortedList, and then we dereference that pointer to get ls->sortedList[ls->size-1]

Answer 53

for min you would do something like:

move [r0] to A
move [A] to r0

which is simply dereferencing ls twice; *ls gives the sortedList pointer, and dereferencing that gives its first element sortedList[0]

Answer 54

honestly i would just compile the C code you had and then use -S to look at gcc's assembly output; it should show you exactly how to do what you need to do

Answer 55

OMG THANK YOU GUS SO MUCH. <3

Answer 56

@oldrin.bataku Hey, so i wrote the get max function in assembly. But how do I call it from my main? also, when I compile, do i just include it like i include all my other .c files?

Answer 57

@e.mccormick

Answer 58

I do not use asm or c. But here are some references that look promissing.

https://courses.engr.illinois.edu/ece390/books/labmanual/c-prog-mixing.html
http://www.nongnu.org/avr-libc/user-manual/group__asmdemo.html

Answer 59

you need to tell the C compiler than you have external symbols:

extern int get_max_ARM(list *);
extern int get_min_ARM(list *);

Answer 60

then link the C, ASM object files together

Answer 61

you should not be passing that assembly code to gcc; presumably you need to use a separate assembler (not GCC's as)

Answer 62

well i have a header file, that has it, is that the same thing as having external symbols?

Answer 63

well, by default the C compiler assumes the storage class of a declaration is extern, so that's fine

Answer 64

those errors are because you're using a non-ARM GCC toolchain to build the code; use the set-up for compilation you were instructed to use for ARM

Answer 65

oo! kk. and does my asm code look aprox right?

Answer 66

and you said, when i'm compiling the files, i don't need to include the ,s files?

Answer 67

so right now, i'm doing gcc -o array array_test.c arraysort.c. Do i need to add any other falgs ?

Answer 68

what did your professor say to use for writing code for your raspberry pi?

Answer 69

what do you mean? like what language?

Answer 70

no, what toolchain? you're using a build of gcc that does not target ARM

Answer 71

oh, i'm trying to ssh into my school's account, i didn't hook up my raspberry pi rn. But, i'm guessing i need to hook it up, and compile the file locally on that, and if it works, then submit that?

Answer 72

yeah, you need to build using a toolchain that targets ARM since the assembly code is made for that; presumably you can hookup your raspberry pi and it'll have an appropriate toolchain on it?

Answer 73

that's what I'm guessing to. Haven't really tried it. What exactly is a toolchain?

Answer 74

as in the GCC toolchain of utilities; you need the version that is made to target ARM devices, so connect your Raspberry Pi and build the code on there

Answer 75

I compiled my code. So in my main, my code is
int x = get_max_ARM(ls); /*ls is already initialized and all the regular c functions work fine with it*/

in my header file, the function is defined as int get_max_ARM(list *ls);

this function is also in my regular c file and is also defined as int get_max_ARM(list *ls);

However when i compile it on my raspberry ppi, i get an error that says
(text+0x58) undefined reference to 'get_max_ARM' ld returned 1 exti status

Answer 76

@oldrin.bataku

Answer 77

yeah, you need to assemble the files and link the object code from gcc and your assembler or better yet just gcc -o <C files> <as files> like you had before

Answer 78

er gcc -o array array_test.c arraysort.c whatever-else.as

Answer 79

can i not deference the first operand?

Answer 80

and should it be mov r1, [r0+4]? instead of r1 being the 2nd operand?

Answer 81

mul only takes one operand, but i don't understand how it'd know which register to take from AND how much to multiply by.

Answer 82

gas uses mov dst, src i believe

Answer 83

mul doesn't take only one operand; can you show your error?

Answer 84

as a variable outside a function argument, int[5] is not reduced to a mere pointer

Answer 85

and it's arrays are never lvalues in C so you can't assign to them like that

Answer 86

so if ar[] was passed in as a function argument, then it'd be ok to do that? Because then it's treated just as int *ar? And just to really clarify, when doing int ar[5] it's only treated like an array and not a int pointer? cause i thought doing int ar[5] is actually doing the following diagram.