Thread: saving an address in two bytes

Hi everyone I need to assign the address of my function to the
first two bytes of my stackPointer.

How do I do this?

Code:

void function foo()
{
}

stackPointer = acWorkspaces + (WORKSPACE*idx);

stackPointer = low bytes of address function foo

(stackPointer - 1) = high bytes of address function foo.

any help appreciated,

Ted.

Hmm that didn't work

Code:

 	 StackPointer = (&Myfunc & 0x0000FFFF);
StackPointer - 1 = (&MyFunc & 0xFFFF0000) >> 16;

I'm probably not using it correctly, I forgot to say.. the adress is only 16 bits long, so I tried the following:

Code:

    stackPointer = ((currProc->pc) & 0x00FF)
    (stackPointer - 1) = ((currProc->pc) & 0xFF00) >> 8;

pc is a function pointer so I'm thinking the way I'm using it is correct. no?

typedef struct pcb_struct 
{
 ..
    void        (*pc)(void); // function pointer
   ..
} ProcCtrlBlock;

I get this error though:

os.c:247: invalid operands to binary &
os.c:248: invalid operands to binary &

Ted

As Salem noted, this is messy. Endianness will play havoc with this code if you decide to port it to a little endian system.
Besides that I am still unsure why you are trying to do this?
You're getting that error because the bitwise operator "&" expects both integral operands.

Originally Posted by Salem

So why not just assign a function pointer then?

Yes, but how? the address has a size of two bytes. If I assign it to one byte, it will only store part of the address?

Besides that I am still unsure why you are trying to do this?

I'd have to write paragraphs to explain what I'm trying to do, and I don't have the time. In short, I need to build an operating system, and I'm trying to get the stuff in the right place for an assembly instruction.

Ted

Originally Posted by afflictedd2

Yes, but how? the address has a size of two bytes. If I assign it to one byte, it will only store part of the address?

How many bytes of storage does a pointer variable take-up as per your compiler?

Originally Posted by itCbitC

How many bytes of storage does a pointer variable take-up as per your compiler?

I'm not sure what command I need to execute to get that information for you?
I'm using the m6811-elf-gcc compiler. I was told it is really primitive and the sizeof an int is like 16 bits.

Ted.

Originally Posted by afflictedd2

I'm not sure what command I need to execute to get that information for you?
I'm using the m6811-elf-gcc compiler. I was told it is really primitive and the sizeof an int is like 16 bits.

Ted.

As a pointer variable contains the address of an object it doesn't matter whether it is the address of a function or a simple object type like an int or char.
To store the address of your function in the stack pointer you can as well do

Code:

void (*pc) (void); 
char *StackPointer;
StackPointer = (char *) pc;

Originally Posted by Salem

How is like this:

Code:

void foo ( int a );

// later on
void (*fnptr)( int );

// then assign it
fnptr = foo;

// then call it
fnptr( 123 );

That's not what I'm looking for though. I don't want a pointer to a function. I need to store the address of that function in the bytes (stackpointer) and (stackpointer -1).

Ted

First of all, you can't access the real stack pointer on the 68HC11 using C.
The 68HC11 uses a special register "SP" and to access it you have to drop down to the assembly level.
And what is the type of your StackPointer variable i.e. is it a pointer to a char or ant int?
Also, are you trying to simulate the hardware stack pointer SP in software? Still can't make sense of what you're trying to do.