NOVARSEG and the quest for _MEM

[NOVARSEG]

Sidetracking a bit.
 I want to devise some test code to see if the order of data items in TYPE structures are important.

TYPE ABC
    Z AS INTEGER
    Y AS INTEGER
END TYPE

DIM b AS _OFFSET

DIM c AS ABC

c.Z = 100
c.Y = 1000

b = _OFFSET(c)

so the memory address of TYPE ABC is stored in b. Now how do you read the entire 4 bytes in that TYPE into an LONG ,  using b ?

[SpriggsySpriggs]

@NOVARSEG
Check the Wiki for MEM

[NOVARSEG]

OK yes MEM   . It looks like the order is important when referencing   TYPE data structures by address but want to make sure with some tests

I was reading on some site that an option to padding was rearranging the items in a data structure and  thought how is that?

[SpriggsySpriggs]

@NOVARSEG
That would only be an option when it is a type that you are making for your own functions. For anything like WinAPI, you can't reorder the variables unless you WANT the data to go to the wrong variables

[NOVARSEG]

@SpriggsySpriggs

Makes sense thanks

DIM L AS LONG

DIM m AS _MEM

_MEMGET m, m.OFFSET, B

_MEMPUT m, m.OFFSET, L

PRINT L

get a memory not initialized error.

[SMcNeill]

@SpriggsySpriggs

Makes sense thanks

get a memory not initialized error.

Makes sense, as you haven’t initialized your memory.  It points at nothing.

Here’s what you’d be doing with file access:

m = FREEFILE
GET m, 0, B

And, you’d get an error then too, as you’ve never actually opened any file on your drive to access.  Start with wiki examples, or my tutorials, and learn the basic syntax of the commands, before just shooting off blindly with them.

What you’re doing is almost as bad as trying to PRINT HELLO WORLD without using quotes, and then complaining when it doesn’t work as you imagine.

[NOVARSEG]

Ok IM totally stupid when it comes to mem

DIM m AS _MEM 

DIM N AS STRING * 4

N = "abcd"

m = _MEM(N)

_MEMPUT m, m.OFFSET, N

N = ""
_MEMGET m, m.OFFSET, N

PRINT N

does not print abcd

[SMcNeill]

Of course it's not going to print abcd.  You just reset N to "".

Start a topic on mem, and ask questions there about it.  I don't have a clue what you're trying to do, but it's definitely not related to the topic at hand.

[NOVARSEG]

yes off topic did not want to start a new thread

Will get back on track but MEM to me is an abstraction on so many levels


 I thought N is now stored in m with
_MEMPUT m, m.OFFSET, N

N = ""

then do the reverse

_MEMGET m, m.OFFSET, N

see how I don't understand MEM?. At least I know what I don't understand

PRINT N


****

In C there is

The & is a unary operator in C which returns the memory address of the passed operand. This is also known as address of operator. The * is a unary operator which returns the value of object pointer by a pointer variable. It is known as value of operator.

Ok so  i got the address of TYPE ABC structure in B

TYPE ABC
    Z AS INTEGER
    Y AS INTEGER
END TYPE

DIM b AS _OFFSET

DIM c AS ABC

c.Z = 100
c.Y = 1000

b = _OFFSET(c)

In C there  is the "value of operator."

Do I really need MEM stuff to do a "value of operator" on  B"

[SpriggsySpriggs]

@NOVARSEG
Like Steve said, you might want to go ahead and make a new thread and discuss MEM there. You might find more help there since the title will at least match the topic being discussed. People who understand MEM will see your question and respond.

[NOVARSEG]

If I do that then more people will see how stupid I am when it comes to MEM

[RhoSigma]

If I do that then more people will see how stupid I am when it comes to MEM

Not stupid, just blind seeing the obvious. you think your m is another variable, which you can exchange the value of N with and wise versa, but NO, m is not another variable, it's just ANOTHER WAY TO ACCESS your variable N.

Ie. m is a pseudonym for N, doesn't matter if you access N directly (N="") or via _MEMGET/PUT, you always change N and only N.

m is a pointer to the memory, where the value of N is stored, not more, not less.

EDIT:
If you're likely more familiar with C/C++, it's like taking the reference of a variable here:

char N;
char *m = &N;

[NOVARSEG]

I have no idea how/why this works.   Finally got it to print out.
Z is LSW,  Y is MSW. Could someone explain how it works

Code: QB64: [Select]

1. TYPE ABC
2.     Z AS INTEGER
3.     Y AS INTEGER
4. END TYPE
5.  
6.  
7. DIM c AS ABC
8.  
9. c.Z = 400
10. c.Y = 0
11.  
12. DIM m AS _MEM
13. m = _MEM(c)    
14.  
15. _MEMGET m, m.OFFSET, N&    
16. PRINT N&

with
 c.Z = 400
c.Y = 0

print N& = 400

****

with
c.Z = 1
c.Y = 1

print N& = 65537

[NOVARSEG]

_MEMPUT does the reverse of that

Code: QB64: [Select]

1. TYPE ABC
2.     Z AS INTEGER
3.     Y AS INTEGER
4.  
5. END TYPE
6.  
7. DIM C AS ABC
8.  
9. DIM N AS LONG
10.  
11. DIM m AS _MEM
12.  
13. m = _MEM(C)
14.  
15. N = 2 ^ 32 - 1
16.  
17. C.Y = 0
18. C.Z = 0
19.  
20. _MEMPUT m, m.OFFSET, N
21.  
22. PRINT C.Z
23. PRINT C.Y

print C.Z = -1

print C.Y = -1

****
This proves that variables in a TYPE list, are contiguous in memory

[NOVARSEG]

What happens when a variable length string is added to the TYPE list

Code: QB64: [Select]

1. TYPE ABC
2.     S AS STRING
3.     Z AS INTEGER
4.     Y AS INTEGER
5.  
6. END TYPE
7.  
8.  
9. DIM C AS ABC
10.  
11. C.S = "abcdefghijk"
12. DIM N AS LONG
13.  
14. DIM m AS _MEM
15.  
16. m = _MEM(C)
17.  
18. N = 2 ^ 32 - 1
19.  
20. C.Y = 0
21. C.Z = 0
22.  
23. _MEMPUT m, m.OFFSET + 4, N
24.  
25. PRINT C.Z
26. PRINT C.Y

print C.Z = -1
print C.Y = -1

The "S AS STRING" takes 4 bytes in the TYPE list, but there are 11 bytes stored in S

Now when the variable length string is changed to a fixed length string like

Code: QB64: [Select]

1. TYPE ABC
2.     S AS STRING * 11
3.     Z AS INTEGER
4.     Y AS INTEGER
5.  
6. END TYPE
7.  
8.  
9. DIM C AS ABC
10. 'print len(c)
11. C.S = "abcdefghijk"
12. DIM N AS LONG
13.  
14. DIM m AS _MEM
15.  
16. m = _MEM(C)
17.  
18. N = 2 ^ 32 - 1
19.  
20. C.Y = 0
21. C.Z = 0
22.  
23. _MEMPUT m, m.OFFSET + 11, N
24.  
25. PRINT C.Z
26. PRINT C.Y
27.  

print C.Z = -1
print C.Y = -1

Then  "S AS STRING * 11"  takes up 11 bytes in the TYPE  list. So we know that "abcdefghijk" is stored in memory just before Z and Y integers.   That is not the case with the variable length string.