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c++ - __far Pointer

reply IR <IR_member pathlink.com> writes:
Hi all,

I was hoping someone could help me with a question.  I want to place data at a
specific location in memory.  I am writing this program with no run-time
support, so dynamic memory allocation is out of the question.  The location I
want to place data in is in a seperate segment, so I believe I will need to use
a far pointer.  I searched this site and found the __far pointer keyword.  I
figure I need to create a pointer using the __far keyword and place the address
in it following the instructions I found on this site.  I am going to need to do
this in both real mode and protected mode.  For example, say I am executing code
in real mode located in segment 0900h and I want to place data in segment 1200h
at offset 0000h.  How would I do this?  I know protected mode is different with
its global descriptor table but is this also possible in protected mode.  I
don't know if I explained this carefully enough, so tell me if I need to
clarify. 

Thanks!
Nov 06 2004
parent reply "Walter" <newshound digitalmars.com> writes:
"IR" <IR_member pathlink.com> wrote in message
news:cmj6r3$2fld$1 digitaldaemon.com...

 Hi all,

 I was hoping someone could help me with a question.  I want to place data


at a

 specific location in memory.  I am writing this program with no run-time
 support, so dynamic memory allocation is out of the question.  The


location I

 want to place data in is in a seperate segment, so I believe I will need


to use

 a far pointer.  I searched this site and found the __far pointer keyword.


I

 figure I need to create a pointer using the __far keyword and place the


address

 in it following the instructions I found on this site.  I am going to need


to do

 this in both real mode and protected mode.  For example, say I am


executing code

 in real mode located in segment 0900h and I want to place data in segment


1200h

 at offset 0000h.  How would I do this?  I know protected mode is different


with

 its global descriptor table but is this also possible in protected mode.


I

 don't know if I explained this carefully enough, so tell me if I need to
 clarify.


You'll need to use a locator to map the relocatable executable to a specific
address. Also, use the 'L' memory model so each source file's code and data
are placed in separate segments.
Nov 06 2004
parent reply IR <IR_member pathlink.com> writes:
Is there any way just to write data to a far segment using a far pointer?


In article <cmj7qc$2h2n$1 digitaldaemon.com>, Walter says...

"IR" <IR_member pathlink.com> wrote in message
news:cmj6r3$2fld$1 digitaldaemon.com...

 Hi all,

 I was hoping someone could help me with a question.  I want to place data


at a

 specific location in memory.  I am writing this program with no run-time
 support, so dynamic memory allocation is out of the question.  The


location I

 want to place data in is in a seperate segment, so I believe I will need


to use

 a far pointer.  I searched this site and found the __far pointer keyword.


I

 figure I need to create a pointer using the __far keyword and place the


address

 in it following the instructions I found on this site.  I am going to need


to do

 this in both real mode and protected mode.  For example, say I am


executing code

 in real mode located in segment 0900h and I want to place data in segment


1200h

 at offset 0000h.  How would I do this?  I know protected mode is different


with

 its global descriptor table but is this also possible in protected mode.


I

 don't know if I explained this carefully enough, so tell me if I need to
 clarify.


You'll need to use a locator to map the relocatable executable to a specific
address. Also, use the 'L' memory model so each source file's code and data
are placed in separate segments.
Nov 06 2004
parent reply "Walter" <newshound digitalmars.com> writes:
Using MK_FP(), you can create a far pointer to any memory location, then
read and write that location.

"IR" <IR_member pathlink.com> wrote in message
news:cmjcjc$2nnu$1 digitaldaemon.com...

 Is there any way just to write data to a far segment using a far pointer?
Nov 06 2004
parent reply IR <IR_member pathlink.com> writes:
Im sorry to keep bothering you but I am still new to this and am trying to
learn.  

I know how I would read data from that location but how would you write.  Thanks
again.




In article <cmjt3m$au8$1 digitaldaemon.com>, Walter says...

Using MK_FP(), you can create a far pointer to any memory location, then
read and write that location.

"IR" <IR_member pathlink.com> wrote in message
news:cmjcjc$2nnu$1 digitaldaemon.com...

 Is there any way just to write data to a far segment using a far pointer?
Nov 06 2004
parent reply "Walter" <newshound digitalmars.com> writes:
char __far *p = MK_FP(0x1200, 0x40);
*p = 6;    // writes 6 to 1200:0040


"IR" <IR_member pathlink.com> wrote in message
news:cmk0u6$fl9$1 digitaldaemon.com...

 Im sorry to keep bothering you but I am still new to this and am trying to
 learn.

 I know how I would read data from that location but how would you write.


Thanks

 again.




 In article <cmjt3m$au8$1 digitaldaemon.com>, Walter says...

Using MK_FP(), you can create a far pointer to any memory location, then
read and write that location.

"IR" <IR_member pathlink.com> wrote in message
news:cmjcjc$2nnu$1 digitaldaemon.com...

 Is there any way just to write data to a far segment using a far






pointer?







 Nov 07 2004



  parent reply IR <IR_member pathlink.com>  writes:


The program has to include dos.h in order to use MK_FP().

Do you know if MK_FP() uses any DOS interrupts or any other DOS functions?

The reason I ask is because I'm writing a program with no underlying OS support.

If it does use some type of DOS support, can you tell me how to set the far
pointer without using MK_FP().

Thanks





In article <cmlsjq$56a$1 digitaldaemon.com>, Walter says...

char __far *p = MK_FP(0x1200, 0x40);
*p = 6;    // writes 6 to 1200:0040


"IR" <IR_member pathlink.com> wrote in message
news:cmk0u6$fl9$1 digitaldaemon.com...

 Im sorry to keep bothering you but I am still new to this and am trying to
 learn.

 I know how I would read data from that location but how would you write.


Thanks

 again.




 In article <cmjt3m$au8$1 digitaldaemon.com>, Walter says...

Using MK_FP(), you can create a far pointer to any memory location, then
read and write that location.

"IR" <IR_member pathlink.com> wrote in message
news:cmjcjc$2nnu$1 digitaldaemon.com...

 Is there any way just to write data to a far segment using a far






pointer?









 Nov 08 2004



  parent  "Walter" <newshound digitalmars.com>  writes:


From dos.h:

#define MK_FP(seg,offset) \
        ((void __far *)(((unsigned long)(seg)<<16) | (unsigned)(offset)))

So, no, it does not use DOS functions.


"IR" <IR_member pathlink.com> wrote in message
news:cmo00b$kjq$1 digitaldaemon.com...

 The program has to include dos.h in order to use MK_FP().

 Do you know if MK_FP() uses any DOS interrupts or any other DOS functions?

 The reason I ask is because I'm writing a program with no underlying OS


support.

 If it does use some type of DOS support, can you tell me how to set the


far

 pointer without using MK_FP().

 Thanks





 In article <cmlsjq$56a$1 digitaldaemon.com>, Walter says...

char __far *p = MK_FP(0x1200, 0x40);
*p = 6;    // writes 6 to 1200:0040


"IR" <IR_member pathlink.com> wrote in message
news:cmk0u6$fl9$1 digitaldaemon.com...

 Im sorry to keep bothering you but I am still new to this and am trying






to

 learn.

 I know how I would read data from that location but how would you






write.

Thanks

 again.




 In article <cmjt3m$au8$1 digitaldaemon.com>, Walter says...

Using MK_FP(), you can create a far pointer to any memory location,








then

read and write that location.

"IR" <IR_member pathlink.com> wrote in message
news:cmjcjc$2nnu$1 digitaldaemon.com...

 Is there any way just to write data to a far segment using a far






pointer?











 Nov 08 2004