struct pointing to RTC in far memory location.

I think that would be a reasonable thing to do, yes. Something like:

#define RtcAddr  0x180000

typedef struct
    {
    U16 year;
    U8 month;
    U8 day;
    } RtcReg;

#define Rtc  FVAR (RtcReg, RtcAddr)

...

Rtc.month = 8;

If you peek at the definition of FVAR, you'll see that it's a macro that really just adjusts an address for the Keil tag byte coding scheme, casts it to a pointer to the type given, and dereferences that pointer. So it's a way to claim that a variable of a particular type lives at a particular address, without actually declaring a variable.


The way I read the manual, you should also be able to use the _at_ extension to declare a variable at the right location:

RtcReg far Rtc _at_ 0x180000UL;

Rtc.month = 8;

But I've never done it that way myself.

A variation would be to put the variable in a source file by itself, so that it will wind up in a data segment by itself, and then use the linker to specify the location of that data segment. A bit more trouble, but it avoids the use of one non-standard extension if portability is a concern.

(The first method is, IMO, the most portable; people tend to forget about the linker configuration files, and there's no commonality at all between linker control file / directive syntax from one linker to another. Most projects I've worked on use the cast-an-address route to physically locate a variable.)

I think that would be a reasonable thing to do, yes. Something like:

#define RtcAddr  0x180000

typedef struct
    {
    U16 year;
    U8 month;
    U8 day;
    } RtcReg;

#define Rtc  FVAR (RtcReg, RtcAddr)

...

Rtc.month = 8;

If you peek at the definition of FVAR, you'll see that it's a macro that really just adjusts an address for the Keil tag byte coding scheme, casts it to a pointer to the type given, and dereferences that pointer. So it's a way to claim that a variable of a particular type lives at a particular address, without actually declaring a variable.


The way I read the manual, you should also be able to use the _at_ extension to declare a variable at the right location:

RtcReg far Rtc _at_ 0x180000UL;

Rtc.month = 8;

But I've never done it that way myself.

A variation would be to put the variable in a source file by itself, so that it will wind up in a data segment by itself, and then use the linker to specify the location of that data segment. A bit more trouble, but it avoids the use of one non-standard extension if portability is a concern.

(The first method is, IMO, the most portable; people tend to forget about the linker configuration files, and there's no commonality at all between linker control file / directive syntax from one linker to another. Most projects I've worked on use the cast-an-address route to physically locate a variable.)