Memory mapped hardware registers

From: Simon Paradis (see.end.of.message_at_globetotter.net)
Date: 01/23/05 

Date: Sun, 23 Jan 2005 16:03:29 GMT

Hi,

I'm developping board support code for an embedded m68k system. The actual
MCU is a MC68331 that has a CPU32 core (more or less a 68020). This MCU
define hardware registers at specific memory locations. My toolchain is
composed of GCC 3.4.3 and binutils 2.15.

It possible to define a variable that would be placed at specific location
using gcc or linker script hacks ? It would avoid using a pointer to access
a register. I don't care if this is non portable and/or gcc/binutils
specific.

For example, the CPU32 has the SIMCR 16-bits register placed at 0xFFFA00.

typedef unsigned short word;

/* SIMCR (0xYFFA00): SIM Configuration */
typedef struct _SIMCR {
    word exoff : 1;
    word frzsw : 1;
    word frzbm : 1;
    word _UNUSED_1 : 1;
    word slven : 1;
    word _UNUSED_2 : 1;
    word shen : 2;
    word supv : 1;
    word mm : 1;
    word _UNUSED_3 : 2;
    word iarb : 4;
} SIMCR;

How can I place a single SIMCR structure at 0xFFFA00 ? I guess some linker
magic is involved ? Also, the actual variable must be left uninitialized;
you don't want init code to overwrite the register with zeros as this will
crash the MCU.

Thanks,

Simon Paradis
simon _dot_ paradis _at_ usherbrooke _dot_ ca

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