EXAMPLE for device support that uses VME, PCI, ..
on an operating system like vxWorks that allows direct register access

Assume an analog input board with registers.
These are typically memory-mapped to some base address
in memory.
Details depend on the hardware and OS.

We want to be able to attach one or more records
which each read one of the 'channels' from the board.
The INP link provides the channel number.

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record(ai, "$(user):reg0")
{ 
    field(DTYP, "hwreg")
    field(INP,  "0")
    field(SCAN, "1 second")
}

record(ai, "$(user):reg2")
{ 
    field(DTYP, "hwreg")
    field(INP,  "2")
    field(SCAN, "1 second")
} 


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# dbd file that links DTYP="hwreg" to devAiHW
device(ai, CONSTANT, devAiHW, "hwreg") 

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#include <stddef.h>
#include <stdio.h>
#include <string.h>
#include "alarm.h"
#include "dbDefs.h"
#include "dbAccess.h"
#include "recGbl.h"
#include "recSup.h"
#include "devSup.h"
#include "devLib.h"
#include "link.h"
#include "aiRecord.h"
#include "epicsExport.h"

static long init_record(aiRecord *rec)
{ 
    // Read channel number from INP link
    short channel; 
    recGblInitConstantLink(&rec−>inp, DBF_SHORT, &channel);

    // Exact base address would depend on hardware,
    // jumper settings, OS, ...
    // The manual for the board would help figure this out.
    // For Linux, you might have to do this:
    //
    //   int fd = open("/dev/that_piece_of_hardware", O_RDWR);
    //   short *base = mmap(...., PROT_READ, ...., fd, ....);
    //
    // Assume this is the base address:
    short *base = (short *) 0x12344532;


    // Assume the first register is a version code,
    // and we skip that.
    // Then follow the registers for channel 0, channel 1, ...
    short *register = &base[1 + channel];

    // Just in case, we use an EPICS call for vxWorks and RTEMS
    // that checks if there is actually anything at that address
    short test;
    if (devReadProbe(2, register, &test) != 0)
    {
        printf("Dude, there is no AI board at 0x%x\n", base);
        rec->pact = 1;
    }

    /* device private (dpvt) is where we can park our device data */
    rec−>dpvt = register;
    return 0;
} 

static long read_ai(aiRecord *rec)
{ 
    short *register = (short *) rec−>dpvt;

    // Two basic options:
    // Just read the register.
    // FAST, but risks bus error (CRASH) when
    // board removed at runtime
    rec−>rval = *register;

    // Other option would be to always use
    //  if (devReadProbe(2, register, &rec->rval) != 0)..
    // which is SAFE, but slower

    rec−>udf = FALSE;
    return 0;
}

/* Create the device support entry table  */
struct 
{
    long        number;
    DEVSUPFUN   report;
    DEVSUPFUN   init;
    DEVSUPFUN   init_record;
    DEVSUPFUN   get_ioint_info;
    DEVSUPFUN   read_ai;
    DEVSUPFUN   special_linconv;
} devAiHW =
{ 
    6,
    NULL,
    NULL,
    init_record,
    NULL,
    read_ai,
    NULL
};
epicsExportAddress(dset, devAiHW);