Manipulating the Pointer

Every time you call viMapAddress(), the pointer you get back is valid for accessing a region of addresses. Therefore, if you call viMapAddress() with mapBase set to address 0 and mapSize to 0x40 (the configuration register space for a VXI device), you can access not only the register located at address 0, but also registers in the same vicinity by manipulating the pointer returned by viMapAddress(). For example, if you want to access another register at address 0x2, you can add 2 to the pointer. You can add up to and including 0x3F to the pointer to access these registers in this example because we have specified 0x40 as the map size. However, notice that you cannot subtract any value from the address variable because the mapping starts at that location and cannot go backwards. The following example shows how you can access other registers from address.

#include "visa.h"
#define ADD_OFFSET(addr, offs) (((ViPByte)addr) + (offs))
int main(void)
{
ViStatus    status;            /* For checking errors */
ViAddr      defaultRM, instr;  /* Communication channels */
ViUInt16    address;           /* User pointer */
ViSession   value;             /* To store register value */ 
    /* Begin by initializing the system */
    status = viOpenDefaultRM(&defaultRM);
    if (status < VI_SUCCESS) {

       /* Error Initializing VISA...exiting */
       return -1;
    }

    /* Open communication with VXI Device at Logical Address 16 */
    /* NOTE: For simplicity, we will not show error checking */
    status = viOpen(defaultRM, "VXI0::16::INSTR", VI_NULL, VI_NULL, &instr);

    status = viMapAddress(instr, VI_A16_SPACE, 0, 0x40, VI_FALSE, VI_NULL, &address);

    viPeek16(instr, address, &value);
    /* Access a different register by manipulating the pointer. */
    viPeek16(instr, ADD_OFFSET(address, 2), &value);

    status = viUnmapAddress(instr);

    /* Close down the system */
    status = viClose(instr);
    status = viClose(defaultRM);
    return 0;

}

Private Sub vbMain()

   Dim stat    As ViStatus
   Dim dfltRM  As ViSession
   Dim sesn    As ViSession
   Dim addr    As ViAddr
   Dim mSpace  As Integer
   Dim Value   As Integer

    Rem Open Default Resource Manager
    stat = viOpenDefaultRM(dfltRM)
    If (stat < VI_SUCCESS) Then

       Rem Error initializing VISA...exiting
       Exit Sub

    End If

    Rem Open communication with VXI Device at Logical Address 16
    Rem NOTE: For simplicity, we will not show error checking
    stat = viOpen(dfltRM, "VXI0::16::INSTR", VI_NULL, VI_NULL, sesn)
    
    mSpace = VI_A16_SPACE

    stat = viMapAddress(sesn, mSpace, 0, &H40, VI_FALSE, VI_NULL, addr)

    viPeek16 sesn, addr, Value
    Rem Access a different register by manipulating the pointer.
    viPeek16 sesn, addr + 2, Value

    stat = viUnmapAddress(sesn) 

    Rem Close down the system

    stat = viClose(sesn)
    stat = viClose(dfltRM)

End Sub