The xExt register class is used for reading and writing bit fields of simple digital registers on the Resource Manager external bus.

Whole words can be accessed by setting the length and bit attributes to zero.
The class accepts values in decimal or hex format.

All address variables are VME addresses and are thus expressed in units of bytes (8 bits). It is assumed that all VME accesses are 16 bits wide.

Attributes :-

-s    VXI slot number

specifies the slot number (1-12) within the VXI crate. Manditory - no default value.

-a    address

specifies the address offset of the register on the RM external bus.

-p    access permission (rw, ro, wo)

specifies the data access modes permitted to the hardware register. Options are ro (read only); wo (write only); rw (read-write).

-l    length of bit field

specifies the length of the data field for this register in bits.
The server code permits access to VXI hardware registers at the bit level and implements in software objects which may be any subfield of a hardware register. For write access to a subfield of a hardware register it is required that the hardware be readable.
The attribute value -l 0 implies that the data field for this object is the same as the hardware access width (16 bits). Optional. By default -l 0 is assumed.

-b    bit position

specifies the position of the least significant bit of the data within the hardware register. The hardware is addressed with the least significant bit having the value 0.
This attribute will normally always be used in conjunction with the -l attribute to define the length and position of a software object which is a subfield of the hardware register. Optional. By default -b 0 is assumed.

-f    data format

specifies the data format to be used by READ operations. Data can be return in decimal (d) or hexadecimal (x). The default is hexadecimal.

-i    initial value

specifies the data value to be used by the INITIALISE operation. The register must also have write permission.