"Rom" / "rom byte" keywords?

[Robotics Guy]

I'm trying to program a microcontroller in C to communicate with my laptop over a USB connection. The code examples that I've been able to find have the descriptors written either like this:

ROM USB_DEVICE_DESCRIPTOR device_dsc=
{
    0x12,    // Size of this descriptor in bytes
    USB_DESCRIPTOR_DEVICE,                // DEVICE descriptor type
    0x0200,                 // USB Spec Release Number in BCD format
    0x00,                   // Class Code
    0x00,                   // Subclass code
    0x00,                   // Protocol code
    USB_EP0_BUFF_SIZE,          // Max packet size for EP0, see usb_config.h
    0x04D8,                 // Vendor ID
    0x003F,                 // Product ID: Mouse in a circle fw demo
    0x0002,                 // Device release number in BCD format
    0x01,                   // Manufacturer string index
    0x02,                   // Product string index
    0x00,                   // Device serial number string index
    0x01                    // Number of possible configurations
};

Or this:

	rom byte deviceDescriptor[] =
	{
		0x12, 0x01, // bLength, bDescriptorType
		0x00, 0x02, // bcdUSB (low byte), bcdUSB (high byte)
		0x00, 0x00, // bDeviceClass, bDeviceSubClass
		0x00, E0SZ, // bDeviceProtocl, bMaxPacketSize
		0xD8, 0x04, // idVendor (low byte), idVendor (high byte)
		0xFF, 0x01, // idProduct (low byte), idProduct (high byte)
		0x01, 0x00, // bcdDevice (low byte), bcdDevice (high byte)
		0x01, 0x02, // iManufacturer, iProduct
		0x00, 0x01  // iSerialNumber (none), bNumConfigurations
	};

The problem is, I have no idea what the "rom" and "rom byte" words mean. I've looked through my C programming book and have searched Google but have come up with nothing. Can anyone point me in the right direction?

Thanks

[BobS0327]

Info from a USB manual...

Information about a USB device is stored in segments of its ROM (read-only
memory). These segments are called descriptors. An interface descriptor can
identify a device as belonging to one of a finite number of classes.

[Robotics Guy]

I believe the answer to my question lies in one for the example project's included files:

#define ROM      const

typedef union 
{
    UINT16 Val;
    UINT8 v[2] __PACKED;
    struct __PACKED
    {
        UINT8 LB;
        UINT8 HB;
    } byte;
    struct __PACKED
    {
        __EXTENSION UINT8 b0:1;
        __EXTENSION UINT8 b1:1;
        __EXTENSION UINT8 b2:1;
        __EXTENSION UINT8 b3:1;
        __EXTENSION UINT8 b4:1;
        __EXTENSION UINT8 b5:1;
        __EXTENSION UINT8 b6:1;
        __EXTENSION UINT8 b7:1;
        __EXTENSION UINT8 b8:1;
        __EXTENSION UINT8 b9:1;
        __EXTENSION UINT8 b10:1;
        __EXTENSION UINT8 b11:1;
        __EXTENSION UINT8 b12:1;
        __EXTENSION UINT8 b13:1;
        __EXTENSION UINT8 b14:1;
        __EXTENSION UINT8 b15:1;
    } bits;
} UINT16_VAL, UINT16_BITS;

So it looks like ROM really just means "const." But the structure which I believe corresponds to the "byte" keyword totally confuses me. Can anyone shed some light on the above structure?

[hoxsiew]

It's a union so all members point to the same memory location. Within the union there is a bit field structure, a word and a struct of 2 bytes. It's sort-of a shortcut to access individual bits or individual bytes in a 16 bit word.