How does .Byte[] function on a specific byte?

I am working on the following lines of code:

#define WDOG_STATUS             0x0440
#define ESM_OP                    0x08

and in a method of my defined class I have:

bool WatchDog = 0;
bool Operational = 0;
unsigned char i;
ULONG TempLong;
unsigned char Status;

TempLong.Long = SPIReadRegisterIndirect (WDOG_STATUS, 1); // read watchdog status
if ((TempLong.Byte[0] & 0x01) == 0x01)
  WatchDog = 0;
else
  WatchDog = 1;


TempLong.Long = SPIReadRegisterIndirect (AL_STATUS, 1);
Status = TempLong.Byte[0] & 0x0F;
if (Status == ESM_OP)
  Operational = 1;
else
  Operational = 0;

What SPIReadRegisterInderect() does is, it takes an unsigned short as Address of the register to read and an unsigned char Len as number of bytes to read.

What is baffling me is Byte[]. I am assuming that this is a method to separate some parts of byte from the value in Long ( which is read from SPIReadRegisterIndirect ). but why is that [0]? shouldn't it be 1? and how is that functioning? I mean if it is isolating only one byte, for example for the WatchDog case, is TempLong.Byte[0] equal to 04 or 40? (when I am printing the value before if statement, it is shown as 0, which is neither 04 nor 40 from WDOG_STATUS defined register.)

Please consider that I am new to this subject. and I have done google search and other searchs but unfortunatly I could not found what I wanted. Could somebody please help me to understand how this syntax works or direct me to a documentation where I can read about it?

Thank you in advance.

Solution

Your ULONG must be defined somewhere. Else you'd get the syntax error 'ULONG' does not name a type

Probably something like:

typedef union {unsigned long Long; byte Byte[4];} ULONG;

Check union ( and typedef ) in your C / C++ book, and you'll see that this union helps reinterpreting the long variable as an array of bytes. Byte[0] is the first byte. Depending on the hardware (avr controllers are little endian) it's probably the LSB (least significant byte)