Can anyone explain me the meaning of the following code block

[srikanthpadava]

Can anyone please help me in understanding the following code:
UD1RX is referred to in the application code (foreg: RX = UD1RX) and i have found the following defines for the same, but
couldnt quite understand why they mean.

I understand it is being casted as a pointer to a pointer. But it is confusing. I appreciate if any one could explain it.

#define	UD1RX            CAST_UC(0xfffffa16UL)

#ifdef __LANGUAGE_ASM__
   #define CAST_UC(x) x
#else
   typedef volatile unsigned char*   pU08necDevFile;

   #define CAST_UC(x) ( *(pU08necDevFile)(x) )

[weaknessforcats]

I can't see what CAST_UC does but if the flag __LANGUAGE_ASM_ _ is defined, then CAST_UC will do something with the value in parentheses. If __LANGUAGE_ASM_ _ is not defined, then your x will be type cast to a volatile unsigned char* and then de-referenced to a volatile char and then pass to CAST_UC.

You would think you could just cast the x to a volatile char and omit the typedef but that typedef may be used elsewhere in the programif __LANGUAGE_ASM_ _ if is not defined.

[donbock]

Your example

...
RX = UD1RX;

expands to the following if __LANGUAGE_ASM_ _ is defined:

...
RX = 0xfffffa16UL;

otherwise it expands to this (the typedef is not part of the expansion, I wrote it below to provide context):

typedef volatile unsigned char*   pU08necDevFile;
...
RX = ( *(pU08necDevFile)(0xfffffa16UL) );

.
I suspect RX is itself a macro whose expansion depends on __LANGUAGE_ASM_ _.

[srikanthpadava]

hi weaknessforcats and donbock,

Thanks for your responses.
I have understood to some extent. But i still couldnot understand the following: (I am quoting a few lines out of weaknessforcats 's response)
"If __LANGUAGE_ASM_ _ is not defined, then your x will be type cast to a volatile unsigned char* and then de-referenced to a volatile char and then pass to CAST_UC. "

i.e in my example if x=0xfffffa16UL, at first it is being typecast to volatile unsigned char* and then derefenced, in the sense that the value at this adress (oxfffffa16UL) is stored in RX. Have i understood it correctly?

[donbock]

@srikanthpadava : Refer to line 3 of the third code block in my earlier reply post for the case when __LANGUAGE_ASM_ _ is undefined.

RX = ( *(pU08necDevFile)(0xfffffa16UL) );

First, 0xfffffa16UL is cast to a pU08necDevFile (volatile unsigned char*). Then, the asterisk deferences the pointer. That is, the value of the unsigned char at location 0xfffffa16UL is stored in RX.

Is RX a macro?

[srikanthpadava]

hi donbock,

thanks for the explanation.

No RX is not a macro.
RX is defined as static T_UINT8 RXTemp without any ifdef
and takes directly the value of UD1RX which is i believe is a register

[Banfa]

My guess would be that UD1RX is a register and that it may be stands for something like

UART Device [b]1[b] Receive

RX is a common abbreviation of receive(r) as TX is for transmit(er).

If __LANGUAGE_ASM_ _ is not defined my guess is that this is reading the receive buffer of a UART device. For some reason if __LANGUAGE_ASM_ _ is defined it does not; a reason for this is that the software can build built on difference platforms, may be a target platform and a simulator or test platform. On the simulator platform there is no UART device so the coder wanted to ensure the program didn't read a random in invalid memory address and since he needs to return something he just returns the value of the address.

Why they have used __LANGUAGE_ASM_ _ as the symbol to control this I do not know, I would use something that was more meaningful but it is possible that they just looked for a symbol that is defined by the compiler of the simulator build and is not defined by the compiler of the target build. Personally I would say that wasn't very good practice.


BTW if you don't know a UART or USART device controls communication using a serial line such as an RS232 port. Modern USARTs normally support multiple different serial protocols RS232, I2C, SPI etc

[srikanthpadava]

thanks for the response banfa.

Yes UD1RX is the register of the device (UART) controlling the serial communication you were mentioning.