Question about variable definitions

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I'm a VC++ programmer doing my first bit of embedded development in C
for a 80C52 compatible chip.  I have the use of some API's and
included is an .h file containing the following typedefs:

typedef unsigned char  data Ucd;
typedef unsigned char idata Uci;
typedef unsigned char pdata Ucp;
typedef unsigned char xdata Ucx;
typedef unsigned char  code Ucr;
typedef unsigned char       Uc;

Code samples available to me use Ucr as a parameter type for a
function if the data being passed is a "string" rather than a
variable.  I understand vaguely that different data may be stored in
different memory spaces, but I guess I need to know the following:

1)    Is there a good book, document, web-site, or what ever that will
help me understand the basic concept of why these typedefs are needed
2)    Do I need to use these typedefs or can I simply use unsigned char?
3)    If there is a choice as to whether or not these typedefs are
required, what are the advantages/disadvantages, etc. of using them

Many thanks in advance for any help with this.

Re: Question about variable definitions
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Re: Question about variable definitions
Hi, Julia.

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I'd suggest you download .pdf datasheet of part you are using (ie. Atmel
from manufacturer's site and learn about memory organisation in

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Of course you can use unsigned char.

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The only advantage of typedefs is you don't have to write "unsigned char
which is 18 characters instead you write "Ucd" and that is 3 characters.
When you
have functions with a lot of arguments it becomes pretty large with unsigned
char data ....

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