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Trying to understand the workings of a Memory Mapped I/O

... snip ...

All of which brings up the ancient lore - all casts are suspicious, therefore eschew them as the plague.

I think nobody has pointed out that the __R macro lies in the implementors name space, and thus has no standards requirements. Even so, its actual wording would probably be instructive.

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Chuck F (cbfalconer@yahoo.com) (cbfalconer@worldnet.att.net)
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Reply to
CBFalconer
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You're still with us! Your old email address(es) don't work when I tried to contact you.

Reply to
Everett M. Greene

Of course it's also possible that the __R is a compiler specific keyword rather than a macro.

Robert

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Reply to
R Adsett

Las,

down

Las,

Here's my take on it. Firstly, these are C language preprocessor macro definitions. The preprocessor's job is to substitute the token P0 with the expression given in the definition; ((*(__R volatile unsigned char *)(unsigned char)0x0)) The C preprocessor is, in essence, a text substituter.

As to the meanings of the definitions, it can be analysed by proceeding from the right towards the left. The numerical values, 0x0, 0x1, etc. are integer constants.

Each of these constants is typecast to an unsigned char. This has two effects:

  1. To take only the least signifigant 8 bits of the integer constant. In the case of your values, this effects no change. It would if the values exceeded 0xFF.
  2. Suppress any sign extension. It's the "unsigned" that does this. In the case of your values, there is again no effect.

The typecast preceding this; "volatile unsigned char*" means regard the expression to the right of this typecast as a pointer to an unsigned char. i.e. the expression is to be regarded as an address. The "volatile" word tells the compiler not to assume that the unsigned char pointed to will retain its value between accesses; i.e. it might be changed by external events.

The "__R" qualifier is non-standard and I don't know what it means. I would guess it might mean register, as the names P0, P1, etc. seem to be register names.

Finally the '*' preceding this typecast means the contents of the address location; the value at the specified address.

P0,P1,P2,P3 are the names of the ports on a standard 8051 processor, but their respective addresses are 0x80, 0x90, 0xA0, 0xB0 respectively, so if the code were to refer to an 8051, there leaves something about the definitions yet to be understood!

David Bardon, Avocet; snipped-for-privacy@avocetsystems.com

Reply to
David Bardon

Good analysis. My guess is that the locations 0x1... are loaded with the real P0, ... data by "another process" - in other words they are letting this process think it is dealing with the ports. - RM

Reply to
Rick Merrill

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