Am 22.01.2003 22:43:14, schrieb "Mark Stokes" <m.sto...@ieee.org>:
>What is the preferred method for allowing byte (or even nibble) access
>to unsigned long variables?
>
>example:
>a setup program that is taking a number off a screen (in BCD) on a per
>character basis (say with a button that steps to each character) and
>places the result in an unsigned long int (preferably still in BCD).
>
>I thought of three methods:
>1. Use a union.
>2. Use a character array and then use shift assignments to initialize
>the proper variables before and after
>3. Try to access the RAM directly (access the unsigned long as if it
>were a character array).
that's the same as 1) but without the help of the compiler
>implementation:
>1. should be obvious
or a union with bitfields for the nibbles, but that will lead to large code
>2.
>unsigned char testlong[4];
>
> // initialize from RAM
> testlong[0] = ramval & 0x000F;
> testlong[1] = ( ramval & 0x00F0 ) >> 4;
> testlong[2] = ( ramval & 0x0F00 ) >> 8;
> testlong[3] = ( ramval & 0xF000 ) >> 12;
>
> // Mess with testlong using loops, etc
> // Copy value back to RAM
> correctraw = ( testlong[3] << 12 ) + ( testlong[2] << 8 ) + (
>testlong[1] << 4 ) + testlong[0];
>
>3. Access the unsigned int as a string. The compiler complains about
>this. But one way around the complaints is to use a pointer.
>unsigned long int *longpointer;
>
> longpointer = *ramval;
* -> &
> longpointer[0] = (uchar)something;
> etc.
or stress typecasts even more...
unsigned long value;
((char *)&value)[offset] = y; //offset = 0..3
but consider that typecasts are a bad thing, cause you tell the compiler to
shut up, even if you doing insane things...
>The reason for wanting to access the variable using byte operations is
>that it makes it easy to "step across" ram val with an index. The only
>alternative I can think of is to hard code each "byte" update of the
>ramval (lots of code).
work with bytes or better just a binary number for your display routine. then
if you realy want, convert to BCD to store it in your settings flash/ram
whatever.
that way you'd simply need a bin2bcd() end bcd2bin()
but i must say that i dont like BCD encoded digits very much. it makes
code only complicated... (it may be resonable for arbitrary precision
integers, where you add digits as needed, but it i dont think that you're
using this on a MSP ;-)
i'd probably work on a normal number (short variable or whatever) and
only convert to BCD if needed. (you're using a F4xx with LCD driver?)
i'm usualy working with character LCDs and there i can use printf to
output nicely formated numbers :-)
>Thanks for any ideas.
>One last thing: is there a way to force the compiler to use a RAM
>location for a variable as opposed to the Stack (I know, same thing), or
>a register?
void f(void) {
static int local1; //like a global var but not known to other
funcs.
register int reg1; //*try* to use a register
int auto1; //register or stack, same as auto int auto1 (?)
}
the compiler will try to use the registers for local variables, so you don't
realy
need the register keyword.
chris
