Lpc2000

David Hawkins wrote:

> 
>  > Hey guys, I've started working with the LPC2138. I'm still working
>  > through the docs, but one question I have is this:
>  >
>  > PLLCFG  = (4 + (2 << 5));
>  >    E3A0320E   mov r3, #0xE0000000
>  >    E283397F   add r3, r3, #0x001FC000
>  >    E2833084   add r3, r3, #0x00000084
>  >    E3A02044   mov r2, #0x00000044
>  >    E5C32000   strb r2, [r3]
>  >
>  > I'm using CrossWorks with a Olimex JTAG (works great so far BTW) with
>  > optimization turned off.
>  >
>  > ARM assembly still isn't clear to me yet, so this is probably buried in
>  > the technical docs somewhere.
>  >
>  > Why doesn't this compile to something like:
>  >
>  > mov r3, #0xE01FC084
>  > mov r2, #0x00000044
>  > strb r2, [r3]
>  >
>  > Can't the ARM load a 32 bit immediate?
> 
> The opcode is 32-bits, so it can't also contain a 32-bit immediate.

That's what I suspected. How big of a number can you fit into a single MOV?

> But in this case, your number is only 8-bits, so it could probably
> code an op-code for it.

Yeah, the value to store is fine since its only 7 bits.

> 
> The way I answer this type of question is to write C-code and
> build it as you have done, and then recode what I think should
> be possible in assembler and see if that builds. In the case
> of
> 
> mov r3, #44
> 
> it'd probably compile, but
> 
> mov r3, #12345678
> 
> it probably would not, but
> 
> mov r3, PLLVAL
> PLLVAL: .word #12345678
> 
> would (or whatever the assembler syntax is :), since this
> is loading a value using indirect addressing.

Another related question - what's the difference between a LDR and a 
MOV? I understand the LDRH and LDRB for 16 and 8 bit loads, but don't 
see why there is a MOV if a LDR does the same thing?

Thanks,

Brian



-- 
-----------------------------------------------------
Brian C. Lane (W7BCL)                      Programmer
www.shinemicro.com   RF, DSP & Microcontroller Design