Lpc2000

bdmlpc wrote:

>--- In lpc2000@yahoogroups.com, "rtstofer" <rstofer@p...> wrote:
>  
>
>>The thing I don't understand is the address calculation.  I am 
>>using -O0 so I understand it won't do much optimizing but, to me, 
>>the 3 instructions used to load a register with a constant value 
>>seems excessive.  It gets a little better with -O3 but still, the 
>>address is a constant, why calculate it?
>>    
>>
>
>ARM instructions can only take 12bit of immediate operands with a bit
>displacement. So it is not possible to load a full 32bit immediate
>value into a core register directly, except that the value can
>separated into a single 12bit value and a shift value. If you turn on
>optimization GCC may use a trick to minimize cycles. Immediate values
>are stored in memory in a location near (< 2^12) to this instruction.
>Now GCC can load this "immediate" value out of memory relativ to its
>program counter in one cycle for example.
>
>     LDR R0, [PC, #+15]
>
>  
>
Exactly, ARM instructions _always_ are 32bits long.  That includes the 
operation code and operands.   An X86 or 68K processor uses variable 
lenght instructions.

Having said that, I will say that reading ARM assembler makes my head 
ache.  It may be a simpler set of instructions but it is very rich in 
addressing modes.

What will drive me crazy is attempting to debug fully opimized code: -Os

>>Now, since the compiler writers are very bright folks and I am just 
>>a user who has never read the standard (how come I have to pay for 
>>it?), I assume they do this calculation for a reason.  Does anyone 
>>know why?
>>    
>>
>
>Compiler guys are totaly crazy! ;-)
>  
>
You can download all the information about ARM from their website: 
arm.com.  However, you will pay for printed material (book form vs. PDF).

Actually, do you really want to know? heh.

TomW

-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
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