[fpc-devel] Re: closing of bug 5042 or general unalignment problems with arm compiler

Marc Weustink marc at dommelstein.net
Mon Apr 24 23:01:34 CEST 2006

Felipe Monteiro de Carvalho wrote:
> Hello,
> What exactly are those exceptions of Misalligned data access on windows CE?
> And why that doesn´t exist on x86 architecture?

It depends all on how a CPU accesses memory.

Suppose a CPU has a 8bit data bus to memory and you want to read a byte. 
   Since you have a 8bit bus, you are able to adress all individual 
bytes, so reading is not a problem.
Now imagine a CPU with a 32bit databus and reading a byte. Data is read 
in chuncks of 32bits (=4 bytes) and it is adressed by multiples of 4. 
For getting the exact byte, the CPU "shifts" the right byte.

Now we do the same for reading a 32bits integer. On a 8 bit cpu, you 
have to read 4 times one byte and then you have the integer.
On a 32bits CPU it can be read in one go if the adress is aligned to 4 
bytes. If not, it has to be read in 2 halves and then combined to one 
integer. The x86 supports reading like this, alltough it is inefficient. 
The SPARC and some implementations of the ARM (*) don't support this and 
raise a buserror if you read data like this.

(*) there are extentions to the ARM core which can handle it, but this 
depends on what functionality the manufacturer of a specific core has 

Hope this made some thing clear.


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