Re: C compiler pointer management on DSPs

On Monday, September 30, 2019 at 3:06:36 PM UTC-7, David Brown wrote:


(snip)


> Some processors have larger access sizes to simplify the hardware. The
> first DEC Alpha, and some ARM designs, had no instructions for reading
> or writing 8-bit or 16-bit data. In effect, these had 32-bit (maybe on
> the Alpha it was 64-bit) "byte" sizes. But smaller access sizes could
> be easily simulated in software.


Alpha isn't quite that bad.


The load/store instructions work on 32 or 64 bit units, but they ignore
the low bits when doing it.


So, you take a byte address, and use a load instruction to load its
word into a register. (I forget now the names of the memory units.)


Then there are instructions for operating on bytes
in a register which ignore the high bits. So, you can load a byte
from memory into a register with two instructions. To store a byte,
I believe you load the word, replace the byte, and write it back,
so three instructions.


Note that those operations are what CISC processors do without
you thinking about them on many machines, as memory is often
much wider than 8 bits.


Machines not so well designed require masking off the appropriate
bits before operating with them, though many machines ignore high
bits on shift operations. (The 8086 allows shifts up to 255 bits.)

Return to the comp.compilers page.
Search the comp.compilers archives again.