[Gllug] IA32? IA64? x86...

[Andrew Halliwell]

> This is my simple understandung..can someone more versed in this tell me if I 
> have hit-the-nail-on-the-head or missed-the-point-completely or am somewhere 
> in between.

I'm afraid you missed the nail... 
:)
There is no "search for instructions" as such, but there is a decoding
cycle.

The instruction is actually a set of bits, the combination of bits hits the
decoder, flips some switches inside the processor, and those switches then
activate certain circuits the perform the operations on the operands that
follow the instruction.

CISC decoding is much more complex, and sometimes needs to go through
several stages, as instructions have varying length. The first stage might
decode the first byte of the instruction to determine the "family" of the
rest of the instruction and the length of the rest of the instruction block,
for example.

A pretty good example of this is the Z80 (8 bit processor).
Instructions could be anything from 1 byte to 4 bytes in length.
Some bytes just flipped a switch to an alternate instruction set or
alternate register. Some Z80 instructions took a LONG time to perform. (in
relative terms)
 
With RISC, most of which use VLIW (very long instruction word) all the
decoding can be performed in one cycle as the length of the instruction is
fixed.


-- 
Gllug mailing list  -  Gllug at linux.co.uk
http://list.ftech.net/mailman/listinfo/gllug