22 Apr
2024
22 Apr
'24
6:26 a.m.
On Apr 21, 2024, at 9:17 PM, Will Cooke via cctalk
<cctalk(a)classiccmp.org> wrote:
Cycles per instruction is one aspect of RISC vs. CISC but there are more, and cycles per
instruction may not be the most significant one.
Given enough silicon and enough brainpower thrown at the problem, CISC machines can be
made to run very fast. Consider modern x86 machines for example. But the key point is
"given enough silicon...".
I think the significance of RISC isn't so much in cycles per instruction but rather in
simplicity of implementation (for a given level of performance). It's not just single
cycle instructions. In RISC architectures it is often easier to achieve pipelining and
parallelism. Consider what's arguably the first example, the CDC 6600 with its
parallelism, and its sibling the 7600 which made the rather obvious addition of
pipelining.
Simplicity of implementation means either lower cost for a given level of performance, or
higher achievable performance for a given level of technology, or lower power per unit of
performance, or easier power management optimization, or any combination of the above.
Consider ARM machines vs. x86. It's not so much that ARM machines go faster but that
they do so on a fraction of the power, and that they require only a small amount of
silicon to do so.
One other factor is that RISC machines rely on simple operations carefully arranged by
optimizing compilers (or, in some cases, skillfull programmers). A multi-step operation
can be encoded in a sequence of RISC operations run through an optimizing scheduler more
effectively than the equivalent sequence of steps inside the micro-engine of a CISC
processor.
paul
On 04/21/2024 7:06 PM CDT Peter Coghlan via
cctalk <cctalk(a)classiccmp.org> wrote:
Why is that? Did the Z80 take more cycles to implement it's more complex
instructions? Is this an early example of RISC vs CISC?
Regards,
Peter Coghlan
I'm certainly no authority, but I have programmed both processors in assembly and
studied them somewhat. It took many years for me to believe that the 6502 was
"faster" than the Z80, but now I'm (mostly) a believer. So here is my
take.
First, yes, the Z80 takes roughly 4 times as many clock cycles per instruction. Where
the 6502 can complete a simple instruction in a single clock, the Z80 takes a minimum of
four.
The 02 certainly has a simpler architecture, but calling it a RISC machine would probably
make the RISC believers cringe. It is simple, but it doesn't follow the pattern of
lots of registers (well, maybe) and a load/store architecture. But that may be its
strongest point. The zero page instructions effectively make the first 256 bytes of RAM
into a large (128 or 256) register file.
...