question

hi.
picmicro 18f452 is a RISC family but it has 75 instruction set.why?
does a RISC micro can have 75 instruction set?

 

http://en.wikipedia.org/wiki/RISC

 

Some of the early PIC's did have very few instructions maybe thats why
RISC has stuck ?

They are great cheap microcontrollers, I have used them since 1985.

 

It's just hype from microchip, PICs aren't RISC, never were.

Bye.
Jasen

 

What's your definition of RISC?

That was really the problem back in the '80s/'90s -- everyone was clammoring
to tell you that "RISC is better!" and "we're RISC!" but, of course, there
isn't any solid definition of what "RISC" and "CISC" were actually supposed to
mean. How "reduced" is RISC? How "complex" is CISC?

The textbook answer I know is basically... "something that looks like the
original ARM CPUs -- pipelined, only load/save instructions can access memory
directly, all other instructions use registers, of which there are lots" is
RISC.

With microcontrollers it's rather pointless to debate RISC vs. CISC anyway...
with big-iron CPUs, the idea was that RISC let you ramp up the clock to a rate
that more than made up for the "less complex instructions," under the
assumption that both CPUs had access to the same fab and power consumption
wasn't more or less equal. In a microcontroller, however, usually clock rate
is limited far more by which silicon process the manufacturer really *does*
have access to, power consumption, or other critieria. Indeed, for
microcontrollers these days "work per joule" is probably one of the best
metrics, as memory and clock cycles are both pretty cheap, but battery life
isn't.

(A good example here is the AVR vs. the MSP430... both are low-power, the AVR
is RISCier and runs at a faster clock rate, but the MSP430 often performs
better in benchmarks since it's actually a 16 bit CPU with a unified address
space... although it costs more too...)

---Joel Kolstad

 

lots of registers,
no single op-code read-modify-write (RAM).
single-clock execution of most op-codes

http://cse.stanford.edu/class/sophomore-college/projects-00/risc/whatis/index.html
http://cse.stanford.edu/class/sophomore-college/projects-00/risc/risccisc/

there's a wikipedia page that basically says the same thing too.

This pic looks like an 8-bit processor like the 6800 or 6500 families with
half the op-codes and some of the registers dropped and a harvard
architecture bolted on. more of a sawnoff CISC than risc.

Bye.
Jasen