15321RE: [SeattleRobotics] how much time single instruction takes?
- Nov 1, 2003I am a big 8051 fan, and am starting to switch over to the Atmel AVR also.
As someone who produces boards for a living, the only downside to the AVR
is single source.
I agree with almost everything Larry writes. However there is a slight mistake.
A register is not an ALU. An ALU is a Arithmetic / Logic Unit. Registers
are like memory locations. In the Atmel AVR there are 32 registers, and 1 ALU.
The ALU performs the math functions like add, subtract increment (usually)
and logic operations like or, and, not. The output of the ALU usually goes
back to a of the registers. During the calculation, it also updates the
status register, for the Z, C, N, V, H flags.
Most of the ALU operations are 1 clock cycle. To save hardware and make
the chip smaller and cheaper, they made the multiply a 2 clock
operation. Most cheap microcontrollers do not have a hardware multiply and
an assembler routine to do it can take 40 to 100 machine cycles. So having
a hardware multiply is a big advantage even if it takes 2 clocks versus
doing it in software.
The advantage of a register, is that it is hard wired to the ALU so two
registers can be an input and one register can be an output. There is no
"put the address on the bus, wait for the data lines to get the data, and
then present it to the ALU". This way it can be done in 1 clock cycle.
At 11:09 AM 11/1/03, you wrote:
>In the AVR all 32 registers are complete ALU's. So there is no shuffling of
>data: just operate on the register and you are done. That is how they get 1
>cycle instructions. Leading edge: read instruction, falling edge execute
>instruction (e.g. add, sub, mov, etc). That is why load/store instructions
>and branches take 1-2 extra cycles since they have to go outside the core
>for data (branch address or SRAM data).
>Contrast that with the PIC four cycles (in the '16 manual): read
>instruction, fetch operands, do it, store results. Most PIC instructions
>are one machine cycle, but, again, go outside the core and it is 2 cycles
>(e.g. branches). I am guessing, clock for clock, good Assembly or C coders
>will get roughly 4x the performance out of an AVR than a PIC.
>I think folks should get over the Microchip marketing of the PIC as a simple
>processor. If Atmel called out instructions like Microchip does, the AVR
>would have only ~30. The PIC is NOT simple, just archaic. On the other
>hand, the PDP-11 was complex, with a beautiful and elegant instruction set.
>So simple you could program and read it in octal.
>Microchip does have a couple things on Atmel: early market lead and a huge
>support system and, in fact, their chips are totally suitable for what most
>small systems need. Plus you can get OTP chips for very cheap which for
>toys is everything.
>However, for hobby robotic work where one-off designs are common and where
>the development cost is everything (i.e. our time spent designing, coding
>and debugging) I still think the AVR wins hands-down.
>From: Brian Dean [mailto:bsd@...]
>Sent: Saturday, November 01, 2003 7:25 AM
>Subject: Re: [SeattleRobotics] how much time single instruction takes?
>On Fri, Oct 31, 2003 at 11:58:40PM -0500, David VanHorn wrote:
> > >As far as I know Atmel AVR is using Harvard
> > >Architecture, RISC and Pipelining to reach that speed.
> > >Could anybody explain more detail about this?
> > I don't know that it's pipelined.
>I think you are right - this document says it is not:
>It says: "For all intents and purposes, the CPU has no pipeline. It
>retrieves both source operands, executes the instruction, and stores
>the result in a single clock cycle. Branch latency is one clock for
>taken branches. All operations are register-to-register; the chip
>follows a strinct load/store model."
>I recall hearing something similar at a recent Atmel seminar.
>Visit the SRS Website at http://www.seattlerobotics.org
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