topic Re: Force variable to Register in ColdFire/68K Microcontrollers and Processors

Force variable to Register

gbaars — Mon, 21 Jul 2008 22:46:41 GMT

Is it possible to declare a variable (in C) assigned

not to RAM but to a processor register so it

runs faster?

Re: Force variable to Register

RichTestardi — Mon, 21 Jul 2008 23:31:02 GMT

Hi,

You can use the "register" keyword to suggest (not force) that the compiler

put the variable in a register. But this will depend not only on the optimization

level you choose (under project settings -> Code Generation -> Global

Optimizations), but for CW7.0 at least, also possibly on if you enable "Register

Coloring" (under project settings -> Code Generation -> ColdFire Processor).

To use the "register" keyword is straightforward -- declare a variable like:

Then build and check your disassembly (right-click on the file and select

"Disassemble" ).

In general, I would also add that if you turn up the optimization level and enable

compiler to use a register -- it will often do so on its own. However, this really

depends on the exact version of CodeWarrior you are using (OK, and I even

*assumed* you were using Code Warrior!).

Good luck.

-- Rich

Re: Force variable to Register

gbaars — Tue, 22 Jul 2008 01:40:57 GMT

Thanks for reply. Version of CW = 5.9.0
With register code indeed translates
avoiding slower RAM.
With a 1000x for loop containing 23 asm
instructions and RPGIO_TOG 600 uS is
measured for 23000 asm instructions.
This results into near 38 MIPS.
(coreclk = 50 MHz, busclk 100 MHz intern).
Is near 100 MIPS somehow possible?

Re: Force variable to Register

gbaars — Tue, 22 Jul 2008 01:42:00 GMT

-err that is busclk = 25 MHz.

Re: Force variable to Register

RichTestardi — Tue, 22 Jul 2008 04:01:26 GMT

What processor are you using?

Re: Force variable to Register

gbaars — Tue, 22 Jul 2008 17:34:30 GMT

The processor is a MCF51QE128CLK (80 pins).
The internal clock is set with processor expert
to 50 MHz and busclk divider is 2.

quote:

• 32-Bit Version 1 ColdFire® Central Processor Unit (CPU)
– Up to 50.33-MHz ColdFire CPU from 3.6V to 2.1V, and
20-MHz CPU at 2.1V to 1.8V across temperature range
of -40°C to 85°C
– Provides 0.94 Dhrystone 2.1 MIPS per MHz
performance when running from internal RAM
(0.76 DMIPS/MHz from flash)

unquote

I am not sure how DMIPS translate to MIPS but I would
like to see it run as fast as 2.1 x 50 MIPS from RAM.
Can Codewarrior do this or has it to be done by code?

Re: Force variable to Register

RichTestardi — Tue, 22 Jul 2008 19:53:42 GMT

Hi,

I'll start with the disclaimer that I'm a ColdFire performance novice!

If I understand everything you are saying correctly (you execute 23000 instructions in 600 us running at 50 MHz), it looks like you're running at about 1.3 clocks-per-instruction on a V1 core.

23000 instructions; 600 us; 50e6 clocks/sec ? clocks/instruction
= 1.3043478 clocks/instruction

From the reference manual (7.3.4 Instruction Execution Timing) clocks-per-instruction tables that does not seem unreasonable.

From the datasheet, I'd interpret the .94 DMIPS/MHz (i.e., 1.06 clocks-per-"sort-of instruction" ) as being a "not to exceed" number -- my experience with DMIPS is they are usually optimistic compared to real workloads, as the "instructions" they measure are not real instructions -- see http://en.wikipedia.org/wiki/DMIPS#Criticisms

It might be you have to recode your 23 instruction loop in assembly to go faster (i.e., to use less instructions)... Or maybe you have already done that?

-- Rich

Re: Force variable to Register

gbaars — Tue, 22 Jul 2008 20:39:38 GMT

If from previous overview (0.76 DMIPS/MHZ)
the 0.76 is multiplied with 50 MHz the result
is 38 (DMIPS?). This may be a coincidence or not.
I measured 38 MIPS with the 1000x loop consisting of
23 mainly 1 cycle asm instructions.

If 2.1 MIPS/MHz is possible this can be due
to executing on both positive and negative
edges of the mcuclk.

Each time I run the debugger the command
window finally shows:

RESET
done .\cmd\CFV1_BDM_P&E_Multilink_CyclonePro_postload.cmd

Postload command file correctly executed.
main 0x410 T
Frequency change to ~24056572hz.
STARTED
RUNNING
Breakpoint

in>

this overrules the settings made with the processor
expert. If the setting was 20 MHz also this is app.
halved then to 10 MHz. (?)

Re: Force variable to Register

RichTestardi — Tue, 22 Jul 2008 20:51:54 GMT

Oh! I don't think 2.1 MIPS/MHz is possible on that core...

I believe 2.1 is actually the Dhrystone version.

The line in the data sheet says:

Provides 0.94 Dhrystone 2.1 MIPS per MHz
performance when running from internal RAM
(0.76 DMIPS/MHz from flash)

Which I interpret as meaning 0.94 DMIPS/MHz (DMIPS are "Dhrystone 2.1 MIPS ").

A DMIP is *almost* a real MIP, but usually on the optimistic side.

Re: Force variable to Register

gbaars — Tue, 22 Jul 2008 22:17:01 GMT

Maybe not in that case if

0.76 dhrystone 2. 1MIPS.

Since fastest instr. do 1 cycle and (somsort of) average. 1/0.76

= 1.3 c/inst. A lot of instructions though take 4 or more.

The goal was to evaluate the DEMOJM which came as a gift

and the Coldfire cpu. I am beginning to think XMEGA AVR

running a true 32 MIPS programming in ASM might

perform at a much faster average than a V1 coldfire

in 'C'. MIPS are important because my applications

involve DSP on audio.

Re: Force variable to Register

RichTestardi — Tue, 22 Jul 2008 23:29:36 GMT

Hi,

I have no reason to believe you are not seeing a true 38 (real) MIPS... I suspect your code may be running from flash, and written in C as well. So that seems reasonable.

I don't understand why you think a 32 MIPS processor (which is only 8/16 bit) might be faster? I'd probably be surprised if that was the case.

I think the real trick will be coding your performance loop in assembly (whether using ColdFire *or* AVR) -- I'd be surprised if you could not get a factor of 2 speedup there...

Good luck!!!

-- Rich

Re: Force variable to Register

gbaars — Wed, 23 Jul 2008 21:38:21 GMT

38 Mips was measured with a loop

whith 1 cycle instructions and a branch.

With the average of the entire inst.set

AVR has much

lower cycles than MCF51 running at

50MHz even performance will be less then

a 20 MHz tiny AVR.

Re: Force variable to Register

RichTestardi — Wed, 23 Jul 2008 22:51:53 GMT

Hi,

OK, I'll have to take your word for that since I have no experience at all with AVR.

I'm not sure what the best way is to "average" an instruction set, since the majority of code use a small subset of the instruction set.

(I admit I am partial to the ColdFire processors! Even from the 68k days! So I am glad you're at least trying them!!!)

-- Rich